Modding contacts on 300/4D for teleconverter

So, I just got me a Tamron X1.4 TC (the new one) to be used with my also relatively new Tamron 70-200/2.8 (the latest version).

It works fine, however, I was kinda hoping that the same TC would work with my good, old 300/4D Nikkor. At first I was like: Wait a minute, this Tamron TC is good only for electronic diaphragms! But then I thought: Hey, the 300mm is a "D" model, so I'll be able to just use the aperture ring. Worst case scenario: "It doesn't work", though I didn't see why it wouldn't. I shoot with the D800, BTW, and I knew for a fact that it could be used with plenty of old, totally manual lenses. So, I was like: If it don't work - I'll make it work! And if THAT don't work - no biggie. I needed the X1.4 TC to cover the gap I had between 200 and 300mm and the extra reach out of the 300/4D would be a bonus.

After attaching the 300/4D to the TC and switching the camera on, the first thing I noticed was a "F EE" error message. Then I looked through the viewfinder and went: How come it's so dark? But I quickly realized that the diaphragm was closed. I manually opened it up and the picture became what it should be. So, there's no way to control the aperture with the camera with this TC (duh!) but the AF works just fine (!). There's no way to release the shutter though and take a picture, because of that persistent error message. Setting up a "manual lens" did not help either. And then I decided to do something "drastic".

By releasing a couple of tiny screws I was able to jam a piece of plastic in front of the contacts on the lens. And guess what.. I can now take pictures! I can dial in any shutter speed I want and to change the aperture I just have to rotate the aperture ring, which now, is even more accessible because of the TC. I took some test shots and, boy, am I glad I went the extra mile. The image quality is nothing to brag about but, given everything, it is up there, even by today's standards. The 'bare' 300/4D is a pretty good lens to begin with. So, to me this little project was definitely worth the hassle.

The elephant in the room is obviously the loss of Auto-Focus... Yep, that's a bummer but nothing I can't live with, if I had to, that is and I need to explain:

So, I only use the 300/4D for shooting panoramas (and the 70-200mm, mostly). Often I do need to go that long because a photo that one would normally capture with, let's say, a 100mm lens, I shoot with a 300mm because I have to break it up into several pieces. And there were plenty of times when I wished I had something even longer with me BUT even if I was given another huge telephoto lens for free, I just wouldn't carry it on my back... That is why I went the TC route. So, basically, if I'm shooting with the 300mm, TC or not, the camera is on a tripod and my ass is on a foldable stool. I can take my time and focus manually in Live View. However, I would really want to have the AF for the obvious reasons, one being that in broad daylight it is very hard to see anything on that LCD. Also, by permanently having the contacts blocked, the 300mm becomes a manual-focus, MANUAL APERTURE lens even when I'm not using it with the TC (which would, actually, be like 90% of the time).

Then I thought: Hey, maybe don't block ALL the contacts, just the ones that are screwing it up for me. I was able to find the schematics of what each of the contacts was responsible for but it gets very confusing with a teleconverter in the equation. So this is where I need YOUR help.

How do I get the AF to work w/o the camera going crazy on me? I must add that this weird setup is something that works for me and I really do need this.

Thanks a whole bunch for reading all this!

Exactly which 300mm/f4 do you have?

There is not really a 300mm f4 D. The earlier 300mm/f4 that uses screwdriver AF is a pre-D lens, and it has 82mm filter thread. The 300mm/f4 AF-S from 2001 uses 77mm filters. And the latest AF-S with VR is an E lens and is also a PF so that it is quite small for a 300mm.

It sounds like you may have that screwdriver AF version, hence it is not AF-S.

I am sorry. Given the variety I should have been more clear on the model. The lens in question is the 300mm F/4D AF-S, reviewed and discussed on this page:

Nikon 300mm f/4D AF-S Review

(no affiliation whatsoever, you can delete this link if it violates any policies).

If the teleconverter is just passing through the electronics from the lens (possibly with an adjusted maximum aperture), I guess it's expecting to be "G" in that it's not moving the aperture ring, but the electronics aren't telling the camera that it's G. Have you tried wedging the aperture ring (perhaps with a bit of paper behind the teleconverter) so the camera thinks there's a non-G lens attached?

Interesting, though - I didn't realise Tamron had made an e-aperture teleconverter.

If the OP is talking about this TC: Tamron Teleconverter 1.4x for Nikon F TCX14N700 B&H Photo Video then it appears that particular one is only compatible with three Tamron lenses: TAMRON | Teleconverter (150-600 G2, 70-200 G2 and 100-500 (A035); technically these are E lenses with an electromagnetic diaphragm). Examining whatever images of the TC I could find, there doesn't appear to be an AI tab (or Ai-tab feedthrough) - so the lens can't communicate it's maximum aperture to the camera. There's also no protrusion at the 8 o'clock position to depress the switch that indicates to some Nikon cameras (not the D800 though) that the lens is set to its minimum aperture. And there doesn't seem to be a mechanical provision to communicated with the aperture control in the Nikon lens - the lens will be stopped down to the set aperture. All this seems consistent with the lens/TC combo acting as if there is no communication at all with the camera - because there isn't since the TC is designed to work with E lenses only.

Interesting, though - I didn't realise Tamron had made an e-aperture teleconverter.

Me neither.

I am sure one can figure out which contacts need to be "unblocked". I'd rather go and find a used TC-14E (or TC-14EII) and use that one instead on the 300.

If the teleconverter is just passing through the electronics from the lens (possibly with an adjusted maximum aperture), I guess it's expecting to be "G" in that it's not moving the aperture ring, but the electronics aren't telling the camera that it's G. Have you tried wedging the aperture ring (perhaps with a bit of paper behind the teleconverter) so the camera thinks there's a non-G lens attached?

 

Interesting, though - I didn't realise Tamron had made an e-aperture teleconverter.

They made it in Japan too. Anyway, I am sorry for causing a confusion. I have a D, not a G-lens. It has both: a dedicated aperture ring and a mechanical lever for the camera body to control the diaphragm in that manner. The Tamron TC does not provide any mechanical coupling for aperture or focus control. In the case of AF I lucked out, well, sort of, because my lens has a built-in AF motor. In theory, at least, I should be able to do what I need, because the AF DOES work when all the contacts are connected.

I have a D, not a G-lens.

Nikon did not make a 300mm f/4D

Nikon did not make a 300mm f/4D

I've had this puppy for 12 years, at least. And gee... I didn't realize it could still be purchased new today... for the same price too, after all these years:

Nikon AF-S NIKKOR 300mm f/4D IF-ED Lens 1909 B&H Photo Video

So, I don't know which 300mm f/4D lens Nikon "didn't make"... Sorry for the earlier confusion...

To clear up the terminology, the AF-S lens, like all AF-S lenses(?), is "D" - it returns (coarse) distance information electronically, which is used for some flash metering. "D" often gets used as a shorthand to distinguish lenses described as "AF-D" from "AF-S" (the latter using the Silent Wave Motor, the former using screwdriver drive), but several AF-S lenses are labelled "D" as well. Screwdriver focus is not needed for AF-S, AF-I or AF-P lenses (possibly or F3AF, but that's weird and I don't claim to know). AF-D is strictly intended to distinguish from plain (screwdriver) AF, as seen in the 80-200 f/2.8 mk1 AF (push-pull), which is not "D" and doesn't return distance information.

Anyway... Okay, you have the AF-S. It will be trying to tell the camera that it's an AF-S but not G lens (I don't know what exactly in the protocol says "G", but something does); this may or may not be getting mangled by the teleconverter's electronics. If that information goes through, you would normally expect to be able to control the aperture via the aperture lever with the aperture set to its minimum (but not with this teleconverter, which doesn't mechanically link the aperture lever), or for the aperture lever to stop down to its fullest extent and have the aperture controlled by the aperture ring (but the teleconverter isn't passing on the aperture ring position).

My theory was that the camera could be complaining because the lens claims to have an aperture ring, but the aperture ring on the camera isn't being moved at all by the attachment of the teleconverter. I think, but could be wrong, that the aperture ring moves slightly even when the lens is wide open. If you can find the position that corresponds to f/4 and wedge the aperture ring there (with paper or a bit of gum), it might be less intrusive than trying to block the electronics - if it works. Then if the camera thinks the lens is set to wide open (and you'll need to tell the camera to set the aperture via the aperture ring), it will effectively expose at whatever aperture the lens is set to - although the matrix meter might behave a bit oddly if the actual aperture is different, so spot metering might behave better.

This could be nonsense, but it feels worth a try. (As others have said, I'd be more confident with a cheaper DSLR with an EE switch rather than the aperture following ring).

I could be wrong (and googling "F mount pinout" will give you some information), but I believe quite a lot of the protocol is serial, and not as simple as physically blocking a pin. So this might be easier, if it works.

Bear in mind though that the AF system assumes it's got wide open to play with. If you stop the lens down so that you're trying to focus at something effectively smaller than f/5.6, phase detect AF might struggle. You might have more luck with live view, although the D800's live view is a bit iffy.

I agree that getting a Nikon TC is simpler, and the 300 f/4 AF-S does teleconvert quite well (this is my combination when I can't be bothered to carry something the size of the 200-500 or want a bit more light when I only need 300mm) - but it's not free, and I appreciate that "buy the right component" isn't always an option.

Good luck? I'll be interested to know whether this works.

To clear up the terminology, the AF-S lens, like all AF-S lenses(?), is "D" - it returns (coarse) distance information electronically, which is used for some flash metering. "D" often gets used as a shorthand to distinguish lenses described as "AF-D" from "AF-S" (the latter using the Silent Wave Motor, the former using screwdriver drive), but several AF-S lenses are labelled "D" as well. Screwdriver focus is not needed for AF-S, AF-I or AF-P lenses (possibly or F3AF, but that's weird and I don't claim to know). AF-D is strictly intended to distinguish from plain (screwdriver) AF, as seen in the 80-200 f/2.8 mk1 AF (push-pull), which is not "D" and doesn't return distance information.

 

Anyway... Okay, you have the AF-S. It will be trying to tell the camera that it's an AF-S but not G lens (I don't know what exactly in the protocol says "G", but something does); this may or may not be getting mangled by the teleconverter's electronics. If that information goes through, you would normally expect to be able to control the aperture via the aperture lever with the aperture set to its minimum (but not with this teleconverter, which doesn't mechanically link the aperture lever), or for the aperture lever to stop down to its fullest extent and have the aperture controlled by the aperture ring (but the teleconverter isn't passing on the aperture ring position).

 

My theory was that the camera could be complaining because the lens claims to have an aperture ring, but the aperture ring on the camera isn't being moved at all by the attachment of the teleconverter. I think, but could be wrong, that the aperture ring moves slightly even when the lens is wide open. If you can find the position that corresponds to f/4 and wedge the aperture ring there (with paper or a bit of gum), it might be less intrusive than trying to block the electronics - if it works. Then if the camera thinks the lens is set to wide open (and you'll need to tell the camera to set the aperture via the aperture ring), it will effectively expose at whatever aperture the lens is set to - although the matrix meter might behave a bit oddly if the actual aperture is different, so spot metering might behave better.

 

This could be nonsense, but it feels worth a try. (As others have said, I'd be more confident with a cheaper DSLR with an EE switch rather than the aperture following ring).

 

I could be wrong (and googling "F mount pinout" will give you some information), but I believe quite a lot of the protocol is serial, and not as simple as physically blocking a pin. So this might be easier, if it works.

 

Bear in mind though that the AF system assumes it's got wide open to play with. If you stop the lens down so that you're trying to focus at something effectively smaller than f/5.6, phase detect AF might struggle. You might have more luck with live view, although the D800's live view is a bit iffy.

 

I agree that getting a Nikon TC is simpler, and the 300 f/4 AF-S does teleconvert quite well (this is my combination when I can't be bothered to carry something the size of the 200-500 or want a bit more light when I only need 300mm) - but it's not free, and I appreciate that "buy the right component" isn't always an option.

 

Good luck? I'll be interested to know whether this works.

Wow... Thanks!

Well, first, jamming something in the camera's f-mount is a no-go for me. Even if it does make it work, I would need to do this procedure "in the field" every time I'd want to use the 300mm with that TC. I'm not even going to attempt something like that just for the reason of having to keep the mirror box exposed for more than the few mandatory seconds it normally takes to change the lens on a DSLR.

And, actually, I'm not sure that I would need to even argue with the camera: "Oh, so you think this is a G-lens? No! It's actually a D-lens!". I'm not getting into that because I don't care what kind of serial signals the TC is sending back! I just block those pins and let the camera argue with itself... All I want is for the power to get through to the AF motor and it really may be that simple... As witnessed before (by me): the AF motor don't care about the mounts, apertures and what not. The camera is blinking an error, refusing to take pictures, yet the AF works just fine, in Live View too!

The beauty in this apparent "mess" is that it just works for me: So, I'm on a tripod, doing some multi-row HDR panoramas, naturally I would use the LV to focus anyway, it being iffy or not. And the metering? I always run a whole bunch of test shots before committing to a 180-shot or so sequence, so no, I don't care all that much about the metering because I don't trust it anyway. I need my histograms to figure out the exposures.

As of now, I'm tempted to just, you know, chance it. I've got the pin layout, so I'll probably just go for it: expose the ones that I think should be in contact and if that doesn't work, then I'll give it a couple more go's with different pins. It is a bit of a hassle though, so, before I actually do it, I'd like to get some more input on the matter.

This being a quite simple, totally reversible hack, I assumed that people here would have at least some experience with those basic internal workings. I probably should just wait a couple of days and unless someone talks me out of it, I'm totally doing it.

Well, first, jamming something in the camera's f-mount is a no-go for me. Even if it does make it work, I would need to do this procedure "in the field" every timeI'd want to use the 300mm with that TC. I'm not even going to attempt something like that just for the reason of having to keep the mirror box exposed for more than the few mandatory seconds it normally takes to change the lens on a DSLR.

Just to be clear, I'm talking about jamming the aperture following ring around the mount - the spring-loaded tab that hooks on the back of the aperture ring on AI lenses (in which I'm including everything from AI until G) and which is pushed when you change aperture. You can push that with your finger while the lens is attached - it doesn't involve exposing the mirror box in the field. You would have to wedge it in place (unless you stuck something to the back of your teleconverter to make it permanent), but it's less intrusive than trying to cover the electrical pins without damaging them; I'd use blu-tak, or whatever the US equivalent is. Of course, it's only a theory - but F-EE often means the aperture isn't set correctly, and if you're already telling the body the aperture is set from the lens, this is the main way I can think of that there could be confusion.

Is your belief that you can block the serial pins and the AF will still work? I'm doubtful, although I've not tried it - I suspect the AF is working because it knows a lens is attached due to the serial connector, but not releasing the shutter because of the aperture confusion. But good luck, and please report back. Bear in mind the connector pins are spring loaded, and they can be a little iffy if you do nasty things to them (having dismantled a TC-16A in the past to convert it), so don't be too enthusiastic!

Just to be clear, I'm talking about jamming the aperture following ring around the mount - the spring-loaded tab that hooks on the back of the aperture ring on AI lenses (in which I'm including everything from AI until G) and which is pushed when you change aperture. You can push that with your finger while the lens is attached - it doesn't involve exposing the mirror box in the field. You would have to wedge it in place (unless you stuck something to the back of your teleconverter to make it permanent), but it's less intrusive than trying to cover the electrical pins without damaging them; I'd use blu-tak, or whatever the US equivalent is. Of course, it's only a theory - but F-EE often means the aperture isn't set correctly, and if you're already telling the body the aperture is set from the lens, this is the main way I can think of that there could be confusion.

 

Is your belief that you can block the serial pins and the AF will still work? I'm doubtful, although I've not tried it - I suspect the AF is working because it knows a lens is attached due to the serial connector, but not releasing the shutter because of the aperture confusion. But good luck, and please report back. Bear in mind the connector pins are spring loaded, and they can be a little iffy if you do nasty things to them (having dismantled a TC-16A in the past to convert it), so don't be too enthusiastic!

Now I got. Thanks, man! Playing with that outer aperture ring just might do the trick, I'll definitively try it tomorrow, because it'd be like: Problem solved!

I should have some time to play with this thing tomorrow. Who knows how that serial data link really works... Not us, apparently.

I'd rather go and find a used TC-14E (or TC-14EII) and use that one instead on the 300.

I have used Nikon's TC-14EII with the 300mm f/4 AF-S on a crop body (D7100). With good technique it works well, with very sharp results, if not quite as sharp as the less magnified images from the 300mm lens alone.

Problem with jamming the aperture follower is that you would have the dim viewfinder, and probably very bad AF performance, because the aperture would be closed already at the time of focussing. And you'd probably loose any aperture info on the body (and in EXIF). Whether that would affect metering, not sure, but for matrix metering I wouldn't be surprised. So even if it works, it sounds like a workaround with some serious potential disadvantages to me.

If you'd fine with those disdavantages - if your body supports non-CPU lenses, you should be able to "override" the FEE error by setting the camera to allow aperture-ring use on CPU lenses. I believe in menu entry F9, or thereabouts.

As Hector, I can confirm this lens works very fine with the TC14EII - if you feel you'd use a TC often enough, I'd really just get that. I also have the TC17EII, and that's a far less happy marriage so I'd avoid that TC.

Problem with jamming the aperture follower is that you would have the dim viewfinder, and probably very bad AF performance, because the aperture would be closed already at the time of focussing. And you'd probably loose any aperture info on the body (and in EXIF). Whether that would affect metering, not sure, but for matrix metering I wouldn't be surprised. So even if it works, it sounds like a workaround with some serious potential disadvantages to me.

If you'd fine with those disdavantages - if your body supports non-CPU lenses, you should be able to "override" the FEE error by setting the camera to allow aperture-ring use on CPU lenses. I believe in menu entry F9, or thereabouts.

 

As Hector, I can confirm this lens works very fine with the TC14EII - if you feel you'd use a TC often enough, I'd really just get that. I also have the TC17EII, and that's a far less happy marriage so I'd avoid that TC.

OMG!!!!!!!! It just works! The shutter, the focus, I mean: WOW!... How come no one came up with this idea? Thanks so much, man, but actually, I have a much bigger problem now... I'm starting a new thread...

Just to be clear, I'm talking about jamming the aperture following ring around the mount - the spring-loaded tab that hooks on the back of the aperture ring on AI lenses (in which I'm including everything from AI until G) and which is pushed when you change aperture. You can push that with your finger while the lens is attached - it doesn't involve exposing the mirror box in the field. You would have to wedge it in place (unless you stuck something to the back of your teleconverter to make it permanent), but it's less intrusive than trying to cover the electrical pins without damaging them; I'd use blu-tak, or whatever the US equivalent is. Of course, it's only a theory - but F-EE often means the aperture isn't set correctly, and if you're already telling the body the aperture is set from the lens, this is the main way I can think of that there could be confusion.

 

Is your belief that you can block the serial pins and the AF will still work? I'm doubtful, although I've not tried it - I suspect the AF is working because it knows a lens is attached due to the serial connector, but not releasing the shutter because of the aperture confusion. But good luck, and please report back. Bear in mind the connector pins are spring loaded, and they can be a little iffy if you do nasty things to them (having dismantled a TC-16A in the past to convert it), so don't be too enthusiastic!

Well, the solution to this problem is above this post, however, I've got myself a much bigger problem now... Anyway, I did try covering up select pins, with no progress. I then removed the plastic piece completely and put everything back they way it should be. I mounted the lens and tried that thing with the aperture ring, with no success. Then I tried to detach the 300mm from the TC... Yes,... "tried"... It won't come off, man!!

I'm starting a new thread in the forum..

I agree with Andrew:

The FEE error is triggered and cancelled by the position of the AI coupler tab on the camera, when a non-G lens is fitted.

There should be no need to jam bits of plastic in the camera contacts. The AI tab can simply be manually moved clockwise (as seen from the back of the camera) until the FEE error disappears, and held in place with a small piece of adhesive tape or Blu Tak.

"Problem with jamming the aperture follower is that you would have the dim viewfinder, and probably very bad AF performance, because the aperture would be closed already at the time of focussing."

- There's no connection between the AI follower and aperture actuator lever Wouter. The two work completely independently and the lens aperture would stay wide open until the aperture ring on the lens was moved manually. Slow to use, like an old preset lens, but doable.

I just o not understand all of this..

Risking an expensive camera , an expensive specialised TC, an expensive lens, just to avoi buying a 1.4 TC ..

Even the first version Nikon TC-14E ( AF-I TC-14E) works perfectly well with that version of the 300mm F/4 , an if that is to expensive on flee-bay then also the Kenko1.4x teleplus Pro 300 GX does the job perfectly well ( i use both of them an i do not see any diffrence in image quality..)

Just my 2 cents..

There's no connection between the AI follower and aperture actuator lever Wouter.

I know, but in this case, the aperture actuator isn't being operated at all (the TC has no linkage for it), so except for using the lens wide-open, the aperture is closed down to the value set on the aperture ring, all the time. My comment on dim viewfinder and lacklustre AF performance assumes using the lens stopped down, which I should have stated.

The AI follower only came into play to stop the FEE error. I did not imply the two were related, apart from the OP following the suggestion to trick the AI follower tab into submission. My point was it solved one issue, and introduced others.

Ah, OK. I see what you mean now Wouter. It shouldn't be a problem when using Live View though.

I've just been playing with a few Nikon and M42 lenses on a Sony a6000 - my latest toy. That presents the same issue of making all non-native lenses manual focus and preset aperture. For static subjects it's not a problem, but if the subject moves you might as well forget it.

It's fun to see how different makes and mounts of lens stack up against each other on the same sensor though. Worst performer so far is a Wray Unilite 50mm f/2. I never had a high opinion of Wray lenses, and this just confirms it.

To be fair, I hope we're mostly talking about shooting wide open with a teleconverted lens this long, and with the teleconverter that's going to be at f/5.6 (effective). You could lose a stop (in daylight) and go down to f/8 for possibly a little more sharpness - and that's not that dim, and would still work with the centre few AF points on a D800. Going beyond that, you're going to have diffraction issues anyway.

The Nikon teleconverters are a bit on the expensive side, unfortunately. So I sympathise with trying to trick things, especially if you already have a TC-14 of sorts. Prodding the aperture ring into place probably won't break anything if done carefully.

Of course, if you wedge the lens on the teleconverter with a bit of plastic, that's another matter (which is one reason I was nervous about it). :-) At least it's not stuck on the body. For what it's worth, I've accidentally mis-mounted my TC14E-II on my 200-500 (by attaching it at the wrong rotation - I'm sure there's a good reason that this is sometimes possible and the mount is a bit symmetrical, and I'll have words with the engineer who designed it as such if I find him) and it took me several increasingly frantic minutes to get it to let go. Fortunately, with no apparent harm to either part.

I've accidentally mis-mounted my TC14E-II on my 200-500 (by attaching it at the wrong rotation - I'm sure there's a good reason that this is sometimes possible and the mount is a bit symmetrical

I'm surprised that works since the bayonet on an AF-S lens is definitely not symmetrical - one prong has a cut-out and it's overall length is longer than the other two. The smaller bit prevents mounting those TCs to lenses not designed for them.

I'm surprised that works since the bayonet on an AF-S lens is definitely not symmetrical - one prong has a cut-out and it's overall length is longer than the other two. The smaller bit prevents mounting those TCs to lenses not designed for them.

It didn't fully mount. But it did latch on and refuse to come off without a lot of jiggling and some degree of panic from me. (I have a third party tilt-shift with a scary mount that is good at getting hooked in the wrong orientation, and is made more complex because, due to the shape of the lens hitting the prism overhang, you have to press the lens on the camera and then rotate its mount under it.) It feels like there should have been a better way to do this, but I'm a few decades late. Though I suppose I should grumble publicly in the hope that any mirrorless mount learns from the experience.

If I'm going to complain about fitting shapes into the appropriate holes (on a family-friendly site, at least) I'm first going after the people who made it possible, when very tired and at an unfamiliar filling station, to put petrol into a diesel tank. Which was fortunately only quite expensive, not catastrophically so. If only every three year old didn't know a way to solve this problem!

If I'm going to complain about fitting shapes into the appropriate holes (on a family-friendly site, at least) I'm first going after the people who made it possible, when very tired and at an unfamiliar filling station, to put petrol into a diesel tank. Which was fortunately only quite expensive, not catastrophically so. If only every three year old didn't know a way to solve this problem!

At least in the US, for cars made since '75 or '76(I forget when exactly) it's impossible to put diesel in a gasoline tank, but not the other way around. Neither situation is ideal, but gasoline cars can at least digest diesel in small doses(I've put kerosene or diesel in my tank a gallon or so at a time for specific reasons) but the lower pressure gasoline in 22:1 or better diesel can be catastrophic.

The reason I mention the '75 cut off date is that it's when catalytic converters were mandated for the US market. Since leaded gas with a catalytic converter is a no-no, unleaded pumps used a smaller nozzle and cars with catalytic converters were fitted with a smaller diameter filler neck that wouldn't accept an unleaded nozzle.

I'm an MG(or specifically MGB) nut, and have a 1970. Of course, I have a big filler neck on my car. In typical British Leyland cost cutting fashion, though, when they started fitting catalytic converters all they did was put a plastic cone in the neck that an enterprising owner could remove if they were so inclined. I know a few folks who owned one "back in the day" and did so along with punching out the honeycomb in the catalytic converter(the latter of course was and still is highly illegal). By the time all the necessary emissions "plumbing" was added to the head castings on late cars, the timing was advanced like crazy(and the engine intentionally run lean) to reduce CO emission at the cost of high cylinder temperatures, and the use of induction hardened valve seats about 80% of late engine heads are cracked these days. Using leaded gas probably saved a few of them. I'm running a late® head on my car for other reasons, but found one uncracked and had it "properly" fitted for unleaded gas with hardened valve seats. I also have a pre-68 advance curve in my distributor(best for performance) and run my car a tiny bit on the rich side(again, performance).