Fork U

[Mike B.]

I need an education on clutch forks - thus "Fork U.

I have been in search of a confirmed correct fork for my '61. I have one that works, but the spring is partially broken so I have hesitated to repair it until I know that it is correct. Unfortunately forks of the period were not stamped with part numbers.

Yesterday while culling through a mountain of parts at the Hershey, PA meet, I came across two very similar forks. At $5 each I figured that it was worth the chance, or at the least, the education.

I cleaned and glass bead blasted each before paint and offer the photos below. I am inclined to think that #1 is correct for '56? - '62; #2 for '64-'81. But I would certainly appreciate any comments from the learned peanut gallery including any telltale things to look for and/or specific measurements for each.

Thanks!
Mike

[Joe L.]

Re: Fork U

I need an education on clutch forks - thus "Fork U.

I have been in search of a confirmed correct fork for my '61. I have one that works, but the spring is partially broken so I have hesitated to repair it until I know that it is correct. Unfortunately forks of the period were not stamped with part numbers.

Yesterday while culling through a mountain of parts at the Hershey, PA meet, I came across two very similar forks. At $5 each I figured that it was worth the chance, or at the least, the education.

I cleaned and glass bead blasted each before paint and offer the photos below. I am inclined to think that #1 is correct for '56? - '62; #2 for '64-'81. But I would certainly appreciate any comments from the learned peanut gallery including any telltale things to look for and/or specific measurements for each.

Thanks!
Mike

Mike-----


It's true that GM clutch forks used at least through 1981 don't have part numbers on them. That was a VERY dumb thing for GM to do since there are so many forks that are very similar. But, that's what they did, so it's done.

Over the 1955-81 period, Corvettes used a total of 6 different clutch forks.

1955-56 used fork GM #3727737. I know very little about this one. It may have been a cast or forged type piece. It MAY have used a ball-type articulation between fork pushrod and fork.

1957-61 used fork GM #3741460. This is a STAMPED type fork. It uses a pin-type fork pushrod attachment. It was also used for 55-56 SERVICE but required the change of other parts.

1962 used fork GM #3739043. This is a STAMPED type fork and is similar or identical to the 3741460. I don't know what the differences are between the two, if any. The 3739043 became the SERVICE fork for 55-61 in May, 1962 when the 3741460 was discontinued.

1963 used fork GM #3820008. This is a stamped type fork but it differs markedly from earlier or later stamped Corvette forks. It uses a ball-type articulation between the fork pushrod and the fork. It does not use a pin.

1964-E66 used fork GM #3844493. This is a stamped type fork which uses a pin for the push-rod attachment in the same manner as all 57-62 forks. I THINK it has a greater off-set than 57-62 forks. Otherwise, I don't know much about it. I wish I did. It is POSSIBLE that your pictured fork identified as "1" is a 3844493.

L1966-81 use fork GM #3887177. This is a stamped type fork which also uses a pin for push-rod attachment. It has a wider beam than the 57-62 forks and may or may not have a wider beam than the 3844493. I suspect it does but I don't know that. I've never been able to confirm by comparison with a KNOWN example of a 3844493. The 3887177 became the SERVICE fork for 64-E66 when the 3844493 was discontinued in February, 1967.

All of the above-referenced forks are Corvette-only pieces EXCEPT the 1963 fork. It was used for other passenger car applications.

Passenger cars and, likely, some trucks used forks very similar to the Corvette forks. Through about 1962, most passenger car applications used stamped forks with a pin-type pushrod attachment like Corvettes. So, these may be difficult to distinguish from Corvette. From 1963 to about 1965, SOME passenger cars continued to use pin-type attachment forks while many others changed to non-pin style forks. After 1965, most, if not all, passenger cars used non pin style forks.

It's very difficult to determine just what your measuring protocols are. Since the measurements are so close in most cases, that makes it difficult for me to use your measurements to identify the 2 forks you have pictured. However, the fork that you have identified as "1" has, depending upon how it's measured, a similar offset to the 64-81 forks. However, it's definitely NOT the L66-81 fork since the beam is too narrow. It's either the 64-E66 Corvette fork, the 57-62 fork, or some other 57-65 passenger car (or truck) fork.

Pictured below are examples of the GM #3887177 fork. The upper example is a new GM piece. The lower is an original GM piece used in PRODUCTION and SERVICE. There are very slight differences between the two but, basically, they are the same. Note the wide beam section which is unlike either of the ones in your photos.

The way I see it, the ones you have pictured MIGHT be any of the 57-62 Corvette forks, the 64-E66 Corvette forks, or any of the similar 57-65 passenger car forks.

By the way, what does the fork that you've been using on your car look like (the one with the broken spring retainer) and how does it compare to these other forks you have pictured?

[Bill M.]

Re: Fork U

My '59 is disassembled. I measured 10-1/8 long, 2-7/8 deep, and it looks more like contour "1" than contour "2".

I think I replaced this part in the mid 60s, but I would have had the original '59 part to compare to, so I think this is the same configuration as the original '59 part.

[Mike B.]

Re: Fork U

Joe,

Thank you very much for responding and the excellent overview. I knew that 1963 was unique regarding the year and style but I did not know that all the others were also unique to Corvette only.

I just took a look at the fork that is currently installed (thank goodness for having a lift). I could not compare it exactly, but it clearly has the more pronounced gooseneck and bottom radius curve of #1 in the photo. This corresponds with Bill's comparison. And sorry that the dimensions are not very defined. It was hard to find absolute reference points anywhere.

The nut on the bellcrank-to-fork adjusting rod is at about 1/3 from the end of the threads. I think that if I were to use the other fork, the adjusting nut would be have to be very close to the end to make up for the less pronounced bend in the neck.

Do you know if the critical dimension from the pivot ball to the centerline of the output shaft differed between the listed forks? If not, then besides the need for different adjusting rod lengths, and perhaps a slightly different mechanical advantage at the fulcrum, how much difference could there be between them all? More puzzling is why on earth did GM make so many varieties? Was perhaps the pressure plate style/fingers different between passenger cars and Corvettes?

These may remain unanswered questions but to be honest, it is things like this that keep me addicted to the hobby!

Regards,
Mike

[Joe L.]

Re: Fork U

Joe,

Thank you very much for responding and the excellent overview. I knew that 1963 was unique regarding the year and style but I did not know that all the others were also unique to Corvette only.

I just took a look at the fork that is currently installed (thank goodness for having a lift). I could not compare it exactly, but it clearly has the more pronounced gooseneck and bottom radius curve of #1 in the photo. This corresponds with Bill's comparison. And sorry that the dimensions are not very defined. It was hard to find absolute reference points anywhere.

The nut on the bellcrank-to-fork adjusting rod is at about 1/3 from the end of the threads. I think that if I were to use the other fork, the adjusting nut would be have to be very close to the end to make up for the less pronounced bend in the neck.

Do you know if the critical dimension from the pivot ball to the centerline of the output shaft differed between the listed forks? If not, then besides the need for different adjusting rod lengths, and perhaps a slightly different mechanical advantage at the fulcrum, how much difference could there be between them all? More puzzling is why on earth did GM make so many varieties? Was perhaps the pressure plate style/fingers different between passenger cars and Corvettes?

These may remain unanswered questions but to be honest, it is things like this that keep me addicted to the hobby!

Regards,
Mike

Mike------


I do not know if the ball stud centerline-to-main shaft centerline dimension differed between the forks. I do did not think it differed for any of the 63-81 forks. Whether it differed between the 55-62 forks and later forks, I do not know.