C2 Clutch Fork Rod Length

[Ed R.]

I have a 66 327/300 4-speed with an April build date. Engine and trans are out of the car and I'm in the process of getting everything back together. Installing new clutch, clutch fork, ball stud in the bellhouseing, etc. The clutch is Delco OEM. I bought a new clutch fork rod because there was a lot of ware on the end that attaches to the cross shaft. My old rod is 9" from fork end to inside of the bend. The one I bought is 10 9/16" to the same point.
My question is the length of the old fork rod. All the reading I've done in the archives say it should be 8 1/2 or 10 9/16 with 10 9/16 for late 66 years. Any ideas what the 9" rod is? Everything worked well before this project and it sure would be nice not to have clutch adjustment issues. One other point. I'm replacing the CE 327 that was in the car originally with the correctly dated 327/300 engine. I don't know if I should keep the longer rod and try it or get the shorter rod. I'm about 2 weeks from starting this engine up. What are your recommendations? I really don't know what to do now.

[Joe L.]

Re: C2 Clutch Fork Rod Length

I have a 66 327/300 4-speed with an April build date. Engine and trans are out of the car and I'm in the process of getting everything back together. Installing new clutch, clutch fork, ball stud in the bellhouseing, etc. The clutch is Delco OEM. I bought a new clutch fork rod because there was a lot of ware on the end that attaches to the cross shaft. My old rod is 9" from fork end to inside of the bend. The one I bought is 10 9/16" to the same point.
My question is the length of the old fork rod. All the reading I've done in the archives say it should be 8 1/2 or 10 9/16 with 10 9/16 for late 66 years. Any ideas what the 9" rod is? Everything worked well before this project and it sure would be nice not to have clutch adjustment issues. One other point. I'm replacing the CE 327 that was in the car originally with the correctly dated 327/300 engine. I don't know if I should keep the longer rod and try it or get the shorter rod. I'm about 2 weeks from starting this engine up. What are your recommendations? I really don't know what to do now.

Ed-----


There was no 9" fork pushrod used for Corvettes except, POSSIBLY, 1963 and 1963 uses a completely different end configuration that would not work on any other year Corvette.

The 64-81 rods are 10-9/16" and 8-37/64". So, either you have one of those or you have a non-Corvette rod. The problem with the latter is the Corvette uses a rather unique rod-to-fork articulation. So, I don't know of any other rod that would work with a Corvette fork (clevis pin type attachment). 1957-62 Corvette uses a rod configuration similar to 64-81, but that rod is 9-13/16" in length. So, it's not that one, either.

[Michael H.]

Re: C2 Clutch Fork Rod Length

Ed-----


There was no 9" fork pushrod used for Corvettes except, POSSIBLY, 1963 and 1963 uses a completely different end configuration that would not work on any other year Corvette.

The 64-81 rods are 10-9/16" and 8-37/64". So, either you have one of those or you have a non-Corvette rod. The problem with the latter is the Corvette uses a rather unique rod-to-fork articulation. So, I don't know of any other rod that would work with a Corvette fork (clevis pin type attachment). 1957-62 Corvette uses a rod configuration similar to 64-81, but that rod is 9-13/16" in length. So, it's not that one, either.

Were these rods measured O.L. or possibly from the C/L of the pin hole? Can't remember. That could be the difference if GM used the dim to the hole.

[Joe L.]

Re: C2 Clutch Fork Rod Length

Were these rods measured O.L. or possibly from the C/L of the pin hole? Can't remember. That could be the difference if GM used the dim to the hole.

Michael-----


That could very well be where the confusion originates here. GM specified that the published rod lengths were OVERALL LENGTH. However, that is inaccurate. The 66-81 rod, GM #3872960, is specified to be 8-37/64" OVERALL LENGTH. But, that specification is not met for EITHER the center-to-center (pin center to eyelet center) NOR the overall length. The center-to-center dimension is about 8-3/4" and the overall length is right at 9-1/8".

Very early 1966 small blocks probably used the 64-65 rod, GM #3844209. Just when this changed, I don't know, but it was probably about September or October, 1965.

[Michael H.]

Re: C2 Clutch Fork Rod Length

Michael-----


That could very well be where the confusion originates here. GM specified that the published rod lengths were OVERALL LENGTH. However, that is inaccurate. The 66-81 rod, GM #3872960, is specified to be 8-37/64" OVERALL LENGTH. But, that specification is not met for EITHER the center-to-center (pin center to eyelet center) NOR the overall length. The center-to-center dimension is about 8-3/4" and the overall length is right at 9-1/8".

Ok, thanks Joe. Strange that GM shows a dimension that doesn't seem to apply to anything on the actual part. Vendor error?

[Joe L.]

Re: C2 Clutch Fork Rod Length

Ok, thanks Joe. Strange that GM shows a dimension that doesn't seem to apply to anything on the actual part. Vendor error?

Michael-----


For some strange reason, GM often used some "very obscure" convention for specifying lengths and other dimensions for parts. I've come up against this time and time again. I'll take a known-original part of a known part number, measure it extremely carefully and it does not have the dimensions that GMSPO says it has. I've never been able to figure out why this occurs.

[Michael H.]

Re: C2 Clutch Fork Rod Length

Michael-----


For some strange reason, GM often used some "very obscure" convention for specifying lengths and other dimensions for parts. I've come up against this time and time again. I'll take a known-original part of a known part number, measure it extremely carefully and it does not have the dimensions that GMSPO says it has. I've never been able to figure out why this occurs.

I've seen this many times over the years too. I suppose it's a communication problem between engineering/print and vendor.

If the original rod measured 9", it sounds like Ed needs the short rod instead of the long one that he received.
If the new clutch release bearing and pivot ball are the same dimension as the originals, the clutch adjustment should be correct. I don't remember if the exact same pivot ball is available though. Isn't the currently available ball a slightly different length?

[Wayne M.]

Re: C2 Clutch Fork Rod Length

Michael-----


That could very well be where the confusion originates here. GM specified that the published rod lengths were OVERALL LENGTH. However, that is inaccurate. The 66-81 rod, GM #3872960, is specified to be 8-37/64" OVERALL LENGTH. But, that specification is not met for EITHER the center-to-center (pin center to eyelet center) NOR the overall length. The center-to-center dimension is about 8-3/4" and the overall length is right at 9-1/8"..

Joe, Michael et al -- once again, Noland's Vol 2 to the rescue. GM engineering dwgs shown on pg 207 for the longer 3844209, and on pg 394 for the shorter 3872960.

Ed probably has the short one, which the dwg shows at 8.60" from centerline of forward insert (into clutch Z-bar hole) to centerline of fork clevis pin hole. Add 0.36" to overall end at the clutch fork, for total of 8.96"

Same way of measuring for the long rod, only 10.20" + 0.36".

[Wayne M.]

Re: C2 Clutch Fork Rod Length

...and it sure would be nice not to have clutch adjustment issues.... .

Ed -- thought you and others would like to see what various previous 'Bubba' owners (incl. one lawyer ) endured in my '65 396 -- was one of the first parts I replaced in late '80s.

This is the "9" inch one. Note extreme wear on both ends. The slop was incredible. One of the anti-rattle springs was gone [can't remember which]. The fork lever was eating into the rod diameter as it elongated the clevis hole !

[Joe L.]

Re: C2 Clutch Fork Rod Length

I've seen this many times over the years too. I suppose it's a communication problem between engineering/print and vendor.

If the original rod measured 9", it sounds like Ed needs the short rod instead of the long one that he received.
If the new clutch release bearing and pivot ball are the same dimension as the originals, the clutch adjustment should be correct. I don't remember if the exact same pivot ball is available though. Isn't the currently available ball a slightly different length?

Michael------


Yes, the current clutch fork ball stud is GM #3729000. It is 1-1/2" in length. It was originally used for all 1955 through 1965 Corvettes and also L1975-81.

For 1966-E1975, stud GM #3887159 was used. This stud was 1-3/8" in length. This stud was discontinued in May, 1996 and replaced by the aforementioned GM #3729000. I don't know of any current source for an exact equivalent of GM #3887159. I have found, though, that the 3729000 seems to work fine for applications originally using the 3887159.

[Michael H.]

Re: C2 Clutch Fork Rod Length

Michael------


Yes, the current clutch fork ball stud is GM #3729000. It is 1-1/2" in length. It was originally used for all 1955 through 1965 Corvettes and also L1975-81.

For 1966-E1975, stud GM #3887159 was used. This stud was 1-3/8" in length. This stud was discontinued in May, 1996 and replaced by the aforementioned GM #3729000. I don't know of any current source for an exact equivalent of GM #3887159. I have found, though, that the 3729000 seems to work fine for applications originally using the 3887159.

Thanks Joe. Trying to do the math here but without the dimensions on the three key points on the release lever, I can only guess at what the 1/8" would result in at the outer point on the lever.
If the distance from the contact point at the release bearing to the pivot ball is roughly 2", and the distance to the outer end of the release arm is roughly 7", the 1/8" difference in pivot ball height would only result in somewhere aroung 1/2" ??
I supppose a 1/2" difference at the threaded section of the upper clutch rod wouldn't create any issues.

If I had an original 1 3/8" ball, I would probably not replace with a current 1 1/2" ball unless it showed some damage. One less "issue" to deal with.

[Ed R.]

Re: C2 Clutch Fork Rod Length

Wayne, my fork rod looks just like your second picture. Is that because it is too long?

[Ed R.]

Re: C2 Clutch Fork Rod Length

I just want to thanks to everyone that has helped out here. I will take more measurements and post them. Seems I need to get the shorter rod.

[Wayne M.]

Re: C2 Clutch Fork Rod Length

Wayne, my fork rod looks just like your second picture. Is that because it is too long?

Ed -- no, I don't think this wear is due to your rod being too long; believe it's from normal-to-excessive usage, and maybe lack of lubrication.

I suspect that the clutch fork rods were not hardened, but the clutch cross-bar WAS, so that the rod would be sacrificed first. I say this based on the fact that my Z-bar (suspect original @ 43 years old) is not excessively worn in the mating hole.

[Joe L.]

Re: C2 Clutch Fork Rod Length

Joe, Michael et al -- once again, Noland's Vol 2 to the rescue. GM engineering dwgs shown on pg 207 for the longer 3844209, and on pg 394 for the shorter 3872960.

Ed probably has the short one, which the dwg shows at 8.60" from centerline of forward insert (into clutch Z-bar hole) to centerline of fork clevis pin hole. Add 0.36" to overall end at the clutch fork, for total of 8.96"

Same way of measuring for the long rod, only 10.20" + 0.36".

Wayne-----


The replacements for the 3872960 available for quite a few years actually have a slightly different clevis pin end configuration. The flattened area is slightly wider and there is slightly more material between the clevis pin hole and end of the flattened area. I don't know if this change was by design change or just something the supplier executed "unilaterally". It actually slightly improves the strength of the rod, though.

The rods were not hardened, which was "dumb".