1977 Distributor Shim - same as 55-74?

[Patrick H.]

Does anyone know if the lower shims used for an HEI distributor on a 1977 would be the same as those used on previous non-HEI distributors? Most catalogs show 55-74 as the applicable years for distributor shims, but Paragon says 55-91. If the shims are the same, then I have some in stock I can use for the 77.

Thanks for any information you can provide.

[Joe L.]

Re: 1977 Distributor Shim - same as 55-74?

Does anyone know if the lower shims used for an HEI distributor on a 1977 would be the same as those used on previous non-HEI distributors? Most catalogs show 55-74 as the applicable years for distributor shims, but Paragon says 55-91. If the shims are the same, then I have some in stock I can use for the 77.

Thanks for any information you can provide.

Patrick------


The same shims will work for all 1955-91 Corvette distributors.

[Duke W.]

Re: 1977 Distributor Shim - same as 55-74?

A note of caution. Back in the seventies I shimmed up the dist. end play on my Cosworth Vega's HEI. Not long after I noticed a slight misfire under load that took me awhile to find. It finally went away when I removed the added shims.

The CV's distributor mounting is different than a V8. It has a "distributor drive housing" driven by a fractional horsepower cog belt off an additional sprocket co-centered and in front of the exhaust cam sprocket. The bottom of the dist. shaft has a "piston" that sits in a well in the dist. drive housing that sees full engine oil pressure. This keeps the shaft constantly pushed to the upper limit of travel when the engine is running, so there is no way it can jump around and cause spark scatter, so it doesn't need to be shimmed (as I found out).

My theory is that shimming the end play caused enough vertical misalignment between the stationary and rotating pole pieces to upset the waveform from the pickup coil and cause a weak spark. I've never owned a V8 with HEI and have no experience shimming up the dist. shaft end play, but it's something to watch for when you reinstall the shimmed up distributor.

Maybe someone who has shimmed up a V8 HEI can chime in and report if it caused any problems.

Duke

[Ken A.]

Re: 1977 Distributor Shim - same as 55-74?

I thought the non HEI shafts were .491 & the others .500.

[Joe L.]

Re: 1977 Distributor Shim - same as 55-74?

I thought the non HEI shafts were .491 & the others .500.

Ken-----


Well, the same drive gear and tanged washer was used for both points-type and HEI. So, if the aforementioned work on both types, the shims ought to, too.

[Joe L.]

Re: 1977 Distributor Shim - same as 55-74?

All------


I'm going to repeat something I've said many times before on this subject. GM original specs for end clearance on most point type distributors (except FI) is O.030" to 0.086". All of the NOS distributors I have are factory-set at about 0.060", right in the middle of that range.

The GM specs for end clearance on HEI distributors is 0.010" to 0.112". I don't have any NOS HEI distributors so I can't say what end clearance range they are set to.

Could it be that these end clearance specs represent a "sloppy" range to ease manufacturing "flexibility"? I don't think so. If they were going to do this they would have set the range at something like 0.002" to 0.086" for point type distributors and similarly for HEI. That would have given them even more manufacturing "flexibility". But they didn't do that. They set the MINIMUM end play for point type distributors at 0.030" and the MINIMUM end-play for HEI distributors at 0.010". Anything less than these clearances is out-of-spec. I believe they had good reason for this. I set my point type distributors within the original Delco-Remy specifications and favor 0.060". I don't do HEI distributors but if I did I'd set them to something above 0.010".

[Mark E.]

Re: 1977 Distributor Shim - same as 55-74?

Patrick------


The same shims will work for all 1955-91 Corvette distributors.

One thing I admire about Chevrolet design is their wide spread parts interchangeability.

[Duke W.]

Re: 1977 Distributor Shim - same as 55-74?

All------


I'm going to repeat something I've said many times before on this subject. GM original specs for end clearance on most point type distributors (except FI) is O.030" to 0.086". All of the NOS distributors I have are factory-set at about 0.060", right in the middle of that range.

The GM specs for end clearance on HEI distributors is 0.010" to 0.112". I don't have any NOS HEI distributors so I can't say what end clearance range they are set to.

Could it be that these end clearance specs represent a "sloppy" range to ease manufacturing "flexibility"? I don't think so. If they were going to do this they would have set the range at something like 0.002" to 0.086" for point type distributors and similarly for HEI. That would have given them even more manufacturing "flexibility". But they didn't do that. They set the MINIMUM end play for point type distributors at 0.030" and the MINIMUM end-play for HEI distributors at 0.010". Anything less than these clearances is out-of-spec. I believe they had good reason for this. I set my point type distributors within the original Delco-Remy specifications and favor 0.060". I don't do HEI distributors but if I did I'd set them to something above 0.010".

The 1963 Corvette Shop manual has a statement in bold faced type that distributor shaft end play should be in the range of .002-.007". It does not specify any particular engine, like FI only, and GM sold shims as small as .005" in order to achieve this spec.

Decades ago I shimmed mine up and have done a number of others over the years. This eliminated spark scatter, yielded a steadier dwell angle as revs increased, and a smoother running engine due to less cyclic variation between cylinders because of different spark advance between cylinders.

No one I know of has ever reported any downside to this clearance range, only good.

Aluminum housings used on later non-tach drive distributors should have a higher minimum. This can be easily calculated by computing the thermal expansion of aluminum versus steel over the length of the housing at, say 250F, and adding this to the .002" minimum spec for the cast iron tach-drive Corvette distributors.

Duke

[Gary R.]

Re: 1977 Distributor Shim - same as 55-74?

When I rebuilt my point and HEI GM distributors I set the end play to about 010 if I recall. Certainly tighter then they were, never had any issues. The HEI car was pushed very hard at the track.

[Ken A.]

Re: 1977 Distributor Shim - same as 55-74?

Ken-----


Well, the same drive gear and tanged washer was used for both points-type and HEI. So, if the aforementioned work on both types, the shims ought to, too.

No tanged washer was used on a cast iron distributor. There are 2 different gears.

[Joe L.]

Re: 1977 Distributor Shim - same as 55-74?

No tanged washer was used on a cast iron distributor. There are 2 different gears.

Ken------

Yes, I mis-spoke. The tanged washer was used only for HEI distributors. I meant to say the NON-tanged washer, GM #1837617. That washer is applicable to all 1955-91 Corvette distributors, point type and HEI.

There were several distributor drive gears over the years. However, for 1965-85 (and all 1955-64 SERVICE), only one gear was used for all Corvette applications, GM #1958599. Beginning in 1986 gear GM #10456413 was used. This gear is the same as the 1958599 except it is melonized for compatibility with hydraulic roller cams. It can be used for earlier applications, though.

correction:

The tanged washer was used for HEI distributors but not only HEI distributors. It was also used for aluminum housing 66+ distributors installed on other Chevrolets and across all GM car lines. So, the tanged washer is a feature of aluminum housing distributors rather than just HEI.

[Joe L.]

Re: 1977 Distributor Shim - same as 55-74?

The 1963 Corvette Shop manual has a statement in bold faced type that distributor shaft end play should be in the range of .002-.007". It does not specify any particular engine, like FI only, and GM sold shims as small as .005" in order to achieve this spec.

Decades ago I shimmed mine up and have done a number of others over the years. This eliminated spark scatter, yielded a steadier dwell angle as revs increased, and a smoother running engine due to less cyclic variation between cylinders because of different spark advance between cylinders.

No one I know of has ever reported any downside to this clearance range, only good.

Aluminum housings used on later non-tach drive distributors should have a higher minimum. This can be easily calculated by computing the thermal expansion of aluminum versus steel over the length of the housing at, say 250F, and adding this to the .002" minimum spec for the cast iron tach-drive Corvette distributors.

Duke

Duke------


Yes, it does say that in service manuals. That includes service manuals after 1963. However, sometimes anachronisms find a way of creeping into service manuals. Curiously, the .002-.007" end play is the exact spec for some FI distributors. .002-.010" is the spec for other FI distributors as well as many early Chevrolet V-8 cast iron bowl distributors. However, non-FI Corvette tach drive distributors have a design specification of 0.030-0.086". There's no way around that. That means that no such distributor could have left the factory with an end play of less than 0.030". As a matter of fact, I've never found one that did.

I totally agree that a much smaller end clearance "seems like it would be better". But, for whatever reason, the designers of the distributors didn't think so. Absent being aware of what the reasons were AND absent being able to discount the real necessity of those reasons, I go with the original specs. Others are, of course, free to do as they choose.