C3: Distributor End Play Specs

[Tony H.]

I just want to confirm end play specs for the main shaft gear. Is it 0.002" to 0.007"?

[Duke W.]

Re: C3: Distributor End Play Specs

Two to seven thou would apply to all cast iron housing distributors. I can't remember if GM went to aluminum housing distributors prior to transistioning to the HEI, but I would recommend a minimum of .005" for aluminum housing distributors.

Duke

[Tony H.]

[Tony H.]

Re: C3: Shim Materials

If I eliminate the majority of end play in my distributor, is it possible that the distributor will no longer seat properly in my engine and would I need to shim the distributor position with respect to the engine intake base? My end play is around 0.048". What materials should these shims be made of (end play shims and distributor shims (if necessary)?

[Duke W.]

Re: C3: Distributor End Play Specs

Specs for the "360" can are 0@6", 15@12". This is a good can for all BB including SHP. (SHP BBs develop about 14" manifold vacuum at idle, so a 12" can meets the "two inch less than idle vacuum" for max advance criteria.) This can was used on '66 BBs, both 390 and 425/450 HP. I don't have the specs for '70 engines.

If your engine has "ported" vacuum advance, suggest you convert it to full time vacuum advance.

Duke

[Duke W.]

Re: C3: Shim Materials

No problem with fitting the dist. with minimum end play. You can prove this by measuring end play with a dial indicator with the dist. on the engine. Your reading will be the same, within measuring accuracy, of the end play you measure with feeler gages or a dial indicator with the dist. off the engine.

GMPD used to sell six and thirty thou shims, so some combination of these could always be used to achieve the two to seven thou spec. I don't know if GM still sells them, but I understand the aftermarket and Corvette parts houses do.

Be sure you buy shims that are specifically designed for the distributor end play roll. They are hardened.

Duke

[Joe L.]

Re: C3: Distributor End Play Specs

Tony-----

I'll say this: the GM specification for distributor end play for your distributor was 0.030" to 0.086". I have several NOS Corvette distributors [none for sale] and the end play on all of them is 0.060".

As I've stated on numerous previous occasions, I don't think that this end play specification was a result of a desire to provide a wide range of acceptable end play to accomodate "sloppy" production tolerances. If so, the specification would have been something like .005" to 0.086". However, if you notice, the specification calls for a MINIMUM end play of .030".

[Clem Z.]

the best reason for the closest spec

is to cut down on "spark scatter". since the dist gear is sprial in cut if the shaft moves up and down it will change the timing.

[Joe L.]

Re: the best reason for the closest spec

clem-----

I totally agree that a "tight" clearance is intuitively better. However, I have to figure that the GM engineers knew that, too. In that case, even if they needed to provide a wide range of end play to accomodate manufacturing tolerances, then I would have expected that they would have set the MINIMUM end play at .005", or so. That way, at least some distributors might get built with the preferential tight end play. Setting the MINIMUM end play at .030" insured that NONE would get built with the "preferred" end play. So, why do that?

I figure that there is some other reason that the MINIMUM end play was set at .030". It might not be so intuitive, apparent, or desirable from a performance standpoint, but I think that they had some other good reason for this. I just don't know, for sure, what it was.

[Clem Z.]

i would guess it was because of "stack up"

of the tolerances in machining. at .030 nothing could go wrong as far as bindup and such plus trying to hold the clearace down tight would cause assy problems because when you go to .005/.006 it takes more skill to get the pin hole in the gear lined up with the pin hole in the shaft. what killed the GM olds diesel was they had the compression ratio varying between cylinder too great because of the "stack up" tolerance of the machine parts,piston,rods,crank and deck height. they got it correct just before they shut down the project. that is the trouble with big companies like GM it takes too long to make changes.

[John H.]

Re: the best reason for the closest spec

Virtually all the Delco distributor drawings through 1967 show .002"-.010" clearance (and the "dimple" on the gear aligned with the rotor segment); that clearance spec didn't start opening up until the L-88/LS-6 distributors, and on later HEI distributors, when it opened up significantly. Dunno why.

[Clem Z.]

i would guess the HEI is because

of the aluminum housing and the steel shaft and the growth difference at temp. also the FI distributors needed the close endplay.

[Joe L.]

Re: i would guess it was because of "stack up"

clem-----

I think that the reasons for it are VERY likely related, in one way or another, to the "stack-up" phenomenon that you referred to. However, the factors that influence the selection of specifications to avoid the possibility of "stack up" problems are things that we have to live with unless we are able to "blue-print" an engine and its components to an OVERALL much tighter set of tolerances than the factory did. Otherwise, the "stack-up" effect could have the same "negative consequences" that the factory engineers were trying to avoid with their specs for distributor end-play. Very few engines are so-"blueprinted" and even those that are might not have tolerances
controlled so well that "negative consequences" might ensue as a result of too little distributor end play.

Just because it's possible to set the distributor end clearance to a much tighter spec than the factory specified doesn't change the tolerances for other related engine specifications. For example, how consistent is the installed height of the distributor in the engine which affects the relationship of the cam's distributor drive gear and the distributor's driven gear? Even the thickness of intake manifold gaskets, alone, will affect this, not to mention the machining of the block, cylinder heads and intake manifold. All of the "stack-up" tolerance variations in this whole system might be a good reason, by itself, for specing the distributor end play at the .030" MINIMUM setting.

Also, what are the factory tolerances for the installation of the tach drive (cross) gear? This affects the inter-relationship of the cross and vertical tach gears. A "tight" end play setting "fixes" this relationship in a manner which might not be conducive to long gear life.

Am I saying that a distributor set up for a tight end play specification will fail instantly, or in short order? Absolutely not. In the vast majority of cases, such a set-up will perform just fine. However, the overall durability of the system might not be what the factory was trying to achieve within the validation standards set at the time. In other words, such a set-up might perform well for, say, 25,000 miles but not for 100,000 miles.

[Tony H.]

Re: C3: Distributor End Play Specs

Joe,

My distributor is the original distributor to my 1970 BB. There is one "shim" installed above the main gear where I measured the end play. This shim appears to be original to me. It measures about 0.036"; the remaining end play is about 0.048". Together, this end play is about 0.084" which is at the high end of the "spec" you cite.

Do the NOS Corvette distributors [not for sale] have a single "shim" or washer between the main gear and housing? If so, what is the diameter. Mine appeared to be bronze/yellowed in appearance....could be nitrided???.

[Tony H.]

Oops: washer thicknes NOT diameter *NM*