67 distributor end play

[Rex T.]

I am replacing the distributor gear (bottom gear) on my 67 smallblock distributor and am curious as to how much end play it should have. Also, what is the maximum number of shims allowable to achieve proper end play?

Thanks,
Rex

[Joel F.]

Re: 67 distributor end play

Hi Rex,

I think you want 10 to 20 thou when done, depending on who you want to listen to. Less is better. I'm not sure how many is the limit on shims. I know you can get as big as .030, possibly bigger.

Hope this helps!

[Rex T.]

Re: 67 distributor end play

Thanks Joel. I got .012 with 4 shims. Now I see that my vac advance is not working. I have a good used one that has the numbers 446 15 stamped into it near the canister. What do these numbers indicate and would this be a good candidate for the car?

Thanks,
Rex

[John D.]

Re: 67 distributor end play

Hi Rex,

I think you want 10 to 20 thou when done, depending on who you want to listen to. Less is better. I'm not sure how many is the limit on shims. I know you can get as big as .030, possibly bigger.

Hope this helps!

Joel. Without looking at my specs 10 to 20 seems like an awful lot. On the FI's I believe it is .002/.007. Actual tends to be about .004 but it varies but in no case over .007
I can't answer as to how many shims. Depends on thickness, etc. John D.

[Joe L.]

Re: 67 distributor end play

Thanks Joel. I got .012 with 4 shims. Now I see that my vac advance is not working. I have a good used one that has the numbers 446 15 stamped into it near the canister. What do these numbers indicate and would this be a good candidate for the car?

Thanks,
Rex

Rex-----


The original end clearance spec on non-FI Corvette distributors was 0.025-0.096". Every NOS Corvette tach drive distributor that I have measures right at 0.060" which is right in the middle of that range. Many say that minimal clearance in the .005" range is best. Personally, I go with the original, as-built specification.

The "446" vacuum control is short for GM #1973446. This control produces a maximum of 16 crankshaft degrees advance at 13-15" vacuum. The plunger begins to move at 6-8" vacuum. If your 67 small block is a 300 hp, this control might work ok. If it's an L-79, then it would not be a good way to go.

[Duke W.]

Re: 67 distributor end play

The 1963 Corvette Shop Manual has the following in bold face, page 6Y-29:

Note: Mainshaft end clearance should be .002"-.007". Add or remove shims as necessary.

Nothing in the text of this section distinguishes between FI and non-FI distributors.

Yes, I know, only the FI distributors were assembled to this tight clearance, and various distributor drawings over the years have different end play specs, and most OE-built non-FI distributors are above any published clearance spec.

But I can also tell you from my own considerable experience with single point distributors that the the ignition system works a whole lot better if the end play is as above and everything else is spot on.

Note that this spec applies to only cast iron housing distributors. For aluminum housing distributors I would add .010" to the above range to cover expansion of the aluminum housing relative to the steel shaft. For cast iron distributors the hot/cold clearance will not change by a measureable amount.

One difference you can easily observe when you compare a sloppy to a snug shaft is spark scatter. With a sloppy shaft there is easily +/- 2 degrees scatter, especially at higher revs that you can see with a timing light.

With tight end play the timing notch is dead on stable at any RPM.

Duke

[Joe L.]

Re: 67 distributor end play

The 1963 Corvette Shop Manual has the following in bold face, page 6Y-29:

Note: Mainshaft end clearance should be .002"-.007". Add or remove shims as necessary.

Nothing in the text of this section distinguishes between FI and non-FI distributors.

Yes, I know, only the FI distributors were assembled to this tight clearance, and various distributor drawings over the years have different end play specs, and most OE-built non-FI distributors are above any published clearance spec.

But I can also tell you from my own considerable experience with single point distributors that the the ignition system works a whole lot better if the end play is as above and everything else is spot on.

Note that this spec applies to only cast iron housing distributors. For aluminum housing distributors I would add .010" to the above range to cover expansion of the aluminum housing relative to the steel shaft. For cast iron distributors the hot/cold clearance will not change by a measureable amount.

One difference you can easily observe when you compare a sloppy to a snug shaft is spark scatter. With a sloppy shaft there is easily +/- 2 degrees scatter, especially at higher revs that you can see with a timing light.

With tight end play the timing notch is dead on stable at any RPM.

Duke

Duke-----


I don't doubt that there may be benefits to the tighter end clearance. Certainly, I don't see how it could harm anything in most cases. However, for a Corvette tach drive distributor I think there can also be negative consequences. A tight end clearance necessarily "forces" the alignment of the mainshaft tach gear and the cross gear. I do not believe that Corvette non-FI distributors were ever manufactured with such precision that perfect alignment of the mainshaft and cross gears is assured with a tight end clearance. I believe the system depends upon some slight ability to "self-align" for these gears. If the distributor end play is too tightly controlled, this may not happen to the degree necessary.