Distributor question

[Bill C.]

Re: Distributor question

Scott,

Have you checked the cable in the Housing?
It could be fraying and the popping/noise might be it messing up and rubbing the cable housing.

Just a thought....

[Roger O.]

Re: Distributor question

What started this "adventure" was an erratic snapping or popping noise coming from under the distributor cap. At first I thought it was my tachometer cross gear but it is in new condition.

Make sure the cross gear is not eating into the back of the housing,it can cause the gear to bind. You'll have to use a mirror to check.

[D S.]

Re: Distributor question

Scott,

Have you checked the cable in the Housing?
It could be fraying and the popping/noise might be it messing up and rubbing the cable housing.

Just a thought....

Bill, when I first suspected the cross gear and removed it I also inspected the cable and it looked okay, too. It may well be the cable but it just didn't look stressed in any way. Like I said earlier....when something breaks I'll get it repaired. Right now I'm chasing an elusive noise. It can only be heard with the hood up with the engine idling.

[Joe L.]

Re: Distributor question

Joe,
As you know removing the end play pushes the gear down,dist is permanently longer by around .090 . Do you think its possible the dist housing could be forced up in some cases ?
Roger

Roger------


There is no way the housing could be forced up. The distributor clamp should prevent that from occurring, absolutely on 55-62 and 69-91 and "more-or-less" on 63-68.

As far as end play is concerned, having considered the issue for quite some time, I theorize that the reason for the 0.025" MINIMUM end play as found in the distributor specifications is because there were other engine tolerances to be considered. Block deck height, cylinder head block and intake manifold surface machining, intake manifold machining, and distributor housing machining all had machining tolerances. All of these could affect the final installed relationship between the distributor driven gear and the camshaft's distributor drive gear.

There are only two ways that I can see that this could be compensated for: the first is by having sufficient end play in the gear-to-housing interface which would allow the gear system to "self-align". This is what I believe was done.

The second choice would be to shim the distributor housing-to-gear interface to a tight tolerance AND somehow measure the correct installation height for the distributor. Then, shims could be installed between the distributor housing and the manifold (or, machining of the manifold or distributor housing seat if installed height was found to be insufficient). Obviously, this approach would have been totally impractical in a PRODUCTION environment.

My consideration is that tight shimming of the gear-to-housing interface could result in a situation in which the gears do not mesh properly due to stack-up tolerances described above. I think this situation could be exacerbated in a rebuilt engine in which the block, cylinder heads and intake manifold may be machined.

Aftermarket shims are available to shim the distributor housing-to-manifold interface. However, how would one measure PRECISELY what amount of shimming is necessary for any particular engine?

Attached is a portion of the GM drawing for a GM #1111240 distributor (1967 L-88). Note the specification for the distributor end-play. The specs for other non-FI distributors vary slightly but they are all in the same "ballpark".

[D S.]

Re: Distributor question

The 1963 Corvette Shop manual calls for .002-.007" dist. shaft end play and does not distinguish between FI and non-FI distributors.

GM assembled them to sloopy tolerance because it was cheaper and worked okay with typical low and medium performance engines.

Sloppy end play causes considerable spark scatter, which doesn't help a high output engine.

Achieving maximum output is about assembling the engine and everything associated with it to a set of tight toleraces - tighter than GM used to assemble an average production engine.

Duke

Duke, thanks for the input. I'm not much on mechanical technicalities but the car runs fantastic at road speeds and has awesome acceleration. The engine was professionally rebuilt about six years ago and has less than 5K miles on it since the rebuild. If I were to start the car from a cold start it will idle without dying yet when I drive the car for over 30 minutes it has tendency to die while idling. Not every time so that could be a carburetor adjustment or vacuum issue.

[Joe L.]

Re: Distributor question

GM assembled them to sloopy tolerance because it was cheaper and worked okay with typical low and medium performance engines.

Duke

Duke------


Then, my question is as it has been previously: if 0.002" was ok (or, even ideal) why didn't they then specify a tolerance of 0.002" to 0.096" and give themselves even more production tolerance?

Also, the GM #1111240 distributor, excerpted specs for which I have posted elsewhere in this thread, was intended for anything but a "typical low and medium performance engine". While this distributor was TI, the same or nearly the same end-play specs were specified for other contact point-type distributors used exclusively for SHP applications

[Bill C.]

Re: Distributor question

Scott,

I had a chirping/popping noise happen just recently.
I removed the dist. to do some work on the intake rail gasklets (leaked).

Someway or another I managed to bend the sprung up contact on top of the rotor. It managed to rub the dist. cap when the car ran, made noises kind of like what you are decribing.

I changed it out and the noises were all gone.

I did not think it to be the problem, at first glance it looked just fine. But I noticed inside the cap a funny wear pattern on the bakelite.

[Roger O.]

Re: Distributor question

There is no way the housing could be forced up.

Joe,
Maybe I didn't explain myself well enough.
Is it possible by shimming the dist , which can increase its length by around .090 that when tightening the dist hold-down clamp the dist could be wedged between the hold-down clamp and the oil pump via the shaft ?

Between the oil pump and the hold-down clamp there is only so much room so I would think in a "perfect storm" kind of way , a decked block , milled heads , aftermarket intake and a "made longer" dist due to shimming could create clearance issues.

I would think if the dist was wedged between the hold-down clamp and the oil pump the oil pump housing could be cracked if the clamp was tightened enough.

Thanks Joe,
Roger

[Duke W.]

Re: Distributor question

Duke------


Then, my question is as it has been previously: if 0.002" was ok (or, even ideal) why didn't they then specify a tolerance of 0.002" to 0.096" and give themselves even more production tolerance?

Also, the GM #1111240 distributor, excerpted specs for which I have posted elsewhere in this thread, was intended for anything but a "typical low and medium performance engine". While this distributor was TI, the same or nearly the same end-play specs were specified for other contact point-type distributors used exclusively for SHP applications

I don't know. Someone in engineering must have decided that the sloppy tolerance was okay; .002" is close to zero, and zero won't work, but you can measure 25 to 96 with your fingernail and eye.

I've never heard of anyone having problems with the tight tolerance. One thing you can do is see if the dist. will drop to the manifold with no gasket. If so, then there is no interference with the cam gear and pump drive.

With the tight tolerance there is absolutely no spark scatter.

Duke

[Joe L.]

Re: Distributor question

Joe,
Maybe I didn't explain myself well enough.
Is it possible by shimming the dist , which can increase its length by around .090 that when tightening the dist hold-down clamp the dist could be wedged between the hold-down clamp and the oil pump via the shaft ?

Between the oil pump and the hold-down clamp there is only so much room so I would think in a "perfect storm" kind of way , a decked block , milled heads , aftermarket intake and a "made longer" dist due to shimming could create clearance issues.

I would think if the dist was wedged between the hold-down clamp and the oil pump the oil pump housing could be cracked if the clamp was tightened enough.

Thanks Joe,
Roger

Roger------


Yes. What you are describing here is part of what I was describing as my theory of why the end play I mentioned was specified.

[D S.]

Re: Distributor question

Scott,

I had a chirping/popping noise happen just recently.
I removed the dist. to do some work on the intake rail gasklets (leaked).

Someway or another I managed to bend the sprung up contact on top of the rotor. It managed to rub the dist. cap when the car ran, made noises kind of like what you are decribing.

I changed it out and the noises were all gone.

I did not think it to be the problem, at first glance it looked just fine. But I noticed inside the cap a funny wear pattern on the bakelite.

Thanks, Bill. That seems like an easy and inexpensive fix and might be the problem so when I get through with getting my '66 Impala road ready I will go back to the '70 Corvette. By golly I'm going to have something to drive.

[D S.]

Re: Distributor question

Make sure the cross gear is not eating into the back of the housing,it can cause the gear to bind. You'll have to use a mirror to check.

I don't think it is, Roger. I removed the cross gear and inspected everything I could see. The button is intact and feels smooth and the cross gear is like new. No visible wear anywhere. So I greased everything and re-installed it.