Effects of distributor end play..

[Michael H.]

Re: Effects of distributor end play..

In any event, regardless that there is the 0.002-0.007" end play specification in the service manual, it is entirely inconsistent with the PRODUCTION specifications for the distributors. I have not found a PRODUCTION specification for end play for any non-FI distributor allowing for less than 0.025" end play.

I agree. Brand new cars that had distributors with .060" or more end play didn't run at all bad.

If this was an issue for normal street operation, engineering would not have allowed new distributors to leave Delco with this kind of shaft movement.

When the engine is running, because of the angle of the drive/driven gears at the cam and distributor, the distributor main shaft is forced up to the top of it's travel, eliminating all of the clearance.

Racing engines or street engines that operate at high RPM are a different story though.

[Duke W.]

Re: Effects of distributor end play..

If this was an issue for normal street operation, engineering would not have allowed new distributors to leave Delco with this kind of shaft movement.

So why was my (327/340) ignition always breaking up at over 4500? Everyone said it was "fouled plugs", so I was constantly changing plugs, and new or freshly cleaned used did work a little better.

When I finally figured out the distributor end play deal it quit "fouling plugs".

And most production distributors exceed the loose end play spec on the distributor assembly drawings. Specifying a 15.5" VAC on a 1963 engine that only pulled 12" at idle was another "oversight", which I also figured out and fixed.

GM did miss a few things.

Duke

[Michael H.]

Re: Effects of distributor end play..

So why was my (327/340) ignition always breaking up at over 4500? Everyone said it was "fouled plugs", so I was constantly changing plugs, and new or freshly cleaned used did work a little better.

A very loose distributor shaft along with a worn out timing chain could very well be a factor at certain RPM's.

At a few different engine speeds, a loose timing chain will develop a whip that will transfer back to the distributor and that will probably unload the upward force on on the shaft for an instant. That would certainly cause the timing to jump around.

[Joe L.]

Re: Effects of distributor end play..

...but remember that production tolerances often cover a very wide range. The limits of the range will allow the components to function, perhaps satisfactorily, but not necessarily optimally.

"Blueprinting" is all about assembling components to tighter tolerances so they perform optimally.

The engine main bearing clearance range listed in the '63 Corvette Shop Manual is .0008-.0034". I know I would not complete an engine if any measured less than .001" or more than .003", and I'm sure you wouldn't, either.

My personal "spec" for mains (on a road engine) is .0010-.0015" and .0015-.0020" for the rods, and it's not hard to achieve, but it can take additional time select fitting .001" over and undersized bearings either in sets or individual halves.

Duke

Duke----

Your example of the engine bearing clearance specification has a significant difference from the case of the distributor end play specification. In the case of the bearing clearance, the desirable clearance you prefer is WITHIN the range specified in the specifications. So, using your preferred clearance does not in any way make the installation out of factory specifications. In fact, some factory engines were very likely produced which fell within your preferred range. As it is, your preferred range is only marginally tighter than the factory specs. I would take issue, though, with the ability to always achieve a bearing clearance in your preferred range using generally available bearings even if the upper and lower shells from different sets are used.

In the case of the distributor end play, a clearance of 0.001-0.003" is OUTSIDE of the PRODUCTION specifications for the distributors. It's not just a "tighter" spec within the factory specs, it's well outside the factory specs. In fact, it's up to 25 times lower than the factory MINIMUM clearance.

I have also discovered one other interesting piece of information. Earlier 1955-57 "bowl" type distributors did have a tighter end play spec. Those distributors were spec'd at 0.002"-0.010" and the manufacturing specs called for shimming as necessary to achieve that. In fact, it would be my guess that this is where the 0.002-0.007" spec in the text of the service manuals comes from. In other words, it's an anachronism that found its way from earlier service manuals where it was applicable and more-or-less consistent with PRODUCTION specifications to later service manuals where it is not really applicable and inconsistent with PRODUCTION specifications. Such anachronisms are not, at all, uncommon in GM service manuals of the period. Even much later, C4 era service manuals have them.

[William C.]

Re: Effects of distributor end play..

One other item to watch is for excessive slop in the fit where the pin from the vacuum advance controls the movement of the breaker plate. Some distributors have worn to the point where that fit is loose and it will cause erratic timing.

[Duke W.]

Re: Effects of distributor end play..

A very loose distributor shaft along with a worn out timing chain could very well be a factor at certain RPM's.

At a few different engine speeds, a loose timing chain will develop a whip that will transfer back to the distributor and that will probably unload the upward force on on the shaft for an instant. That would certainly cause the timing to jump around.

My timing chain wasn't loose at 5K miles. In fact, at 115K miles when I took the engine down, it was not loose.

But the very bad crack in the #7 connecting rod at the bolt seat made me thankful that the distributor had misbehaved so many times in the first 60K miles.

Had it revved to 6500 every time I tried that con rod would have never made it that far.

Duke