Effects of distributor end play..

[Tom L.]

I,ve been having a bit of engine trouble with my '72 LS-5 (a highky annoying miss/stumble at cruising speed) but I think I have it sorted out, just waiting for the weather to allow a test.

While sorting through things I noticed that my distributor shaft has excessive end play, about .090. While searching through the archives I only found that .090 is too much (should be under .010), and that the up/down movement can contribute to the distributer drive gear wearing out prematurely.

What I was really looking for is what effect on the way the engine runs will the end play have? I have a few thoughts of my own I'd just like a bit of input from others. Thanks

[Michael W.]

Re: Effects of distributor end play..

The distributor is driven by a helix gear off the cam. As the shaft rises and falls, it changes it's relative position with the cam and therefore the entire rotating assembly.

Net effect is you see the timing 'jumping' under a light particularly at idle.

[Duke W.]

Re: Effects of distributor end play..

Yep, "spark scatter" is one ill effect. Point bouce is another. The excess end play may not be the cause of your problem, but it's certainly worth correcting.

Plus I bet your distributor needs a good overhaul and cleaning. The procedure is easy and in the COM. Be sure it's assembled with the dimple in the drive gear pointed the same direction as the rotor tip, which is NOT mentioned in the COM.

Before you pull it. Check the spark advance map and compare to specs in the CSM or AMA specs. If the max centrifugal is too much the limit bushing could be disintegrated, and you will be able to verify the condition of the bushing upon disassembly.

Another common problem is deterioration of the ground wire on the breaker plate. This can result in intermittent misfires, and eventually no spark at all.

There is lots on information on single point distributor issues in the archives.

Duke

[Tom L.]

Re: Effects of distributor end play..

Initially this was my first thought. However afer re-timing the motor, running it up and down, I found little scatter at idle. At most it moved a degree or two.

[Tom L.]

Re: Effects of distributor end play..

It is obvious that the distributor needs attention, that was not my question. However, you brought up the issue of "Point bounce", I've never heaerd that term. What is it, how is it identified and how is it corrected?

[Joe L.]

Re: Effects of distributor end play..

I,ve been having a bit of engine trouble with my '72 LS-5 (a highky annoying miss/stumble at cruising speed) but I think I have it sorted out, just waiting for the weather to allow a test.

While sorting through things I noticed that my distributor shaft has excessive end play, about .090. While searching through the archives I only found that .090 is too much (should be under .010), and that the up/down movement can contribute to the distributer drive gear wearing out prematurely.

What I was really looking for is what effect on the way the engine runs will the end play have? I have a few thoughts of my own I'd just like a bit of input from others. Thanks

Lynn-----


For whatever it's worth and perhaps to the consternation of some, I repeat something I've mentioned many times before. GM never specified a distributor end play in SERVICE specifications. However, the Delco-Remy PRODUCTION specification for these distributors was an end play of 0.025" to 0.096". Every NOS GM distributor I have has an end play of right at 0.060" which is, curiously, right in the middle of that range.

Also note that the MINIMUM end play specified by GM was 0.025". If they had just wished to provide a wide manufacturing range for themselves, they should have gone with a range of 0.005" to 0.096". That way, at least some distributors might have ended up being manufactured at the low end play that is often recommended. However, they didn't. Presumably, a distributor with less than 0.025" end play would have been rejected as not meeting manufacturing specifications.

[Tom L.]

Re: Effects of distributor end play..

Joe, I appreciate your input. The debate over end play was the "feature" topic of discussion in the archives. This is why I didn't bring it up. My only concern was the effect on my engine. I just want it to run well. I'll fix the end play soon, it just seems that it is not a major engine operational issue. I'ts mostlty a tuning or "making things correct" item. Thanks

[Jim T.]

Re: Effects of distributor end play..

In the early 80's my original owner 1970 350/300 engine started dying coming down the off ramp of an interstate after driving for two hours and the Corvette was running great. No trouble restarting. Driving around town no problems.
Discovered the end play was more than .007. Bought GM part #1837617 distributor shim washers (5 came in the plastic package) and installed two of them to get the end play less than .007. Cleaned up the tack gear drive cavity and applied new lubrication for the tach drive gear.
Did not have any more problems with the engine dying coming off the interstate.
I was satisfied that I had corrected the problem.

[Joe L.]

Re: Effects of distributor end play..

Joe, I appreciate your input. The debate over end play was the "feature" topic of discussion in the archives. This is why I didn't bring it up. My only concern was the effect on my engine. I just want it to run well. I'll fix the end play soon, it just seems that it is not a major engine operational issue. I'ts mostlty a tuning or "making things correct" item. Thanks

Lynn-----


The reason I mention it is that INTUITIVELY I've always figured that shimming for minimum end play was the thing to do. However, for a variety of reasons, including becoming aware of the Delco-Remy manufacturing specification, I began to question that. I've got to figure that the engineers that designed the distributors had the most in-depth knowledge of the overall requirements. Also, I've got to figure they were well aware of problems that could be created by too great of end play. What the reason was for the end play specified I do not know. However, I do have faith in the engineering process that got us to those specifications. So, I accept them.

[Duke W.]

Re: Effects of distributor end play..

Lynn-----


For whatever it's worth and perhaps to the consternation of some, I repeat something I've mentioned many times before. GM never specified a distributor end play in SERVICE specifications. However, the Delco-Remy PRODUCTION specification for these distributors was an end play of 0.025" to 0.096". Every NOS GM distributor I have has an end play of right at 0.060" which is, curiously, right in the middle of that range.

...not true, Joe. The 1963 Corvette Shop Manual (page 6Y-29) has a boldface note that distributor end play should be .002-.007", and it is not specific to FI engines.

I agree that none, except maybe FI distributors were built to this spec, but it sure makes the distributor work better.

Duke

[Duke W.]

Re: Effects of distributor end play..

It is obvious that the distributor needs attention, that was not my question. However, you brought up the issue of "Point bounce", I've never heaerd that term. What is it, how is it identified and how is it corrected?

Point bounce is when the points do no follow the cam. It's similar to valve float. It can be caused by the dist. shaft bouncing up and down through sloppy end play, a worn, wobbly breaker plate, or points with insufficient spring tension in high revving mechanical lifter engines.

The bounce and resulting spark scatter can get so bad that the engine will misfire/backfire through the carburetor.

Duke

[Duke W.]

Re: Effects of distributor end play..

Initially this was my first thought. However afer re-timing the motor, running it up and down, I found little scatter at idle. At most it moved a degree or two.

That's typical for the sloppy as-built end play. Shimmed up to the .002-.007" spec in the 1963 Corvette Shop Manual, the timing mark should be dead nuts on at any engine speed and not show any noticeable scatter.

Duke

[Duke W.]

Re: Effects of distributor end play..

Lynn-----


I've got to figure that the engineers that designed the distributors had the most in-depth knowledge of the overall requirements. Also, I've got to figure they were well aware of problems that could be created by too great of end play. What the reason was for the end play specified I do not know. However, I do have faith in the engineering process that got us to those specifications. So, I accept them.

...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

[Jim T.]

Re: Effects of distributor end play..

My 1963 Corvette Shop Manual page 6Y-29 is where I got my distributor end play information when I shimmed my 70's distributor. The same 002-007" end play is in my 1970 Chassis Service Manual. My 1971 Chassis Service Manual does not have the end play information.

[Joe L.]

Re: Effects of distributor end play..

...not true, Joe. The 1963 Corvette Shop Manual (page 6Y-29) has a boldface note that distributor end play should be .002-.007", and it is not specific to FI engines.

I agree that none, except maybe FI distributors were built to this spec, but it sure makes the distributor work better.

Duke

Duke----


Yes, I see that's correct. In fact, the 1969 service manual also specifies 0.002-0.007" in the text. I was previously referring to TABULAR distributor specifications and I could not find a spec there. FI distributors did have a much tighter manufacturing specification for end play of 0.002 to 0.010". However, FI distributors have a completely different shaft configuration.

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.

Also, I might add that the 0.005" shims, GM #1927529 (i.e. the ones that are "added and subtracted" to adjust distributor end play) were never specified by GM for use on any 62-74 Corvette distributor, except FI.