Distributor shaft end play fail!

[L31MaxExpress]

I noticed my distributor making some weird noises. The rotor tip was contacting the cap electrodes, causing a clicking/tapping sound. I pulled it out and discovered I had 0.055" end play and cracked distributor cap mounting bolt holes despite the thing being aluminum. The gear had some wear too. I pulled a new distributor out of the box and it had greater than 0.040" end play. I shimmed the new distributor down to 0.017", closest I could get to my target of 0.015". I ended up having to shim the distributor body with a 0.060" nylon shim to prevent it from bottoming out on the oil pump drive shaft. When I shimmed the drive gear down, it stacked the oil pump drive shaft solid and the base would not seat in the intake fully without the gasket in place. Oil pump drive shaft needs ~0.050" clearence gap to allow for heat expansion to prevent from shoving the oil pump gear into the housing. Distributor is nearly silent now. That is a 0.015" feeler gauge relative to the end gap of the old distributor. I checked a low mileage OEM GM plastic distributor and it had even more end play. I cannot believe they let these roll out of the factory as loose as they are. No wonder the CMR dances all over the place relative to RPM change and the gears wear as quickly as they do. Straight garbage tolerances, especially the OEM distributors. My CMR does not even vary 1° now snapping the throttle, reving the engine from 1,000 to 6,000 and coasting back down to 1,000. The engine seems to run more smoothly as well, could be placebo, but it seems to none the less.

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[L31MaxExpress]

I finally changed out those last two wires as well, but the click, tap, squeaking noise was all from the cap contacting the rotor.

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[Schurkey]

Chevy has always had lotsa end-play in the distributor shaft.

The gear should always be pushed "up" against the thrust washers on the housing by the oil pump drag transmitted through the helical gears.

Yes, shimming the distributor shaft end-play makes the distributor longer, and can cause problems with the oil pump. Good catch, shimming the distributor up to account for that. Be sure that nylon shim can take the heat (shouldn't be a problem.)

Oldsmobile, on the other hand, needs a thrust-pad cast into the block, the gears spin the opposite direction, the distributor shaft is pulled "down".

 

[L31MaxExpress]

Chevy has always had lotsa end-play in the distributor shaft.

The gear should always be pushed "up" against the thrust washers on the housing by the oil pump drag transmitted through the helical gears.

Yes, shimming the distributor shaft end-play makes the distributor longer, and can cause problems with the oil pump. Good catch, shimming the distributor up to account for that. Be sure that nylon shim can take the heat (shouldn't be a problem.)

Oldsmobile, on the other hand, needs a thrust-pad cast into the block, the gears spin the opposite direction, the distributor shaft is pulled "down".

Nylon shim was built for the purpose, Moroso kit, but I know to keep an eye on it for a while.

 

[L31MaxExpress]

I logged some cool data on the CMR after shimming the distributor. Disregard the actual CMR setting. It was off when I stabbed the distributor with the 4.3L hold down on it and I have not done anything about it yet. I want to slot the 4.3L hold down at the mounting bolt because it holds the distributor much more securely than the SBC hold down, just have not had time to mess with it. I know some guys have rotated the gear 180 degrees on the shaft as well, but I forget how much that changes the CMR. I may pull the distributor, rotate the gear and see where it ends up prior to slotting the 4.3L hold down.

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[Astro]

Cool finds dude. It never even occured to me to check that. I bought a "highly rated" amazon aluminum one for mine and it's been running pretty well. If I have to pull it again, I'm going to check that end play.

 

[Schurkey]

720 crank degrees = 360 distributor degrees in a distributor rotor revolution.

360 / 13 gear teeth = 27.7 degrees per tooth.

Turning the distributor gear half-a-turn moves the distributor shaft half-a-tooth = 13.8 distributor degrees.

 

[89gmc]

I logged some cool data on the CMR after shimming the distributor. Disregard the actual CMR setting. It was off when I stabbed the distributor with the 4.3L hold down on it and I have not done anything about it yet. I want to slot the 4.3L hold down at the mounting bolt because it holds the distributor much more securely than the SBC hold down, just have not had time to mess with it. I know some guys have rotated the gear 180 degrees on the shaft as well, but I forget how much that changes the CMR. I may pull the distributor, rotate the gear and see where it ends up prior to slotting the 4.3L hold down.

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[89gmc]

L31, I have to use a Hei distributor on the motor I am building. If you had to buy one, where would you purchase it at? Also coil in cap or separate? There is a lot of junk out there now.

 

[Schurkey]

I have to use a Hei distributor on the motor I am building.

Why?

If you had to buy one, where would you purchase it at? Also coil in cap or separate?

"I" would buy one, used in good condition, from whatever Treasure Yard was most convenient. Disassemble for cleaning and inspection. Probably replace the pickup coil, and put fresh heat-sink compound under the Genuine GM module that came with it.

I'm kinda fond of the big coil-in-cap distributor, with or without the actual coil in the cap vs. an external coil conversion kit.