The bottom of the distributor gear on my '65 425 has worn in to the block to the point where it should probably be machined flat. Is there a shim or something else that can be put in there to make up for what gets machined off? I'm not sure what caused the wear. It looks like a later distributor, 1974 455? There are 2 shims between the gear and the distributor housing, see pictures. I've read elsewhere on CO that there shouldn't be any shims, is that correct? I'm also not sure if the small hole was in the oil gallery plug that oils the distributor gear. I'm guessing not by the wear. Any help is greatly appreciated...... Thanks


Wear on the bottom of the gear.

 

I have an original 1965 425 distributor for sale on this site: 1965 Oldsmobile distributor # 1111042 5C 10

It has no shims between the gear and the distributor body and the base of the gear is good.

 

I would just remove the gear and mill some off the bottom.

Seems as if the shims are irrelevant to this issue since the roll pin hole is what sets the position of the gear on the shaft.

 

Hmmm that’s not exactly correct.
They leave the shaft/gear with lots of play so that the gear will actually rest on the pad in the block, but not wear into it.
However the shaft can still move up and down which will effect timing consistency.
I shim the dist/gear clearance to .012-.015 and machine the bottom of the gear .040 or so. Then the timing stays steady and nothing touches anything it’s not supposed to.

 

So, after shimming and machining, the bottom of the gear no longer touches the block. I just want to verify.

 

I meant the shims are not going to keep the gear from moving downward and contacting the block since the gear position is set by the roll pin. Those shims limit upward shaft movement. There would need to be a shim on the top of the housing to move the gear away from the block.

 

Regarding the Oldsmobile engine, adding shims can and in most cases will take up the clearance that was built in by the factory to prevent wearing away the "bearing" surface machined into the engine block. The direction of rotation of the camshaft and the location of the distributor shaft and gear on the driver's side of the engine block cause the distributor gear to be thrust downward during normal conditions. When shims are added, the clearance is removed, the mounting flange of the distributor no longer touches the distributor mounting surface at the top of the engine block and when you tighten the hold down clamp you are forcing the bottom of the distributor gear into the machined "bearing" surface of the engine block causing that wear to occur. The folks that add these shims read an article about adding shims to Chevrolet distributors to help prevent spark scatter and think it applies to their Oldsmobile engine. Chevrolet distributor shafts are on the passenger side of the camshaft in the engine block and are thusly thrust upward during normal engine rotation and do require a shim to keep the distributor gear from wearing into the softer aluminum of the distributor housing. The moral of this story is don't add shims to an Oldsmobile distributor.

 

 

the bottom of the gear is worn because someone thought they were smart and stuck an extra shim(s) in there to take up the clearance.

What’s the thrust pad down inside the block look like? It may have a big gouge in it now .

your only option is to take out the thickest shims or all of them and reinstall it. If you put another gear on or machine that one flat, it will probably be a big mismatch with the thrust pad in the block because of the wear pattern and will eat itself up even more.

there is a lot of shaft endplay purposely built into the olds dizzy because there are a bunch of machined surface that come into play when its installed. The distance between the thrust pad down in the block and the pad on top where the dizzy sits when clamped, the distance between the drive gear bottom and the roll pin centerline hole in the gear, the dizzy shaft roll pin hole, the dizzy housing itself from the bottom to the pad that sits against the block etc. none of those are ever exactly the same because of mass production tolerances.

so to make things work with tolerances that are all over the place with so many things having to come together, Olds put some big clearance into the shaft end play . The design puts all the thrust from driving the oil pump and distributor drag between the bottom of the dist gear and thrust pad in the block.

The dist gear only receives downward thrust ..it doesn’t bounce or move up, ever. There could be an inch clearance , it would still function the same.

 

Was just thinking the same thing plus . . . . . there is nothing in the upper part of the distributor to absorb any downwards thrust so if you leave a gap at the very bottom something is going to start rubbing up in the top. I just took apart an Olds distributor to make sure before I opened my mouth

I think best bet in this case is to take the shims out and put a new gear on the distributor and hope for the best. Even though like you said it will be a mismatch between the gear and the block and might make it worse