TR4/4A distributor pedestal shim [packing] quandary

[Dash]

I'm installing the distributor from scratch and checking the pedestal end float. Using a 0.76mm (.030") washer under the gear I get a gap reading of 0.50mm (.020"). According to the illustration on how to figure it, I subtract the gap reading from the washer thickness thereby getting a difference of 0.26mm (.010"). If the target gap is 0.13mm (.005") then according to the book, that's way over insufficient. This is where the book example loses me.. The book says add shims to make up the difference but what in the world would I add to 0.26mm to make it 0.13mm? Is there something I'm forgetting to factor in maybe?
Or do I have to think backwards and still have to add shims? If so, would they need to total .015" (0.38mm)?
Don't ask me where I got that number from cause as I said early on I'm gettin dumber by the minute with this car.

 

[Geo Hahn]

I think the math you want is .020 - .030 = -.010

So to get the float up from -.010 to a +.005 you would need a .015 shim. (and that's where you got that number).

 

[TR3driver]

If adding a .030" washer only gave you a gap of .020", then the clearance without the washer will be .010". This is larger than the specification of .003" to .007" and cannot be fixed with shims under the pedestal. (The book doesn't mention that possibility.)

But it's not much larger, so up to you if you want to try to fix it. First step towards fixing it would be to install a new bushing (Stanpart 56405) under the gear, if you haven't already. If that doesn't bring it down, I would probably try cutting a small shim to fit under the head of that bushing. The other alternative would be to machine a bit off the top of the block opening, or the seating surface of the pedestal.

Then you can go back to measuring the clearance and shimming the pedestal.

 

[Dash]

Randall, is it possible to replace that bushing somehow through the distributor hole itself? Shaving the pedestal may be a good option but what affect would that have in relation to the gear and shaft?

 

[TR3driver]

I've not tried it myself, but I think it wouldn't be too hard to find a way to pull the bushing from the top instead of driving it from the bottom. Something like this, maybe (tho you'd probably have to make one to suit the diameter and length)


Or maybe just make up some extra-long jaws for a typical pilot bearing puller:


But still probably easier to drop the pan and tap it out from underneath. That will also let you clean up any dirt or whatever that gets knocked loose.

As far as the gear mesh goes, from what I've seen the gears are plenty wide so moving that gear up or down even .030" isn't going to hurt anything at all. You just don't want it flopping around, as that will affect the ignition timing. But I've not tried any of this, so YMMV and all that.

 

[Dash]

Ah, could be I'm just now understanding maybe what adjusting the end float actually is doing. Using the specs (.003-.007) is just a way , working blind as they say, to insure that all the cogs are getting a good mate...correct? Then regardless of the specs, as long as they are making full contact with each then it's not really that critical is it?

 

[TR3driver]

I could be wrong, but I think the main issue is that if the driven gear wanders up and down while the engine is running, it will change the ignition timing.

These are simple skew gears (not helical like the rear axle) and relatively lightly loaded. So precise mesh isn't so important.

Besides, shimming the pedestal (as the factory said to do) doesn't actually move the gear. It just controls how far it can come up before meeting the bottom of the pedestal. In operation, the gear floats between the top of the bushing and the bottom of the pedestal; the .003"-.007" spec is for how far the gear can float. All the business with the washer is just a method of measuring that end float, without having to use a dial indicator. (They didn't have HF back in those days, dial indicators were expensive, fragile, and not typically found in a mechanic's tool chest.)

As a side note, I own one of the very last GM V8s to have a distributor. They went to a lot of work to eliminate that skew gear drive, to improve spark accuracy and help meet emission requirements. What they would up with was the distributor mounted right on the front of the camshaft, which is a terrible place to put it! Just to remove the distributor cap, you have to start by removing the radiator, radiator fan, and water pump! Fortunately it only has to be changed every 100,000 miles. (Unfortunately, a tune-up costs close to $1000.)