Distributer end play

[Jack J.]

Had to have my distributor re-built because of internal wear. This caused the horizontal cross shaft gear which drives the tach to get chewed up. The end play on the distributor shaft is roughly .040 up and down. What are your thoughts on this dimension? Jack J.

[Dave P.]

Re: Distributer end play

What are your thoughts on this dimension?

.040 seems ok to me. Some may prefer a little tighter, but I don't think it really makes a difference. When the engine is running the oil pump loads the gears, forcing the distributor gear against the shims. The gear and shaft don't move up and down. The oil pump load takes up the clearance.

[Owen L.]

Re: Distributer end play

A Duke Williams L-79 rebuild article shimmed to 0.004"-0.005".
A Dave Fiedler distributor rebuild article says to aim for ~0.010".
A Joe Fisher rebuild article says 0.007"-0.010".

Duke L-79 Distributor Overhaul.pdf
Distributor Resto.pdf
Distributor Reconditioning - Disassembly and assembly.pdf

When I rebuilt my '72 distributor, it went from ~0.060" down to 0.008" and I've had no problems with it being that small.
You can buy a shim pack from the regular vendors and places like Summit Racing.

[Larry M.]

Re: Distributer end play

Agree with Owen.

0.010 inch is what I have always used.

Larry

[Dave P.]

Re: Distributer end play

A Duke Williams L-79 rebuild article shimmed to 0.004"-0.005".
A Dave Fiedler distributor rebuild article says to aim for ~0.010".
A Joe Fisher rebuild article says 0.007"-0.010".

Do any of these articles suggest to check the clearance after the distributor is installed in the engine it is going to run in? (Rotate the shaft back and forth from above. There must be some clearance in the gear felt as slight play. Just like checking a ring and pinion. )

[Removed erroneous stuff about "backlash" ]

[Larry M.]

Re: Distributer end play

Dave:

Excellent point.......

I typically first install distributor without the distributor-to-intake gasket to see if distributor fits without interference and lays flat on the machined intake pad. Then I pull the distributor and install the gasket. Then I set initial timing and snug the hold-down bolt.

Larry

[Duke W.]

Re: Distributer end play

Do any of these articles suggest to check the clearance after the distributor is installed in the engine it is going to run in? (Rotate the shaft back and forth from above. There must be some clearance in the gear felt as slight play. Just like checking a ring and pinion. )

What the shimming is setting is the back-lash between the cam gear and the distributor driven gear. The dimension between the mounting pad surface on the intake manifold and the cam center line varies from engine to engine and can change with head surfacing, block decking, etc. The distributor dimension from the mounting pad to the bottom surface that the shim contacts can also vary.

Because of the variables mentioned above, the clearance with the distributor out of the engine means squat. Using an arbitrary clearance number is wrong. You're setting the gear back lash that exists after it is installed. Get the clearance on the distributor too tight, and you'll jamb the driven gear into the cam gear with zero backlash when it is installed and clamped down. Accelerated gear wear may result. With the early 'wire' style hold down, the clamp will yield, and there may be an oil leak at the distributor base.

I've never run across any issues with 2-7 thou end play, which is the spec written in bold face type in the 1963 Corvette Shop Manual!

As long as the distributor seats and there is still slight end play there is sufficient backlash in the gears. Thrust from the gears places an upward force on the shaft, but under certain conditions it can vibrate back and forth within the end play range. This is seen as spark scatter with a timing light. It can occur at idle and various speeds above idle as the timing mark jumping back and forth up to a couple of degrees.

With end play in the 2-7 thou range there is no visible spark scatter. The view of the balancer notch illuminated by the timing light is like a still photo... no wiggling balancer notch.

Duke

[Duke W.]

Re: Distributer end play

A Duke Williams L-79 rebuild article shimmed to 0.004"-0.005".
.

Four to five thou was the final end play on this distributor, which is within the two to seven thou range specified in the 1963 Corvette Shop Manual. Prior to disassembly it measured .043", and a 30 and ten thou shim brought it into the 4/5 range. (There's some slight tolerance on shim thicknesses.)

I like to see final end play in the 3-5 thou range. If below three thou I will dress one of the thicker shims on some 220-400 paper on a flat surface with mineral spirits to get at least 3 thou.

If end play is a little over 7 thou, I'll add a five thou shim and dress down a thicker shim as above to get at least 3 thou at final assembly.

I have no idea why production distributors were assembled with such sloppy end play, but it's my understanding that FI distributors were assembled with much tighter end play.

Duke

[Joe L.]

Re: Distributer end play

Had to have my distributor re-built because of internal wear. This caused the horizontal cross shaft gear which drives the tach to get chewed up. The end play on the distributor shaft is roughly .040 up and down. What are your thoughts on this dimension? Jack J.

Jack------


I've said this before and I'll say it again: the GM PRODUCTION spec for tach drive distributors, except those for FI applications, is 0.030-0.086". I have several NOS tach drive distributors and all have end-play approximately in the middle of this range. Especially note that this specification has a LOWER limit as well as an upper limit. That tells me that this specification was not based on providing for "manufacturing convenience". If it were, they would have specified something like 0.001-0.086". But, they specified a MINIMUM clearance of 0.030". Would I ever set an end-play of less than 0.030"? Not a chance.

[Duke W.]

Re: Distributer end play

Do any of these articles suggest to check the clearance after the distributor is installed in the engine it is going to run in? (Rotate the shaft back and forth from above. There must be some clearance in the gear felt as slight play. Just like checking a ring and pinion. )

What the shimming is setting is the back-lash between the cam gear and the distributor driven gear. The dimension between the mounting pad surface on the intake manifold and the cam center line varies from engine to engine and can change with head surfacing, block decking, etc. The distributor dimension from the mounting pad to the bottom surface that the shim contacts can also vary.

Because of the variables mentioned above, the clearance with the distributor out of the engine means squat. Using an arbitrary clearance number is wrong. You're setting the gear back lash that exists after it is installed. Get the clearance on the distributor too tight, and you'll jamb the driven gear into the cam gear with zero backlash when it is installed and clamped down. Accelerated gear wear may result. With the early 'wire' style hold down, the clamp will yield, and there may be an oil leak at the distributor base.

Distributor shaft end play, which is a function of shim stack length, has no effect on backlash. Backlash is a function of the distance between the shaft centerlines, which is fixed by block machining. The same is true of transmission gears (recall the old sliding gear manual transmissions) except in this case the shafts are parallel rather than being normal to each other.

If you drop a bare distributor shaft with the gear attached down the hole without the oil pump installed (or at least the intermediate oil pump drive shaft) it will slide completely past the cam gear rotating about 30 degrees as it does. The gears will never bind.

It's not like a hypoid gear set that has a tapered pinion in which case backlash is a function of pinion depth. The same is true of bevel gear sets like the differential gears inside the rear axle.

Duke

[Dave P.]

Re: Distributer end play

Thank you for the explanation/correction.

I have a 327 in my ski boat that I originally rigged in 1976, still own, and use annually. The guy that "curved" the distributor in 1976 shimmed it pretty tight. Over the decades, heads have been changed, intake manifold milled to repair pitting (marine engine), etc. It developed a nagging oil leak at the base of the distributor. No amount of tightening the clamp helped.

Eventually I figured it out: The distributor was "bottoming" before the base contacted the intake manifold. (When writing my post over the weekend, I came to the conclusion that this was the gear bottoming; I was wrong.) I removed some shims from the distributor, it seated properly, and the oil leak was gone.

So the distributor was bottoming on the oil pump drive. (Not the gears.)

Checking that there is still vertical clearance after installation is prudent. You don't want the distributor bottomed on the oil pump drive which forces the pump's drive gear against the pump cover.

[Duke W.]

Re: Distributer end play

Yeah, if the distributor "bottoms out" before the housing seats on the manifold, it will be on the oil pump intermediate drive shaft slot. I've never run across this issue, but if a block is "zero decked", which is not uncommon and the manifold is milled to keep the ports lined up, the distributor could be lowered by a spark plug gap or so, and this could lead to the problem.

So for sure checking that the distributor shaft still has end play after installation should always be part of distributor installation, especially if end play is shimmed up to the tight spec, and doing a trial installation without the gasket to check that there is still a bit of vertical play like Larry suggested is a good idea.

Duke