70 350/350 distributor advance weights & shaft bearing play

[Stephen B.]

Attached is a picture of the advance weight. Do these appear original? I was planning on installing the springs as enumerated by Duke Williams. The mechanical advance appears to kick in very quickly as in at 1,500 to 2,000 rpms right now.

In addition, there is a slight bit of play in the what bearings. Should there be no play?

Thanks in advance.

[Mark E.]

Re: 70 350/350 distributor advance weights & shaft bearing play

No photo, so can't comment on the weights.

1500 rpm is early unless you're measuring distributor rpm; in that case 1500 (3000 crank rpm) is a bit late.

The service manual specifies shaft end play. I usually keep it on the tight end of the range to minimize timing variation. You definitely don't want NO play.

[Richard G.]

Re: 70 350/350 distributor advance weights & shaft bearing play

Stephen;
You didn't specify it the play was radial or axial in direction.
I attempt to set the axial play to roughly .020. I typically find them in the .030 to 035 range.
I just attempted to look up the bushing radial clearance in the 1936 Supplement "Shop Manual."

I quote:
" inspect the shaft for wear, and check its fit in the bushings in the distributor body. If the shaft or bushing are worn, the shaft and distributor body should be replaced."

As I failed to find the exact specifications I suspect it would be close to .002 clearance in the shaft to bushing fit.
The manual did give shaft run-out as a max of .002.

[Stephen B.]

Re: 70 350/350 distributor advance weights & shaft bearing play

My apologies. Here's a picture of the weights. In addition, let me restate a couple of observations. For starters, the mechanical advance is only advances approximately a maximum of 4 degrees by 1,500 to 2,000 rpms. Secondly, the slight loosen in the distributor shaft is radial, or side to side on a horizontal plane. I've already installed shims in the bottom of the shaft to rectify the axial loosen. Thirdly, when time with the total advance timing light, the time jumps around about 2 degrees at idle and about 4 degrees at 38 degrees to total advance.

Again, I appreciate everyone's help in advance (pun intended).

[Duke W.]

Re: 70 350/350 distributor advance weights & shaft bearing play

The mechanical advance appears to kick in very quickly as in at 1,500 to 2,000 rpms right now.

I can't make any sense out of "kick in". Look at your CSM specs section in the back, Section 6Y or the AMA specs. It will state the start point of the centrifugal, maybe a mid point and a max point. So what are the OE specs and what do you measure.

I can't say one way or another if the weights look OE or not. Check the bottom side for numbers. I'm also not sure about the springs, but they appear to have little if any "preload".

Shaft side play should be just barely perceptible, if at all. Shaft end play should be two to seven thou.

The spark scatter you report is usually due to excess end play. It can also be caused by excess play in the stationary pole piece hole that the VAC link engages. Since the stationary pole piece is a lot heavier than the breaker plate/points, the TI distributor is likely to wear at this point more rapidly.


Duke

[William G.]

Re: 70 350/350 distributor advance weights & shaft bearing play

That bottom weight looks pretty much worn out. I assume the top one is the same but hidden under the spring.

[Joe L.]

Re: 70 350/350 distributor advance weights & shaft bearing play

My apologies. Here's a picture of the weights. In addition, let me restate a couple of observations. For starters, the mechanical advance is only advances approximately a maximum of 4 degrees by 1,500 to 2,000 rpms. Secondly, the slight loosen in the distributor shaft is radial, or side to side on a horizontal plane. I've already installed shims in the bottom of the shaft to rectify the axial loosen. Thirdly, when time with the total advance timing light, the time jumps around about 2 degrees at idle and about 4 degrees at 38 degrees to total advance.

Again, I appreciate everyone's help in advance (pun intended).

Stephen------


Those weights and springs do not appear to me to be GM parts. GM distributor weights were natural steel, not plated. In addition, GM weights usually do not have "beveled" edges as these appear to be. GM springs are usually phosphate finished. I've never seen any that were plated as these appear to be. I'd say that these weights and springs are aftermarket. The configuration of the weights is very similar to the most common GM weights, though.

The GM weights were hardened. Based on the wear that can be seen on one of the weights, I'd say these weights were not hardened

As far as end play goes, I have several NOS GM tach drive distributors. All of them have an end play of right at 0.060". The GM spec is 0.030" to 0.086" and mine are, curiously, just about in the middle of that range. I believe that GM had a good reason for this spec. Note that the GM MINIMUM end play spec is 0.030". That's quite a bit different from the tight spec that many think is needed.

[Duke W.]

Re: 70 350/350 distributor advance weights & shaft bearing play

Joe - the 1963 Corvette Shop Manual calls out two to seven thou end play in bold face type. End play in this range eliminates spark scatter, which results in smoother engine operation. I've set many to this spec and it never created a problem. Aluminum housing distributors should probably have more end play to account for the higher thermal expansion rate of aluminum over steel, but that's not an issue on tach drive distributors that all have cast iron housings.

I believe FI distributors were assembled with much tighter end play, but I don't know why the non-FI distributors were assembled so sloppily.

Duke

[Joe L.]

Re: 70 350/350 distributor advance weights & shaft bearing play

Joe - the 1963 Corvette Shop Manual calls out two to seven thou end play in bold face type. End play in this range eliminates spark scatter, which results in smoother engine operation. I've set many to this spec and it never created a problem. Aluminum housing distributors should probably have more end play to account for the higher thermal expansion rate of aluminum over steel, but that's not an issue on tach drive distributors that all have cast iron housings.

I believe FI distributors were assembled with much tighter end play, but I don't know why the non-FI distributors were assembled so sloppily.

Duke

Duke------


I know why. It's because the specifications for the distributors called for the 0.030"-0.086" spec I mentioned. The distributors were, of course, manufactured to the specifications, not the service manual. I don't know where the service manual specs came from. Certainly not from the distributor specifications.

[Duke W.]

Re: 70 350/350 distributor advance weights & shaft bearing play

All I can say is that they work a lot better with end play in the 2 to 7 thou range, and I always shoot for 2-3 thou. That's easy to do by dressing down a thick shim on some 400 paper wetted with mineral spirits to get 7-8 thou. Then add a five thou shim, and I always place the thin shims in the center of the stack.

The 30-30 cam valve clearance situation is similar. The engineering drawing says .025", and all the service literature is .030". My recommendation is .023" as explained in the Hinckley-Williams valve adjustment paper on the Web.

Duke

[David L.]

Re: 70 350/350 distributor advance weights & shaft bearing play

Stephen,
The weights in the photo below, GM # 1881371 in Gr. 2.388, are from distributor 1112021 dated 0E7 (May 7, 1970). They seem to be very common on various Chevrolet & Corvette distributors. I have other Chevrolet distributors with these same weights. My Delco-Remy Catalog (Oct. 1972) lists these weights on the 1111491 distributor. The weights on your distributor appears to be the same.
Dave

[Joe L.]

Re: 70 350/350 distributor advance weights & shaft bearing play

Stephen,
The weights in the photo below, GM # 1881371 in Gr. 2.388, are from distributor 1112021 dated 0E7 (May 7, 1970). They seem to be very common on various Chevrolet & Corvette distributors. I have other Chevrolet distributors with these same weights. My Delco-Remy Catalog (Oct. 1972) lists these weights on the 1111491 distributor. The weights on your distributor appears to be the same.
Dave

Dave------

Yes, these weights were widely used in Delco-Remy distributors. By far, these are probably the most common weights. Note the sharp edges on these weights. This is how all GM weights I have ever seen appear. The weights on the distributor shown in the original post here are more "beveled" indicating a different sort of stamping process was used to manufacture them.

One thing, though: these weights appear to be zinc plated. I've never seen original GM weights that were plated.

[Stephen B.]

Re: 70 350/350 distributor advance weights & shaft bearing play

Here's a set of weights that were in the car at one time. I owned this car for 32 years and remember that these weights were in the distributor at one time. I wonder is these are original. They have the square corners.

[Stephen B.]

Re: 70 350/350 distributor advance weights & shaft bearing play

I'll measure the weights that I have. Thanks

[Stephen B.]

Re: 70 350/350 distributor advance weights & shaft bearing play

Duke,

I believe the side play may be a little more than barely perceptible. I had recently shimmed the shaft end play to between 2 an 7 thousands. Could the side play cause 2 to 4 degrees of spark scatter. This distributor is a points distributor with Pertronix. I need to check the hole for the VAC.

The mechanical advance is providing only 4" of advance at a maximum rpm of approximately 2,000 rpm's. I wonder why. I posted 2 pictures of advance weights that might be originals. I wonder if I should change to the original weights (if original,, or change to original type weights) and the springs you recommend in other posts.

Thanks