Distributor Part Fits
 

The point gap specification is 0.018" to 0.022". Considering the radial fits of parts that can affect the point gap, I deduce the following fits (radial clearances) are necessary for a rebuild:
Shaft to Bushings 0.0005" to 0.001"
Cam to Shaft 0.0005"
Cam Lobe Variation 0.0005"
Upper Plate Center Hole to Casting 0.001" to 0.002"

The fit that the restorer has control over is the "Shaft to Bushings" because the ID of the bushings is reamed to size. Notice that if all of these clearances become additive, the point gap tolerance becomes zero as the gap would decay to 0.018".

What are your thoughts in this scenario?

Re: Distributor Part Fits
 

Bob, could centrifugal force come in to play at all?

Re: Distributor Part Fits
 

I think my head hurts. I just do the best I can, and don't worry about it.

Re: Distributor Part Fits
 

Bob -
There are several things under the restorers control that can affect distributor performance.
1. As you indicated, the shaft to bushing tolerance
2. The cam itself - I have seen a lot of cams that are out of spec. The restorer has control over this, as the cam can be replaced.
3. The biggest variable, in my opinion, is the fit of the upper plate in the casting. Worn castings and/or worn upper plates that allow the plate to change rotational center alignment during timing adjustment or due to engine vibration. In many cases where the ears of the upper plate engage in the casting, the castings are badly worn causing the upper plate to slightly change angle as it is rotated. But again, the restorer has control over these items, as distributor castings and upper plates can be replaced

Re: Distributor Part Fits
 

Just drive it. One can restore it to like new specs but 10,000 or 20,000 or 50,000 miles down the road is a different story.

Re: Distributor Part Fits
 

I was thinking about the upper plate last week. The wear area would be both the plate and casting centering hole. I have seen suggestions to go around the hole with a punch to close it up some, but came to the conclusion that doing so might close the hole up but keeping the hole centered would be iffy at best. Even replacing the upper plate does not mean it will be centered on the distributor core because of it being worn. I had thought about making a jig to put the plate in then using a die in a press to reduce the hole size to what would be needed to remove slop and keep centered.

Never thought about tilt. All cylinders need to fire at about or very close to the same place, time and intensity, so any time something shifts it changes that.
True the castings can be change, but there are only so many. Most average restorers are only going to have access to maybe one or two to chose from.

Re: Distributor Part Fits
 

If you like to learn and futz with things this is great. If you get frustrated or if you just want things to work right Bert's has great rebuilts.

Re: Distributor Part Fits
 

To quote Rolie Polie Olie, "I did my best because it is the best I can do".

Advice I try to follow.


https://en.wikipedia.org/wiki/Rolie_Polie_Olie

Re: Distributor Part Fits
 

Had to dig that one out of my brain, my kids loved that show.

Re: Distributor Part Fits
 

Look at the distributor casting where the fingers of the top plate rotationally slide. You might be surprised at how worn the casting is, compared to wear of the top plate. Over the years, the top plate has likely been replaced multiple times, but never the distributor casting itself, so most of the wear you will find is in the distributor housing. Any movement that changes orientation of the plate to the center can have an effect on point gap. Enough to be concerned about ??? Perhaps not.

As Bob started this discussion about cumulative tolerances that may be out of the restorers control, I just wanted to point out several that we can minimize or eliminate, such as a good cam (Sipes, perhaps) or finding a good distributor core with minimal wear.

These cars are so hardy that I also think that we can get ourselves overly concerned with point gap that is off by a few thousands, or timing that is off by a few degrees. We all know that these cars will run with the point rubbing block worn to the point the points barely open, and timing that is slightly off and compensated for with the spark advance lever.

Re: Distributor Part Fits
 

I just replaced my "modern" point set with the original upper plate and point set. Old style points are much easier to set precisely with the screw. HOWEVER, when I wiggled the cam back and forth, I can get as much as 5 thousandths play. Despite that, the car runs quite smoothly.

Re: Distributor Part Fits
 

That is aprox. .0025 clearance shaft to bushing.
The spring tension of the points keeps the cam from wobbling while driving.

Re: Distributor Part Fits
 

Yes, and that's one reason I balance my distributor rotors.