Distributor Shaft - Point Opening Question

[Skirted]

In process of rebuilding the distributor on a 48 as well as replacing the distributor driveshaft to eliminate rotational play. Prior to disassembling, I noticed the points would open .006 more on the small lobe compared to the large lobe. I thought it was probably due to wear because it appears to be the original shaft. After installing a new shaft from Greer, it also opens the points more on the small lobe but only about .004 more. This leads me to believe the variation is designed in for a reason but it doesn't seem right and my manuals don't say anything about it. Is this normal?
Thanks

[fillibuster]

In process of rebuilding the distributor on a 48 as well as replacing the distributor driveshaft to eliminate rotational play. Prior to disassembling, I noticed the points would open .006 more on the small lobe compared to the large lobe. I thought it was probably due to wear because it appears to be the original shaft. After installing a new shaft from Greer, it also opens the points more on the small lobe but only about .004 more. This leads me to believe the variation is designed in for a reason but it doesn't seem right and my manuals don't say anything about it. Is this normal?
Thanks

Unless there's a "new math" going on, I'd say you want equal open dimension, and equal dwell (closed time/rotation). And if you can, try to determine whether your interval is 159 degrees, to produce a second spark that is 318 degrees of flywheel rotation after the first spark. Small lobe is for front cyl spark, large lobe is for rear.
You may be having some eccentric rotation of the distributor shaft, due to housing worn housing bushings, or an eccentrically-machined shaft.
Many distributors wobble in the oil pump (and mine is one of them), but the slop in shaft bushings (if still evident) would more likely be your difference.

Okay, next up! somebody add, subtract, or clobber me!

[Skirted]

Thanks for the reply fillibuster. I did install new bushings and the new shaft fit perfectly with zero side to side play. I'll indicate the top of the shaft above lobes this evening and see if its true. I think it probably will be.

This brings me to another related question. Can someone enlighten me why the rear cylinder shaft lobe needs to hold the points open longer than the front? Since spark occurs when they initially break, why not both the same? just curious.

[tfburke3]

Thanks for the reply fillibuster. I did install new bushings and the new shaft fit perfectly with zero side to side play. I'll indicate the top of the shaft above lobes this evening and see if its true. I think it probably will be.

This brings me to another related question. Can someone enlighten me why the rear cylinder shaft lobe needs to hold the points open longer than the front? Since spark occurs when they initially break, why not both the same? just curious.

I think the time that they are closed is whats more important,not how long they are open.
Tom

[tomfiii]

It holds the points open longer ,because it was easier to machine it that way, I suppose or it was to make dwell the same on both cylinders. On a flathead accuracy was akin to Kentucky windage.

[pisten-bully]

... but the point gap should be the same on either lobe.

[fillibuster]

... but the point gap should be the same on either lobe.

Point gap and dwell (the time that points are closed) are closely related. Adjusting for one will affect the other. When engine analyzers first came to our little shop in 1975 I learned what dwell was, and that you could adjust the points gap accurately enough to give proper dwell angle, and also that you could simply eyeball the points gap and adjust with the dwell meter once it was running. You need dwell-time to produce a spark.
But your focus should be on equal-equal-equal, to keep your engine running smooth, in the distributor, in the (dual) carbs, in the valves, the ports, in the cam gallery,.....
An exception: Don't be afraid to replace that burned front piston with a +.020 while remaining standard on the rear. (trying to pick a fight/argument!)

[Skirted]

Indicated the lobes on the new shaft and pics tell the story with large lobe being .005 under. Indicated very top of shaft and there is zero variation/run out. If this variation is not designed in, its seems like too much of a coincidence for the new shaft to measure nearly the same as the old shaft which I am almost certain was original or if not a very early replacement.
I will check with Greer and see what they say.

[fillibuster]

Indicated the lobes on the new shaft and pics tell the story with large lobe being .005 under. Indicated very top of shaft and there is zero variation/run out. If this variation is not designed in, its seems like too much of a coincidence for the new shaft to measure nearly the same as the old shaft which I am almost certain was original or if not a very early replacement.
I will check with Greer and see what they say.

Ya know Jason, points opening a little further on one lobe than the other is not the critical issue, IF the points-breaking interval is a proper 159 degrees. And I guess that any dwell is maybe all you need, considering the milli-second variance in high rpm vs low rpm. I'd say what's critical is the interval, so your timing can be equal for both cyls., approx 33 degrees btdc, or 1/2" for 74's, or 5/8" for 80's.

I'd set the large lobe at .018" and somehow check that rotation interval. .... and I'd also say, inquire of a "spark-head" who knows more than most of us.

[Skirted]

Update - Greer said thats how the shaft lobes are supposed to be and the point gap gets set on the small lobe.

[pisten-bully]

So something has changed, or? Trying to wrap my head around the importance of point gap, and I can agree that dwell is the rule to follow, but the Repair and Overhaul Manual from 1946 to '48 advises you can set the point gap using either lobe.

[tfburke3]

So something has changed, or? Trying to wrap my head around the importance of point gap, and I can agree that dwell is the rule to follow, but the Repair and Overhaul Manual from 1946 to '48 advises you can set the point gap using either lobe.

Seems obvious they should have same gap on either lobe.
I would be more concerned with how this affects the timing.Would be interesting to put a degree wheel on top of shaft with a test light to see if the cylinders will fire in time,0(360) then 159 as previously stated.I have done this on Bosch mag,but never indicated an autolite dist.
Some emery cloth could polish to spec.First to get the correct opening,then to adjust leading edge of appropriate lobe if timing is off.If back cyinder fires late,take some off front lobe leading edge.Cant account for JG statement other than he gave no technical explanation.
Tom

[tfburke3]

So something has changed, or? Trying to wrap my head around the importance of point gap, and I can agree that dwell is the rule to follow, but the Repair and Overhaul Manual from 1946 to '48 advises you can set the point gap using either lobe.

Seems obvious they should have same gap on either lobe.
I would be more concerned with how this affects the timing.Would be interesting to put a degree wheel on top of shaft with a test light to see if the cylinders will fire in time,0(360) then 159 as previously stated.I have done this on Bosch mag,but never indicated an autolite dist.
Some emery cloth could polish to spec.First to get the correct opening,then to adjust leading edge of appropriate lobe if timing is off.If back cyinder fires late,take some off front lobe leading edge.Cant account for JG statement other than he gave no technical explanation
.Maybe he meant just put it together,it'll run fine,and you will plenty of other problems soon enough.
Tom

[RicD13]

Seems obvious they should have same gap on either lobe.
I would be more concerned with how this affects the timing.Would be interesting to put a degree wheel on top of shaft with a test light to see if the cylinders will fire in time,0(360) then 159 as previously stated.I have done this on Bosch mag,but never indicated an autolite dist.
Some emery cloth could polish to spec.First to get the correct opening,then to adjust leading edge of appropriate lobe if timing is off.If back cyinder fires late,take some off front lobe leading edge.Cant account for JG statement other than he gave no technical explanation
.Maybe he meant just put it together,it'll run fine,and you will plenty of other problems soon enough.
Tom

To sound a little ignorant here. If you were going to find out about the rear cylinder opening at the 159 degree point how would you find the zero point to start? Would you make the zero starting point where the .020 point gaped points start to break on the leading edge of the small lobe? Years ago I worked with brand X with their dual point and single distributors and there was a stamped mark on the leading edge of where to start the point timing to open. Or did I just answer my own question.

[tfburke3]

To sound a little ignorant here. If you were going to find out about the rear cylinder opening at the 159 degree point how would you find the zero point to start? Would you make the zero starting point where the .020 point gaped points start to break on the leading edge of the small lobe? Years ago I worked with brand X with their dual point and single distributors and there was a stamped mark on the leading edge of where to start the point timing to open. Or did I just answer my own question.

I think you got it.Set the point gap,turn dist. in direction running,and with a test light mark the point where the front lobe just opensthe points and light goes out.This will be zero.Now turn until rear points open and observe.Should be 159.Or do the same thing with heads off and check both fire the same relative to piston position.
Tom