I am almost finished converting a old style chev distributor for my new 8BA Flathead. The only issue I have left is attaching the drive gear in the correct location. I am just putting the engine together for the first time so I cannot run it and check the rotor phasing on the original distrib, but here is what I have done: Set the points on the original distributor for .015 point gap (approx 28° dwell) and dropped it in the hole. I marked the outside of the housing where the #1 terminal is and then hooked up a meter to the points so I could tell the exact time when the points open. Then I rotated the distrib so the points opened at the 4° advance mark on the crank. I transferred the location of the center of the rotor tip down to the distributor housing so I could measure the distance from the #1 terminal to the "firing location". I repeated this with the vacuum advance manually locked at 24° advance. The outcome was that the centerline of the rotor was .280 AFTER the centerline of #1 terminal at 4° advance and it was .060 BEFORE the terminal at 24° advance. There was still overlap on both ends of the spectrum so the engine should still run fine. However, I would think the distance should be equal at no advance and full advance. Question is, when I go to set up the modified chev distrubutor gear location should I make the distance equal on both ends of the advance curve or is there something I am missing about checking the location this way. The other question, does the spark occur at the instant the points open? Any insight on this would be appreciated. -Dave
There's what, 20 teeth (maybe more) on the gear? And no tang to worry about. Just stick it on at any degree and you will only have to adjust 18 degrees max to correct your timing. You've got room for that, don't you?
Thanks for the feedback There is only 11 teeth on the flathead distributor gear, that means if I move it one tooth it will move the rotor 33 degrees. If the gear is not positioned correctly, the rotor could be totally off the mark. Also, the Chev distributor only can turn about 30 degrees total because it stops against the vacuum module in one direction and against the cap hold down boss the other direction. I have seen aftermarket distributors that stick way above the head where you would not have any problem, but made mine similar to the stock location. I also want the #1 terminal in a different position so the wires can be routed better than the stock way. -Dave
I'm pretty sure the mark on an 8BA pully is 2 degrees BTDC. That's what my truck manual says... Flatman
I had to think about this one for a minute ( makes my head hurt ). I think we are making this harder than it really is , the rotor, and distributor cam are factory indexed for correct relationship. Then on top of that the distributor cap is indexed to the distributor housing. The rotor phasing is done with the distributor cam angle relationship to the rotor. This doesnt change at all when using the factory chevrolet unit. Put the gear on ( somewhere) and install it in the engine with the engine on number one cylinder using the timing mark turn the distributor housing until a test lamp blinks ( points open ) and it should start and run. Also dont forget that all degrees distributor are 2 degrees engine. I have shipped 65 of these units this year and they all have ran great. I just install the gear and run em.. However........ The gear location on the factory chevrolet is indexed to provide proper clearance for accessories as well as camshaft oil feed galley etc. Ever notice the dimple on the gear? it should line up with the rotor tip. Just makes it a little simpler for the zillion chevy V8s and makes service manuals easier to write..
Thanks Bubba, I think you boiled it down nicely. You are correct, the place where the points open is hard set on each distributor. and that location should be the same on all 8 cylinders . The only time this changes is when vacuum advance is applied since this moves the breaker plate and also when the points open. The only thing I have to watch is to make sure the distributor is set in with the initial advance already taken into account so I can still adjust it a few degrees and get it right on when I start the engine. You also answered my question about when the spark occurs, It occurs the moment the points are open. Thanks again !!
Alchemy, You are right to a certain extent, but since I do not have much room for adjustment and I want my #1 terminal in a certain location, I need my gear pretty close to where it needs to be. I don't want to pin it in any location and hope for the best. Chances are it would not be where I want it. Since it is a special modified shaft, I would not want to have to make a new one if I do not have to. Thanks for the input... both you and Bubba helped me solve the problem -Dave
This would explain why when I located TDC on my front pulley, it was about 1-2° after the bump that was on the pulley. "after" meaning that as the pulley rotates CW, it reaches the bump first and then my TDC mark.
The type of phasing you are talking about seems to me to be exactly what Chevy was up to in drilling the phasing dimple in their gear. My take on easiest way to phase those relationships would be to get a tight unpinned fit for drive gear, with endplay adjusted by shim washers in advance. If necessary tighten the slide on fit with a bit of solder coating or prick punch dimples on shaft. With engine at static firing position on #1, put everything together in desired relationships with room to adjust static in both directions. This will twist drive gear to position you want, so you can just yank it and mark position of desired hole for pin. I think that makes sense... Getting a handle on endplay of course affects rotational change...I think around .007 is what I've seen for aluminum Chevy housings.
Sounds like a good plan to me, I have that tight fit on my gear right now and it is waiting for me to decide where to drill the hole for the pin.
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