Can you recommend a service to rebuild a tach drive distributor?

[Joe C.]

Re: Can you recommend a service to rebuild a tach drive distributor?

The shape of the football affects the shape of the curve. The 365/375 HP distributors have a linear 24 degree "curve" from 700 to 2350. The '66-'68 300 HP curve is non-linear from 700 to 5000, but most of it is in by 4500. With lighter springs that bring the 30 in at 3500 all but two or three degrees is in by 3000.

The non-linear centrifugal curves are a refinement that allow optimum advance at high revs without so much advance on the low end that detonation can be a problem, and it took a lot of geometric analysis to achieve.

If you installed a 365/375 HP dist. in a 300 HP engine and set initial to achieve 38-39 total, detonation would be likely, but it doesn't happen on 365/375 HP engines I've tuned using CA 91 PON premium, and the 375 HP engine had single .018" gaskets, which is an indication that it has never been apart, so the true CR is likely at least 10.5:1 and could be about 10.9 if deck height is nominal.

Duke

Of course, I realize that. As far as I know, there are no published graphs of the spark advance/RPM plots for each and every autocam/cam and plate/weight/spring combo (and there were a lot of them) that was used in GM vehicles. This makes it moot as far as programming a distributor using a Sun Machine, unless the rebuilder is using the same autocam that the engine was born with. The L79 that you refer to is the oddball in the group, and probably the only one where the necessary late application of the final few degrees of spark advance is taken (partially) into account. Engine Analyzer Pro shows that for optimal performance, spark advance should continue to increase as revs increase up to and beyond the power peak. Unless one is tuning on the ragged edge of detonation, and I seriously doubt that anyone perusing this website is doing so, then timing can always be tweaked by backing off the initial by a degree or two. The ONLY way to dial in a distributor is ON THE CAR, and road testing. The absolute BEST way to tune a distributor is on a dynamometer, and the second best way (to tune for WOT ignition program) is at the drag strip.

To show how obsolete those old "custom programmed" distributor configurations are, how's this: My modified 331 tolerates 20 degrees initial advance, plus 17 degrees centrifugal, all in by 2100 RPM. Plus 14 degrees vacuum advance. It uses a modified 1111069 distributor.

[Mark E.]

Re: Can you recommend a service to rebuild a tach drive distributor?

To clarify terminology-

Football = Auto Cam = elliptical part brazed on the top of the main shaft

Cam Weight Base Assy (Service Manual's name) = part that slides onto the main shaft and includes the cam for the points

Is this right? At first I thought Auto CAM was another name for the Cam Weight Base Assembly, but these are two different parts.

I see a lot of threads on distributors. Is there a summary Auto Cam and Cam Weight Base Assys numbers by engine application?

[Joe L.]

Re: Can you recommend a service to rebuild a tach drive distributor?

The autocam has to be clocked correctly relative to the roll pin hole for the distributor drive gear. Autocam clocking determines cam/plate clocking which determines rotor clocking. And the rotor clocking must be correctly aligned to the camshaft position.

Joe-------


Yes, the autocam must be brazed to the new shaft in the same relative position to the drive gear pin hole.

[Mark E.]

Re: Can you recommend a service to rebuild a tach drive distributor?

I just installed a rebuilt distributor in my '70 and am very pleased. The engine runs like a scalded cat and the tach now works great. My journey:

- I first sent my tach to Dave Fielder at TI Specialty. The quote to restore my tach was $850 and 12 weeks. I wanted a specific centrifugal advance curve, but Dave wasn't sure he could achieve it because he only uses original (NOS??) main shafts and auto cams. (Which I think is great, but that apparently limits what can be done with the ignition curve.). So after $120 for that evaluation and shipping, I had the distributor returned.

- I then called Don Baker. Great guy and seemed to know his stuff. Don assured me he could dial in about any reasonable curve I wanted. He was willing to look at my distributor but had a backlog of several weeks. Also, he didn't have many parts stocked, so he would need to order whatever was needed, which could delay the work.

- In desperation, I went to eBay and ran across Steve Newsom with Northwest Florida Distributor Service. 850-776-7334; d64sln@gmail.com . Steve was advertising "remanufactured" tach drive distributors for $375 outright. He promptly replied to my email saying he could build a distributor matching my ignition advance specs in a few days. I decided $375 for a complete distributor didn't represent much risk so I ordered one. Within a week a complete distributor was in my hands, including a test report showing my requested advance curve achieved using his distributor machine. The housing was clean, in good condition and painted, and the main shaft, auto cam, weights, springs, points, cond., rotor, looked new. The clearance for main shaft end play and cross gear runout were within specs. After installation, I verified the advance curve with my timing light and it was spot on.

(BTW, Steve offered a choice of housings- with or without a hole for the cross gear thrust button.)

Steve is a semi-retired guy with a couple of Sun distributor machines. What I appreciate is his responsiveness when I emailed with a question and possible issue. He responded immediately and suggested I call so he could talk me through the situation (I had a question related to the fit of the tach cable). He then followed up a few days later to make sure everything was ok!

Needless to say, I'm a happy camper. Steve uses aftermarket parts, so I guess I'll learn about durability as time goes by. He offered to buy my core for $100 making the net cost $275, but I decided to keep my original as a spare.