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ATTN: MSD distributor owners



Last month, I installed MSD's pro-billet distributor with the tach drive and vacuum advance, described in my first thread on this board a while back. Everthing went great and the car runs fantastic, but the tachometer didn't work when I hooked it up. The distributor was shipped with the tach drive facing the opposite direction of the vacuum advance. After looking at the old distributor, I reasoned that it was simply driving the tachometer in reverse, and that I just needed to flip the drive gear around so it's facing in about the same direction as the vacuum advance. But when I did that, the vacuum advance was in the way and it was impossible to connect the tach drive cable!! I can't believe this. I suppose MSD never did any test-fitting.

Anyway, for any of you that own this distributor, how did you get the tachometer to work? Does driving the cable in reverse hurt the tach? Which way does the drive gear have to face? Is there a more compact vacuum advance canister available that will clear the tach cable? Do I have to get the adapter that MSD sells to reverse the drive direction? I really hoped to avoid the adater because the area around the distributor is so compact.

Thanks a lot!
I was looking at the MSD 8572 Dist for my 73. Is that the one you used?
Square Peg In a Round Hole

I can't see your distributor ... but its tach drive PROBABLY came ready to accept a 3/16" (about 0.187") tang drive cable-end ... that end has a round cross-section with a small tab (tang) for engaging the drive. Your vette has a 0.104" square drive cable-end ... its end has a square cross-section and no tang or tab. MSD pn 8491 tach drive adapter creates a union for such a mismatch ... it DOES NOT reverse rotation ... it slips into the inside diameter of distributor tach drive ... then vette cable-end slips into inside diameter of the adapter.

It's possible your vette's small square drive cable-end did not fully engage with the MSD's larger tang-type drive (it should not engage) ... so the tach may not have worked at all or maybe it only jumped intermittantly.

First, I'd check your vette's square drive cable-end for damage (gross rounding off) ... if it's OK then ... GENTLY chuck the cable-end into a reversible drill and spin the drill motor IN REVERSE while checking the tach readout ... RPM on tach should approximate DOUBLE speed of drill motor. If that checks out then tach is OK. Remember, distributor & tach drive & cable run at half-speed of engine ... so tach itself doubles the input from cable. If it doesn't check out, you have problems in addition to any possible distributor problem.

Also, I'd check the rotation of the MSD tach drive itself. With both tach cable disconnected & coil wire REMOVED ... have someone crank the motor while you view the distributor's tach drive ... it should rotate clockwise (just as your GM distributor's td does).

If your MSD has only the 3/16" tang drive (probably does) ... then order MSD pn 8491 tach drive adapter and install it ... but first check your cable & tach & td rotation as I just outlined.

If you need further MSD help I'll try ... also, Mr. Joe Lapille (a tech at MSD) can be reached at jlapille@msdignition.com ... in past he's proven quite helpful to me. Hope this helps.
check yer box for tda

MSD's configuration of td-vac can is correct from factory. Properly installed distributor will have vac can nipple approximately pointing toward passenger front wheel and td drive approxmately pointing toward brake master cylinder ... return your's to factory configuration. Some MSD tach drive distributors ship new from factory with the pn 8491 tach drive adapater included in package ... check your box for a small short metal rod with a square hole inside one end and a tab/tang outside on other end.
MSD 8572 Comes with Adapter

I purchased an MSD 8572 for my 73 from Summit Racing in January. Included in the box was the adapter Jack mentioned.

Without the adapter its like putting a square drive cable into a round hole... literally... and your tach won't work.

Funny, when I installed the distributor in my car I also noticed the incompatability between the cable and the drive and thought MSD forgot something. I found the adapter in the parts bag (with the advance springs).
Thanks for the help!! I did have that little adapter in the distributor box, just as both of you described. I just installed it and it works fine! :) I checked the cable as you suggested, Jack, and its in great condition. What a relief. Funny thing...I came across that little adapter when I unpacked the distributor and I was extremely puzzled. I also noticed the "square peg in a round hole" discrepancy, but I assumed that whatever engaged the cable was deeper in the drive assembly. There's no telling how long it would have taken me to figure this one out. Thanks a lot!!

BTW, Tim, this is the 8572, and I highly recommend it.
Doncha just love it when solutions are cheap & easy? Geez ... does anyone have a sure-fire fix for procrastination? Just can't get focused on my 388 project more than a day or two ... every month or so!

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