lars

I’ve done a lot of posts on this Forum about the constantly-degrading condition of Q-Jet Carbs: The carbs I received for rebuild are in a constant state of degrading condition. The carbs are more damaged, screwed up, altered, and corroded than ever before, and more replacement parts from “donor” carbs are required to keep the carbs “alive.”

But it’s not just the carbs that are getting worse: I’m also seeing original tach-drive distributors (used up through the 1974 model year) in severely declining condition. If you’re rebuilding your own distributor, you need to be aware of the problems than can exist. Common issues that I see on a regular basis include:

• Badly damaged & worn housing crossgear thrust surface, requiring custom machine work and custom-built parts to salvage the housing (failure to repair will result in immediate re-failure of tach drive gears)
• Stripped mainshaft gear (due to the above)
• Stripped crossgear (due to the above)
• Destroyed lower bushings from crossgear thrust surface wear debris in bushing
• Destroyed mainshaft from lower bushing debris
• Missing thrust button
• Missing crossgear washer
• Missing upper shaft seal
• No lube in upper lube well
• Incorrect upper bushing installed with no lube bleed hole
• Distributor drive gear installed backwards
• Severe slop in breaker plate attach point to the vacuum advance rod, causing “timing scatter”
• Incorrect vacuum advance control unit installed (usually too stiff)
• Ruptured vacuum advance control unit (inoperative)
• Missing breaker plate retaining snap ring
• Excessive shaft endplay
• Vacuum advance curve too long (over 18 degrees typically – should be 12-14)
• Centrifugal advance curve too long, caused by missing limit bushing or incorrect cam plate installed (slot too long in plate)
• Centrifugal advance curve too slow
• Centrifugal advance curve too fast
• Bent cam plate weight supports
• Incorrect weights installed
• Seized centrifugal advance
• Worn out weight pivot pins
• Broken or missing breaker plate ground wire
• Broken or damaged primary lead wire

…and that’s just to mention the more common and obvious issue to look for.

Here’s the common state of distributors I’m getting for rebuild now. This one was received disassembled, with severe wear in the housing thrust area. Gears were stripped, and over half of the issues noted above were present. Machine work and replacement parts were needed to bring it back to life:



Once repaired and assembled, the Pile-O-Junk was run and curved out on the SUN machine to produce a perfect performance unit that should run another 40 years:







Be aware of the issues and potential problems, buy good quality parts, and set these things up carefully: Even “junk” can be restored to produce very high performance parts.

Lars

doorgunner

Amazing!

cardo0

Thx for sharing lars. Your distributor work is that of a master craftsman - very hard to find today. Problem is those distributors are fairly rare. I visited most of the popular swapmeets in LA county for last 2 years hoping to find a spare/rebuild-able core. No luck even seeing one. I would collect any sbc mech tach drive dist that i could find even if only for parts.
The dist in my '74 was replaced already with an MSD mech tach drive when i bought it (back around '95) and i guess im lucky for that.

Glad to read they are still repairable and parts are still available.

lars

Actually, you can build an "original" Corvette tach-drive distributor almost from scratch:

There are aftermarket tach drive distributors being sold by all of the major Vette supply places, using all Chinese castings and components. These "new" distributors are not good, and employ housing castings of really bad quality - I've seen a few of them, and I would never use one.

However, you can get badly-worn, clapped-out original tach drive distributor housings at a very reasonable price on eBay and other sources. If you have a $30 junk housing, the rest is cake:

The upper and lower bushings can be replaced with good quality bronze bushings. You have to have the ability to hone them to size, but that's easily done.

The mainshaft and crossgear parts are easily procured. You have to be careful about who you buy them from: I found a major vendor who was selling mainshafts with the advance cam incorrectly "clocked" to the drive gear roll pin hole in the shaft, making it impossible to correctly "clock" the distributor into the engine block. But good quality parts are available.

All the other parts, such as the breaker plate (the sheet metal plate that the points screw onto) and the Cam Plate (the rotating component that the points rub on and that attaches the advance weights) are the same as any other GM points distributor: You can just buy a junk '73 Monte Carlo/Chevelle distributor for $15 and get these parts (they only made a few million of these...). The only thing you have to do is tailor the advance slot length a little to make it a "Corvette" advance curve and retain the GM weights you get out of the Monte Carlo/Nova/Impala distributor - the aftermarket weights are junk. Upper shaft seals and hardware kits are available from many sources, and once you piece it all together, you can fabricate a super-high-performance "Vette" distributor almost from scratch. You can even buy the reproduction ID tag to "prove" that it's correct...

Lars

ctuinstra

I should really send mine it to you to have everything checked and freshened. It really is not in bad shape, but I'm not sure how much slop is slop, etc. I do know the tach gear end is worn and needs to be fixed.

Lupigiato

Lars, based your extensive experience, have you ever seen a TD distributor that had a weight pin come out and the weight ended up in the bottom of the distributer and nicks up the pickup to the point it causes hesitation and stall? Is it possible that you can repair the pickup and install a new weight pin? Thanks.

FlyLS6

Before we get to the major tear down/rebuild stage, any insightful maintenance tips from observations over the years, for units still in good operating condition?

lars

Are you talking about a TI distributor?

dmaxx3500

whats it cost to go thru a dist now a days?[i have 2-3 cores],,id like to do a electronic conversion ,what do you recomend for a kit?,,69' 427/400hp tripower auto trans car

jersey68l36

When I brought my '68 in '72, the tach was not working. The cross shaft and the gear broached in the main shaft were shot.

After getting the replacement parts, (back then from GM) I drilled a hole in the housing, and tapped the hole for a hollow set screw. The set screw dials in the backlash against the cross shaft. Initially I had the back lash set too tight, and once the mtr warmed up a bit it binded and actually shut off the mtr! Anyway, I've been going along ever since w/o any tach drive problems. This set screw idea came from a article in Hot Rod.

cardo0

lars, will all years of chevy tach drive distributors work? I believe the sbc and BB distributors are the same and interchangeable? So im guessing any Chevy tach drive dist for the '60s or '70s will work?

cardo0

Lupigiato

Sorry, I abbreviated for Tach Drive (TD).

LS4 PILOT

Nice post Lars .

My 74 original distributor , starter, alternator ,and water pump and diff were rebuilt by the old school guys , like Mock and Pirkle 25 years ago . ....still going strong .....GM parts were the best .

lars

My base entry price is $110, but that can easily double if the housing thrust surface is damaged and if the distributor needs mainshaft and crossgear. You can e-mail me for my complete pricing and info sheet.
I won't do electronic conversions - they're junk, and there is no good technical reason to do the mod: a good set of points will run through 6500 rpm without issues, and they will last for 20K miles easily with no reliability problems.

Yes, they're all fully interchangeable - big block and small block, and all years. Any Chevy points-type distributor can be set up with an excellent performance curve, so it's irrelevant what you start with.

Next question, then, is what's a "pickup"..? Obviously, if parts have come loose inside the distributor, they can cause damage and short-out of the points, but I've never heard of a "pickup" in a non-TI distributor. Anything can be repaired or replaced - the tach drive distributors use all standard Chevy parts other than the mainshaft, housing and crossgear, so any Chevy '62 - 74 distributor can be used for parts.

Lars

71scgc

Really need to send you mine soon. I'm running an HEI right now, but would love the old one to be rebuilt.
Many of the issues above are present in it.

Carter