I have a 1973 Corvette, L-82. I'm the original owner, so I have a pretty good idea of what has and has not been done to it, but a few things might have slipped my mind over the past 50 years.

With that background, a little while back the tachometer failed, completely, with no hints that a failure was imminent. The cable is still good, although not original, nor do I remember when I replaced it. The tach (instrument) was replaced about three years ago and still operates fine. The cause of the failure was diagnosed to be the tach drive gear, and there was also obvious wear on the gear on the distributor shaft. Since there was obvious wear on the distributor shaft gear, I elected to replace the shaft as well as the tach drive gear.

When I researched which distributor main shaft to get, I noted that at several suppliers, the same shaft was used for the base engine and the L-82 (and even the 454), so that's what I had installed. The mechanic who did my installation pointed out to me (after the installation) that the shaft he removed had a spiral groove machined into a section of the shaft, which he said was probably an oiling groove. The accompanying photo is of the original shaft with the groove plainly visible. The replacement shaft did not have that spiral groove, and was completely smooth in that part of the shaft. I did a little more research after the installation and found that Zip actually shows two different part numbers for the shaft, one for the base engine and one for the L-82 engine. I called Zip and the customer service guy I talked to said he would get both shafts off the shelf and physically compare the two. He found that there is no discernible difference between the two shafts, with no spiral groove on either one. Has anyone else changed from the original distributor shaft to a replacement shaft (with no groove) and has the new version caused any issues as a result of not having the groove?

The photo is of the original distributor shaft.

 

I would surmise that the spiral grove took oil up to the tach drive. Machining that groove would make the shaft more expensive, but since no one has it anymore it may have been deemed superfluous.

 

That's an interesting dilemma. Do you have the number on the band at the top of the distributor.
Is it that the advance stop button is in a different location between the base and high performance shaft?

 

hmm, does the gear have a pin to remove for changing?

 

Here is my original 1970 shaft, no groove. It lasted about 90,000 miles and the tach drive failed. The auto cam was also quite worn.
The groove must have been added later than 1970.

I have had a replacement shaft (no groove) installed now for about 10 years, no problems.
I now do oil the felt once in a while in the top bushing and grease the tach drive.

The original dist. shaft/tach drive had no maintenance for 90,000 miles. My bad.

 

Thanks for the note. Does your 1970 have the LT-1 engine? The reason I ask is, I wonder if the LT-1 might have had the machined groove in the shaft. My engine is the L-82, which is described sometimes as essentially the LT-1 with a milder camshaft, hydraulic lifters, and lower compression. I'm thinking the grooved shaft might be a hold-over from the LT-1.

Disregard that question about your engine. I now see in your sign-in name that you apparently have the LT-1 engine.

Thanks again.

 

If you slide left on the pictures of the standard and high performance shafts you will see the difference is in the shape of the top of the shaft, the cam that the weights ride against.
https://www.corvettecentral.com/c1-5...3%26count%3d18

 

Just to add a little more information, I originally replaced my main shaft with the "standard" shaft, as the "Hi Perf" one was not available at that time. The standard one did not work well. I soon converted to the Hi Perf shaft, it has a much better advance curve.

 

The shape of the cam that touches the centrifugal weights is different on the std shaft and the hi-perf shaft.
The hi-perf shaft cam has a better advance curve profile.


This is what I found on-line describing the two cams & their curves


I actually measured the timing curve on the two I had. The hi-perf cam gave an almost linear advance curve, exactly like the Chevy power book. The std cam would result in 3 more degrees of retarded timing at all rpms, except full advance.

 

The L-82 distributor pulls a lot less mechanical advance than the others too........like 15 as opposed to 21........and the can only pulls like 10.......so I think they wanted this installed with a high initial timing setting like 20 degrees.....which would give it a more aggressive transition as the vac can peters off.........during acceleration.....


Jebby