Workin On 59

I'm trying to determine if an earlier (58-60) tachometer has the same gearing as the later (61-62.....and possibly up to 74 tachs).

Does anyone here know?

Also are the HEI tach drive distributors advertised here for $199 compatible with a 58-60 or a 61-62 tach?

I would like to stay with a stock mechanical tach if it's feasible, but go with an alternator instead of a generator.

Thanks, Doug

62Jeff

I'm 99% sure the two are not the same. The older tachs were driven from the generator, the later tachs driven by the distributor.

I believe the speed difference was a factor of 2.

Others will chime in I'm sure, particularly if I'm wrong.

K2

In years through 62 the tach was driven from the generator except when optioned with SPH engines in which case they were distributor driven. The two are not interchangeable as the driven ratio is different.

dazvet

I think when you pursue the tach drive alternator thing you will find it to be very problematic, it's doable but you will chase it awhile. On my 58 I just went with the tach drive distributor with a 62 tach drive cable and a 0.500 ratio adapter.

wmf62

an instrument rebuilder, such as Jason Oliver, can recalibrate your tach so that you can use it with a tach drive distributor.
Bill

chris ritchie

1962 through 1974 Corvettes drove their tachometers via the distributor. All those distributors will interchange. Most of them were the same. If you're planning on rebuilding one of these distributors before you use it (and you should), then all of them, except the "985" centrifugal advance only one, will work.

These 62-74 distributors will work in pre-62 Corvettes if you don't use the tach drive feature of the distributor. Pre-62 Corvettes drove their distributors off the generator. 62 and pre-62 tachometers don't interchange because one reads 2X the other. I forget which one. They can be fixed to read correctly. But it's probably cheaper to buy the right tach.

HEI distributors will work on 62-74 Corvettes if they have a tach drive. But the HEI distributor won't fit under the RFI ignition shield, and the plug wires will need to be changed to mate with the HEI cap.

Workin On 59

Thanks for all the responses.

It sounds like it is workable to use a stock tach with a gear ratio corrector coupling and a tach drive distributor.....or use a 62 tach.

For a driver I don't plan on using the ignition shielding so the HEI tach drive distibutor would work as well. I want to use an alternator so I'm glad to get confirmation that I can do that and still have a mechanical tach.

timbo79

Here is another option.. To keep period correct and use the shielding, I opted for the MSD tach drive distributor with remote coil. Fits as stock, the tach works great. Kind of pricey, but looks nice..

I got the original brackets and it all works. Mine is a 59, origianl high horsepower, and it works correctly. I am not sure of the gearing change at later dates.

Here is a pic (sorry, only from the side.. it is a ZZ4 with dual quads)

Second pic is from the other side before I got the stock air cleaner fitted.

JohnZ

There were two types of tachs in those years.

The most common was driven from the back of the generator, and the gearbox translated generator rpm to crankshaft rpm; the tach read exact cable speed.

The other tach was calibrated to be driven from the distributor (on solid-lifter fuelies, mostly), and that cable ran at 1/2 of crankshaft rpm; the tach was calibrated to read twice cable speed, and "reads twice cable speed" was ink-stamped on top of the tach case. That tach will work with ANY distributor drive.

If you have the tach that was designed/calibrated to be used with a generator drive and run it from the distributor, it will read half of actual engine rpm.

If you have the tach that was designed/calibrated to be used with a distributor drive and run it from the generator, it will read double the actual engine rpm.

When you decide what drive source you're going to use (generator or distributor), Jason Oliver can set up your tach to match.

DZAUTO

Just to add to John Z's comments.
ALLLLLLLLLLLLLLLLLLLLLLLLLLL tach drive distributors, 57-74, turned at EXACTLY the same speed-1/2 crankshaft speed.

BUT, not all generators turned at the same speed! The gears in the gear reduction adapter on the rear of all generators were the same, ALTHOUGH, not all Corvette generators used the same size pulleys (3 5/8in or 4in). Thus, not all generators turned the same rpm, therefore, generator driven tachs need to be calibrated according to which pulley is on the generator.

Workin On 59

Thanks again, guys. I knew you would have the facts.

BTW, great looking setup there, Timbo79.

dazvet

Hi, I think I gave you some bad information earlier, I have an MSD tach drive distributor and it seems when I hook a test tach to MSD box that it's going to take a 1.500 ratio adapter to get the mechanical tach to read correctly. I have one ordered and will keep you updated on what happens. I had some problems getting the electrical tach to work with the MSD so we'll see what happens but at 2000 rpm on the mechanical tach I was getting a 3000 rpm reading on the electrical and this corresponds to the mechanical tach giving me a reading at idle of around 600 rpm and I know the engine idles at around 900 rpm.
Larry

JoesC5

I disagree. Both the low HP version with the 3 5/8" gen pulley and the hipo versions with the 4" gen pully had the same 2:1 gearing on the rear of the generator. The 4" pulley had the same pitch diameter as the 3 5/8", difference was that the 4" pulley extended out past the belt the extra 3/16" (on the radius) to help keep the belt from flying off at high RPM.

DZAUTO

Joe,
When I get home tonight, I'll have to pull out a 3 5/8 and a 4in pulley and check that. But, I can't respond until tomorrow because the monitor on my home computer died last night!

67L36Driver

Why does no one mention what crankshaft pulley is used?

Crank Pulley P.D. divided by the Gen Pulley P.D. X engine speed = generator speed. (P.D. = pitch diameter)

BTW: Pulley outside diameter has little bearing on Pitch diameter. Best to get yourself a Gates or Dayco engineering catalog.

bweaver999

If you go this route, you might also want to check out a Mallory Unilite. I posted the Jegs part numbers in this thread:

http://forums.corvetteforum.com/show....php?t=1541490

Looks similar to the MSD you don't need to mount the remote coil. My Mallory with the coil and power protection fits under the shielding. Might want to check, mine does not have a vacuum advance. Have had no problems with pure mechanical advance, car runs great. Just an opening in the shielding where it should come out.