jusplainwacky

Hello - I have two tach drives and I'd like to know if either are correct. One has stamp "L 10" and the other has "L 5" with a circle around the "L".
Anyone have any idea's on if these are original, or what year?

DZAUTO

Tach drive what??????????? Distributor? Generator? What?

Tom Parsons

jusplainwacky

Tach Drive units....the assembly that bolts on to the back of the generator. Also I have another question. I have a pulley that is 3 3/8" in diameter (outer edge to outer edge or the pulley). I believe it should be 3 5/8"...can anyone verify?

JohnZ

The tach-drive generator gearboxes were all the same - the L5 and L10 markings are mold numbers. Base engines used a 3-3/8" dia. pulley with a 3/8" groove width, optional engines used a 3-5/8" dia. pulley with a 1/2" groove width.

DZAUTO

And which generators got the BIG 4in pulley?

Tom Parsons

jusplainwacky

I wonder why all the corvette dealers sell 3 5/8"...nobody mentions the 3 3/8". Thanks!

toms silver 60

Would both generators have the same effective diameter and require same tach calibration?
My 60 generator has had a 3" dia pulley with 3/8" width for many years. This might explain the long standing overstating of rpm, even after recent tach rebuild? Presently about 11% high, used to be even more.

jusplainwacky

Being an engineer, all I can say is that having different pulley sizes is going to change your RPM readings...so its confusing to me. Did they have different gearing systems in the tach for the different pulleys??

John (above) apparently is correct...as someone else told me they looked it up and base engines had the 3 3/8" pulley. Strange why they would change the pulley for different engines.

DZAUTO

The guts (known as the head) of a mechanical tachometer and a speedometer are essentially the same. The odometer part of a speedometer (and the rev counter in some early tachometers) is fixed and cannot be adjusted/calibrated because it is directly gear driven by the speedo cable.
BUT, the speedo/tach CAN be calibrated. The indicator needle is moved to a higher position as the tach spins faster. This is caused by a spinning magnet inside the head. The faster the magnet spins, the more the needle is energized (if that is a good description). The amount of increase or rise of the needle can be adjusted by a knowledgeable speedo technician. It is a fairly easy adjustment (provided you know what the heck you are doing) and you need a speedo calibrating machine which spins the speedo/tach head while the adjustment is made (I've watched the guy at the local speedo shop while he did this for me). I have what appears to be a matching tach and speedo in my 51 Chevy. I made the tach from another 51 Chevy speedometer and had the speedo shop guy recalibrate it to indicate rpm instead of mph. After I saw how he did the adjustment, I could have done the adjustment myself, but I don't have the machine to indicate the rpm's that are desired. My home made ("factory") 51 tach is driven by a Vette tach drive distributor, just as it's done in a Vette.

Tom Parsons

jusplainwacky

Thanks Tom....this is what I love about this forum...the things you learn!

Thanks!

JohnZ

The gearbox output rpm was the same with either pulley - the 3-5/8" pulley with the 1/2"-wide groove was a "deep-groove" pulley (to keep the belt from coming off), and the belt rode deeper in the groove than it did with the 3-3/8" pulley which had a narrower (3/8") groove.

The 4"-diameter pulley was used on solid-lifter fuelies with distributor-driven tachometers, to reduce generator rpm under "sustained high-rpm operation".

jusplainwacky

Thank John....your answer, basically answered my question. As an engineer I was scratching my head on how could they have different pulley sizes and yet the same tach (rpm gauge). WOW...I can sleep tonight!

DZAUTO

And that was basically my understanding, BUT, for what years? Did the 57s with a 908 dist get a 4in generator pulley? I've never known of ANY 57 generator to get the 4in pulley (well---------------except maybe for my 56! ).




Tom Parsons

62Jeff

For what it's worth, 62 solid lifter motors have a 4.125" diameter pulley on the generator, which of course is not a tach-drive unit.

toms silver 60

So, it would appear that the tach drive generators run exactly double engine speed, then that is reduced 50% to engine speed so that tach cable turns engine speed. Distributor drive cables run half engine speed. The tachs are all essentially the same but are calibrated to either same cable rpm for gen drive or double cable rpm for tach drive. That was good to know!

JohnZ

That's correct - the distributor-driven tachs have "Reads Twice Cable Speed" ink-stamped on the back of the case to identify them as having a different internal drive ratio.