Tricky Tachometer

[harland]

Thanks to all who helped me with the clock removal. I bid on and won a quartz model for a 55-56 Chevrolet. I have the tach in front of me. I recall that the owner said the tach wasn't reading correctly. On the case of the tach is printed "registers twice cable speed". I wonder if this is common on Corvette tachs as illogical as that sounds. What should one do to get the tach calibrated to being accurate with my engine? Anyone have good experience with a repair facility? where? thanks,Harland

 

[JohnZ]

There were two different sets of guts for Corvette tachs - the ones for use with the cable drive from the gearbox on the back of the generator were stamped "reads cable speed", as the output from the gearbox was the same rpm as the crankshaft. The other type (like yours) was stamped "reads twice cable speed", and was used on cars that had the tach drive from the distributor; that output is at 1/2 crankshaft rpm, as it's driven by the camshaft. If your tach is used on a car with the generator-driven cable, it will indicate twice the actual rpm. If the tach intended for use with the generator-driven cable is used on a car with a distributor-driven cable, it will indicate only half of actual rpm.

Outfits like Corvette Specialties of Maryland, D&M Restorations, etc. can rebuild/restore/re-calibrate tachometers and can convert the guts from one type to the other.

 

[harland]

Hi John, that is such helpful information.I was very puzzled by the printing. thank you. a friend suggested I use a portable drill in reverse (with a known RPM drill speed) and hook up the tach to see if it is reading off quite a bit. I was told these instruments tend to lose their accuracy after the passage of decades. Would you think this would be OK to try?I have a feeling it will be a hundred dollar bill to get the guage calibrated and I want to find out the current status to see if it's critical to go with a rebuld. thanks, Harland

 

[JohnZ]

Tachs work on exactly the same principle as a speedometer - a driven rotating magnet wheel inside a cup with a needle on it, and their accuracy is a function of the strength of the magnetic field (gauss) between the two parts. Part of a proper rebuild/restore of either instrument is re-calibration (re-gaussing) of the magnets; it's possible for them to need re-calibration after many years. The drill trick will work (to show that the tach functions), but you need to know the exact rpm of the drill in order to determine how accurate the tach is. The "reads cable speed" tach should indicate the same rpm as the drill, and the "reads twice cable speed" tach will indicate twice the rpm of the drill.

 

[harland]

Hi John, thanks for taking the time to educate me. I appreciate it! Harland

 

[harland]

Generator drive tach

sorry if I a bit slow in comprehending mechanical thoughts, but please advise. My mechanic tells me my tach runs off the generator. The one I am staring at says 'registers twice cable speed'. Does this mean that I have the newer distributor run style and either I have this one attached to the distributor via some connector, or I need the 'older' style that runs off the generator? Thanks, Harland

 

[JohnZ]

Your generator is on the other (passenger) side, but does it look like the photo below with a gearbox on the back and a drive cable attached to it? If so, you're looking at the wrong tach; if you use that one, it'll read twice actual engine rpm.

 

[harland]

wrong tach

I believe I have the incorrect tach--now recall the seller mentioning something-after money traded hands-about the instrument not being connected correctly or something like that. Anyway, a Corvette guy said I could possibly get a part that would allow this generator driven type to be instead run off the distributor with the addition of a small gear? Does this make sense or should I seek out the correct generator driven one--or have mine modified if it isn't outrageously costly? thanks! Harland

 

[JohnZ]

No such thing as a multiplier gear for it, although there are reducer gears. Don't make it complicated - just have a Corvette instrument specialist replace the guts with the correct internals for a generator-driven tach (Corvette Specialties of Maryland, D&M Restorations, etc.).

 

[harland]

sounds like a sensible approach. thanks. Harland

 

[harland]

6 model tachometers

Hi, wonder if I can get some further guidance. I have found several good sources for recalibrating my 1961 distributor driven tach--so that it will run off the generator, but then learned via the Corvette America Catalog that there was more than one model. Mine runs all the way to 7000 RPMs and I question whether this ever came in a small engine car. I wonder if the 283 tach is supposed to be the unit that runs to 5300 RPM instead. If I have the wrong unit entirely, it might be better to buy a new repro already set up for my generator drive style and resell this very nice one that is distributor run. Please advise fellows. Harland--PS I should clarify "five" choices in the CA catalog depending on which one came in my 61.

 

[JohnZ]

Noland's C1 book has an extensive discourse on which C1 tach face and redline came with which engine, and the '61 Judging Guide should also cover it. I'm in Bloomington this week and don't have access to my references.

 

[harland]

tach issue

Hi John, again thanks for your time and interest. I learned from a Corvette parts vendor that all 1961 tachs had the absurb 7000 top number, but that the 282s had the lower redline. I wonder how many straight axle cars needed replacement engines by careless drivers actually 'redlining' their cars-when I think what RPMs actually signify it is scarey! Anyway, I have the right 'red line' indicator, and have sent my tach offer to Clocks by Roger in Tn who was recommended by my restorer. I do appreciate very much your help. I hope you have/had a great time in Bloomington this weekend. Bet there are some fantastic cars there! Regards, Harland

 

[KANE]

Huh.

I learn something new everyday. I've never seen a tachometer driven off a generator. I'm more familiar with Corvettes from 1974-2004, specifically smog era C3s.

Thanks for the great pic John!!