Frequently Asked Questions answered by individual members of Natural Glass Corvette Association.

C1-C3 Mechanical Tachs and C5/C6 Manual Shifters


The following is a series of questions and answers posted by club members in an effort to help share information with each other. Many of the answers are based on the personal opinions of the individual member, and may or may not produce the results desired by the member who posed the question. Therefore the posting of these answers is not an endorsement by NGCA of the opinions expressed therein. We recommend that you conduct further research and consult a qualified GM expert before attempting to perform any of the suggested techniques or procedures listed below.

Question #3: How do I go about the care and feeding of mechanical tachometers in C1 to C3 cars?

Answers:

I do have a suggestion on the mechanical tach. On motorcycles (older ones) the speedo was mechanically, driven by a cable off the front wheel. 99% of the problems were broken cables as a result of the cable lube leaking out over time and the inner cable then drying out and binding on the protective outer cable guide. Most scooter shops sell a cable lube that is pumped into one cable end with a "syringe-like" injector. That MAY help in preserving functionality on Vette mechanical tachs.

Answer Submitted by – A.E.

My personal experience is confined to C2 & C3 vintages, which utilizes the distributor to drive the Mechanical tach. The main sore spot in the design is the drive gear machined onto the distributor shaft, and the corresponding driven gear and its housing. Typically what happens is the driven gear wears the housing to the point where it touches the housing and seizes, shattering the teeth on both the drive and drive gears. When this happens, the engine timing is dramatically altered, basically killing the engine. Please don't ask how I found this out. The factory used a nylon button to prevent this catastrophe, but I have found that the button quickly wears away. There are a number of fixes used to varying degrees of success. In my '67, I drilled and tapped the mating surface that wears and used a hardened set-screw to set the driven gear back lash. Keeping the driven gear well lubed in its housing is also a good thing. After that, periodically lubing the Tach drive cable with a light lubricant should keep that Tach telling its tales! A quick search of the usual web site suspects, ( CAC, vettefaq, C3VR, etc) will undoubtedly provide excruciating details!

I currently happen to have my distributor out of my 72 at the moment, and if the individual would like, I could bring it to next weeks meeting, and off to the side. dissect it with them, maybe even getting grease under our fingernails! Woooo-Hoooo!!

Answer submitted by – K.M.

There appears to be two areas for care of C1-C3 tachometers. One centers on the circuit board (1975-1982) and the other on the cable.

Circuit Boards

There are a few issues with the board:

Board accumulates dirt and grim which needs to be removed through the judicious use of a pencil eraser and electrical spray.

Make sure all connections are shinny after cleaning

The solder on the board has small cracks which are normal in older cars. Reflowing the solder helps to eliminate the problems and get the tachometer working again

If none of the above corrects the problem the board needs to be replaced. They are available for approximately $ 75.00 from various sources including ZIP which most people seem satisfied with.

Looks like replacing the board is real easy - unscrew the several nuts, put the new board on, put nuts back on. The guys claim it’s a ½ hour job to do.

www.zip-corvette.com provides step by step instructions with pictures

Cables etc.

The cable comes through the firewall and attaches to your distributor. The gear that engages with the distributor main-shaft gear is the typical culprit. Unhook your cable from the distributor (may be easier said than done) and look at the male end inside the cable, it should be square, if it's rounded off you need a new cable. If not, then pull the brass cross gear assembly off of your distributor (it may have come off with your cable). There are several parts that make up the cross gear. The part you want to look at is the internal shaft gear that makes contact with the distributor main-shaft gear. The internal shaft is usually the culprit and needs replaced, $30-40 for a new part. If it looks ok, then if you have a hex bit that will fit in your drill and engage the cable, connect it to the end of the cable and rotate the cable with the drill while someone watches your tachometer. If the internal shaft looked good, and the drill made the tachometer work, then it's probably the main-shaft in the distributor which requires a techie to assist with.

If the drill didn't turn the tachometer, then you should go under the dash and unhook the tachometer end of the cable (a real PIA) and see if it is rounded off. If it's not rounded, do the drill hook up again and see if the cable turns, if not you have a broken cable and need a new one.

Service and Parts Sources:

Instrument Services Co. of Roscoe, IL

Corvette Clocks by Roger of Jackson, TN

corvetteinstrumentservice@worldnet.att.net

www.tonyscorvetteshop.com
www.corvettespecialtiesofmd.com

Answer submitted by – D.W.

Question #4: Will the 6 speed shifter from a 05 C6 bolt into a C5?

Answers:

Here is a post on the Corvette forum reference your second topic. A forum member has purchased the shifter for install. Your interested person can search for further post on this topic in the Corvette Forum. Warranty issues should be checked into as well...

This is from Bob at Fichtner Chevrolet in Montana:
We have received a C6 shifter to compare with both C5 6 speed shifters. We have compared the three shifters and measurements are all the same. The C6 shifter should bolt right up with no modifications required! The only thing is the C5 shift knob will NOT fit the C6 shifter so you will also need the C6 shift knob. We have put a package together with the C6 shifter, C6 shift knob and retainer.

Answer submitted by – A.P.

Yes the C6 shifter will fit but the threads for the shifter knob are different. Thus if you are trying to keep the stock look, the shifter knob probably needs to be changed.

More importantly is why, since it will not provide the crisp shifting an aftermarket can provide such as the Breathless Performance Shifter. Many people still feel that the C6 shifter is still sloppy.

Installation of any of the shifter takes less than 1 hour, assuming proper tools are available. You will need a rag, 2 medium sized flat blade screwdrivers, a Torx 15 screw driver, and various sized sockets, 11mm and I think 15 mm. Also various sized nut runners would also be helpful, a tube of lock-tite, and a plastic cup to hold the bits and pieces until they are replaced.

Also this would be a good time to change the emergency brake boot and/or shifter boot. These changes add approximately 15 - 20 minutes to the shifter change.

Answer Submitted by – D.W.

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The answers may reflect personal opinions of individual members and are not an endorsement by NGCA. Always make sure to consult a qualified GM expert and conduct proper research first.

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