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MG MGA - MGA/MGB rev counters

Is the mechanical rev counter used in the MGA 1600 identical to the one used in the early, 3 main MGB?

I will use a 5 main MGB engine in my MGA coupe project, so I have to use an electronic instrument. And I already have one in my 3 main MGB, which really should have a mechanical rev counter. Can I just swop them around?

Tore
Tore

The gauges are the same size, but usually different manufacturers. The B tach uses a current loop sense which runs in series with the primary side of the coil if I remember correctly. Not hard to wire in, a common swop for those using B engines.
Bill Young

The gauges are dimensionally the same. The drive on the end of the camhaft is the same part number in both applications, so it should work accurately (and the very early B one has a Jaeger face too). Although the cables are different, I believe the threads on the rev counter drive are the same thread in both cases.

The electronic gauge will be fine with the 5 bearing motor - if it's already in the B and working, you know how to wire it for the A.

dominic clancy

Internally they are identical (both 3:1). The case on the mgb early mech tach has a different bulb socket for the ignition light and this may present a problem with the bulb holder in the mga--not sure. The guage will install in the same hole as the case diameter and depth are the same. Brackets may be different. The faces are identical with exception of the sn number. This also applies to early midget Jaeger mechanical tachs.
I think this answers your question, if you are talking only about the using the early B mech tach in your A and not the electric tach (Smiths) in your A. For that refer to Bill Young's comment.
James Johanski

Tore

Be aware of the polarity issues. All MGAs and early MGBs were positive earth, unless changed over by the owners. I am not all that knowledgeable on the MGB, but something at the back of my mind is saying that the MGB electronic tachometors are different depending on whether +ve or -ve earth. I do remember the supplier asking me for the polarity when I ordered an MGB tacho for my car.

Worth asking the question. You don't want to risk damaging the instrument.

Steve
Steve Gyles

Polarity can be changed within Smith's electronic tachometers by making a small wiring change inside. It's very simple and details on the process should be in the archives. Sorry, I can't relate them verbatim from memory so I won't bother trying!
Steve Simmons

Thanks for all your comments. It seems that I can then use the mechanical MGA tacho in the MGB, possibly with some adaptions, and use the electronic tacho that is now in my MGB in the MGA coupe project with the 5 main MGB engine.

My MGB has been converted to negative earth, and I will do the same to the MGA, so that I can have a decent car stereo in it!

Tore
Tore

Tore,
Also the needles are different. It is not very noticable but they are not the same. I have a B tach in my A roadster and was told that you should not interchange the needles. Have a good day!

John
John Progess

I have a 5-main with an electronic tack with a Jaeger face. I used an early MGB electronic tack and took off the Smith face. I then drilled two new holes to mount the Jaeger face, I took the needle from The MGA and took my dremel tool to it to grind off all the excess weight. I then sent it out to MOMA in New Mexico who put in a new German electronic board to make it work. When looking at the car, you have to look very closely to see that it is not an original tack.
JEFF BECKER

This thread was discussed between 18/10/2007 and 21/10/2007

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This thread is from the archive. The Live MG MGA BBS is active now.