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MG MGA - MGA tachometer drive gear ratio

I don't suppose anyone knows or has access to this figure?
I'm guessing it's 6:1. The camshaft speed is half the engine speed, and the Jaeger tachometer dial face in my car has at the top, 3-1.

Why am I asking? Well, having just fitted an 18V engine to my Mk II, I was about to adapt an MGB Smiths RV1 positive earth tachometer to my MGA, and fit the Jaeger dial face and needle onto the Smiths tachometer too. (The former, surprisingly, mounts differently).

I then noticed the Accuspark Dynamotor range, (alternators that look like generators), which included models with a take off at the rear for a tachometer drive. I emailed Accuspark to ask if this would work with an MGA, and was assured it would.
Well, it doesn't, not straight out of the box at any rate. The tachometer drive cable end has to be different, needing a spade like fitting to slide into the alternator. Secondly the screw on collar is a different size too.
And once I sort that out (an automotive instrument repairer is making one up for me), the next issue will be the pulley size on the alternator.
I'm thinking now that I'll need to spin the alternator at three times engine speed. I was going to calculate the pulley size from here:http://www.engineeringtoolbox.com/pulley-diameters-speeds-d_1620.html

There's a local guy who's made several pulleys for various purposes for my local mechanic, who I'm told is very good.

But it sure would make life a bit easier if someone would possibly be able to give me a definite answer on the ratio of the tachometer drive off the end of the camshaft.

Thanks in advance.
T Aczel

For 1500/1600 with 4.3:1 final drive, the tach cable runs same speed as the speedometer cable, 1450 turns per mile or 5/12 as fast as crankshaft speed. Speedometer drive in gearbox is 5/12 reduction ratio. Drive gear at rear end of camshaft to tach drive is 5/6 reduction ratio.

Due to standard pulley sizes, the generator runs about 50% faster than crankshaft speed. To run the standard MGA tachometer from the generator you would need about 5/18 reduction ratio speed reducer gearbox.

Check price of the speed reducer used there on the MG TD, and maybe you will change your mind about doing this.
Barney Gaylord

Thanks for that Barney.
Irritating to put it mildly that a "bolt it in, hook it up, and you're away" task has now turned into a very fiddly and somewhat expensive saga. I'd of course never have started down this path had the information by the supplier been different.
Needless to say I'll be giving Accuspark my feedback in due course.
I'll share the reply on this forum. First I've got to get this conversion up and running properly.
T Aczel

you need this

http://www.ebay.co.uk/itm/MG-Midget-Austin-Healey-Sprite-Rev-Counter-Tachometer-Drive-Gearbox-13H632-/221427553275?hash=item338e1c53fb:g:kQwAAOxyu~lSTpgN

dominic clancy

Thanks Dominic.
While I can check on a Sprite/Midget forums, would you happen to know if the Jaeger tachometers on early Spridgets and MGAs are the same?
I'd not be surprised if they were; the chap I bought my MGA from had owned 17 of them! But, he'd owned even more Sprites and Midgets!!! I've found a few Sprite bits here and there on my MGA already anyway.
T Aczel

Quite honestly, I would just buy a MGB electric RVC tacho and install that. With the difference being minimal on the face, its going to be a lot easier to install, and even if you get the gearbox below, there's no guarantee that all the other gearing ratios - crank- dynamo - this extra gearbox are going to be accurately reflected in RPM on the gauge when its all installed.

AFAIK the the dynamo pulley is the same on Midget and MGA, the crank pulley on the 1275 Midgets I look after are also the same OD as the MGA. But you may have bits from other cars installed.

In any case it's going to be a parts-bin mongrel, its usually best to take the approach that is technically simplest
dominic clancy

Thanks for your advice Barney and Dominic.
It looks like it's back to plan A, fit a Smiths RV1 tachometer. I'd already purchased one in readiness in any case.
The tachometer drive take off from the back of the alternator seemed a very neat solution, but clearly to make it work will take a lot of fiddling and expense, and, at the end of the day, as Dominic describes it," it will still look like a parts bin mongrel".

So I now have an alternator which I really don't need. Needless to say I'll be giving feedback to Accuspark, who when I originally enquired prior to purchase, assured me that the Dynamator would fit straight on to my MGA tachometer drive, and work without needing recalibration of the tachometer.
T Aczel

I would still fit the alternator just to get the improvement in output. If you need details of the wiring at the control boy, just drop me a mail.
dominic clancy

Tom
why not still fit the alternator, it looks right, it will do a much better job of keeping your battery charged up and also will provide a much more stable voltage output which is preferrable if you have an electronic ignition system.

I fiited (a standard Lucas ACR type years ago to help power my twin electric fans.

As Dominic says, the Smiths RVC rev counter is the best one to fit if you have electronic ignition, AFAIK the other models are suited for points igition and if you fit them to electronic ignitions, they either show the wrong revs or break down.

If you look back a few days you will see the thread we had on this subject and this is the link I was iven to sort out the wiring.
http://www.clancy.ch/MGA.html
Colyn
Colyn Firth

Sorry, I think I somehow cut and pasted the wrong link.
This is the one I meant.

http://www.mgb-stuff.org.uk/tach.htm.

The other link is to Dominics website which is always well worth a look at
Colyn
Colyn Firth

Thanks for that information Colyn (and Dominic).
MGB assembly in Australia ceased in 1972, so RVC tachometers are essentially non-existent here though I'm sure they show up for sale overseas on eBay and elsewhere.
The original RV1 tachometer in my MGB always read high, and got worse over the years. Its readings became even higher and less constant when I fitted pointless ignition. So I installed a "Spiyda" circuit board conversion. I found this an extremely fiddly and time consuming job, but I got there in the end, and it now reads accurately and is rock-steady.
As I mentioned, I had managed to pick up another RV! tachometer, so my next option was to do to it what I'd done to my MGB tachometer, plus substitute a Jaeger dial face and indicator needle off a spare Jaeger tachometer I have (though I hate to dismantle a perfectly good instrument for donor parts).
I'll do an eBay search for RVC tachometers and see how I go.
Add: Just looked on eBay, "worldwide"; none currently listed for sale. Starting to look like I'll be buying another Spiyda conversion circuit board!
T Aczel

I've got a converted RVI knocking around if you are interested - email me at scuderiablkhole9ataoldot com
d brenchley

Tom, I have just seen a negative earth RVC Rev counter listed on ebay today at £30 which is quite a bit less than I paid a few months ago. It still has 6 days to go.

http://www.ebay.co.uk/itm/mgb-smiths-rev-counter-RVC-2415-00-/141993941164?hash=item210f7f98ac:g:~KkAAOSwdV1XOIWm

Colyn

Colyn Firth

Thanks Colyn.
I just went onto the link. Unfortunately the seller won't post outside of the U.K. This is probably why it didn't come up when I entered Smiths RVC tachometer/rev counter into eBay , despite adding "worldwide".
Mr D Brenchley, thank you. I'll email you when I get home from work tonight.
T Aczel

Tom,
If you wish, I could buy it for you and post it on to you when it arrives.
I have just fitted an RVC to my car and so I can easily check to see if it works before I forward it.
The only thing I am uncertain of is how much it will cost to post it.

Colyn
Colyn Firth

Colyn, as you know, I've just emailed you directly. Thanks so much for offering to help out.
Mr Brenchley I tried to email you as well, but the email address you gave seems invalid.
T Aczel

try scuderiablkholeataoldotcom
d brenchley

This thread was discussed between 13/05/2016 and 16/05/2016

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