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MG MGA - 1800 block with mechanical tachometer ?

Hi All:
Just wondering if anyone has ever had an 1800 block machined for the mechanical tachometer drive?/Jc
John Crawley


The early 3 bearing engine has it as standard. It will be the 5 bearing engine you are enquiring about. I think it's been done by Dominic Chancy of this forum if memory serves me right. I presume it also needs a new camshaft unless the off-rake was retained on the standard cam.

Steve Gyles

If you check out Barneys website John there is an entry on the "Whats New" section on this very subject and how it was done.

I have just fitted a 5-bearing 18V engine to my car and I would have had my block machined for this if I had seen it earlier.

it will now have to wait for the next rebuild.

Cheers Colyn
Colyn Firth


Interesting bit of machining. Beyond my ability, definitely one for the machine shop:

I presume the drive on the end on the camshaft is a one-off?


PS. My last post should have read off-take, not off-rake. Tablet and mob keyboards are the bane of my life.
Steve Gyles

The drive gear for the tachometer on a MGA camshaft is a separate part from the shaft.
It should not be difficult to fit one to a MGB camshaft.

M F Anderson

Attached is an image of the tachometer drive gear.


M F Anderson

A better image of the gear.


M F Anderson

Steve is mistaken

I have had 1622 engines from other cars modified to take the tacho drive, but the 1622 is fundamentally the same as the early 3 bearing MGB blocks. This is an easy modification, because all that is missing is the machined hole - the threaded holes for the drive are already in place. The tacho drive gear itself is a tight fit onto the camshaft. It is supposed to have a woodruff key, but I have been told many times that the interference fit is enough and that the key is unnecessary. The 5 bearing camshaft seems to be substantially different at the gearbox end, so wouldn't be able to take the tacho drive gear without modification.

I have no experience of 5 bearing blocks, so can't make any contribution on whether the block could be machined if the camshaft issue were overcome. The pictures in the parts list show very different block castings at that location, so I rather suspect the answer is a clear no. For the 3 bearing 1796 engines, the casting should have the boss just like the 1622, so then it is an easy mod.

I think that for all the effort involved, it would be much easier to use an electronic tacho - the car isn't original any more with a 5 bearing engine anyway!

dominic clancy

Thanks for the replies . . . It was mostly out of curiosity that I asked. I own a early 3-main B that blew the engine years ago. I bought three blocks truing to get the proper engine and finally gave up because they were all not rebuildable, so the car has just sat. I guess that a later block will just have to do and I should get the car back on the road.
John Crawley

This thread was discussed between 02/05/2016 and 03/05/2016

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