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Mike Muir
Mike Muir
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#850685 06/04/21 6:46 pm
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Kevin E Offline OP
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Hi all,

I am hoping that someone can give me some help with the tachometer drive on my 1966 Lightning.

I have attached some photographs, which show what I have and this has been on the bike since I first bought it.

It looks to me like someone has filed, or ground off some material from the shaft, which as far as I can make out, should be a collar that fits inside the hole in the inner timing cover. I am assuming that the said hole in the inner timing cover serves as a bush that accommodates whatever has been removed from the drive shaft I have.

Also the O-Ring is baffling me. You can see it in the photographs, and although it is old and has a small 'nick' in it, it fits perfectly on the shaft and in the brass adaptor. In the parts books this O-Ring is listed as BSA part number 68-0588. This is also the same number as the O-Rings that fit in the oil pipe union and they are much smaller than the one on my tacho drive.

The one on my tacho drive measures 31/64" OD x 9/16" ID x 5/64" thick (approximately), which is much larger than the oil pipe union O-Ring.

The 68-0588 O-Ring measures 21/64" OD x 7/32" ID x 3/64" thick (approximately).

Any ideas what the O-Ring is on my tacho drive and am I right in that there should be a collar on the shaft that fits into the hole in the timing cover?

I cannot locate new tacho drive parts anywhere, so if anyone knows where I can get hold of them, or know the exact dimensions so I can make one, it would be a big help.

Cheers,

Kev E

Tacho Drive 01.JPG Tacho Drive 02.JPG Tacho Drive 03.JPG Tacho Drive 04.JPG
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Mine is from a later engine but I think they are the same.
The shaft after the cable hole is 0.282" diam. and 0.740" from the base of the cable square to the shoulder. The shoulder (yours is missing) is 0.437" diam., 0.148" long.. From the pump end of the shoulder to the base of the tang is 1.334" long and 0.250" diam. The sleeve that fits over the shaft is 0.341" O.D., 0.250" I.D., 1.666" long. The tang is 0.236" long and 0.088" thick,.

The O-ring in the adapter is 9/32" I.D. x 0.070" thick. Mine has only one O-ring.
The shoulder on the shaft captures the O-ring in the groove of the adapter. The O.D. of the shoulder fits inside the bore of the inner timing cover.

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Kevin E Offline OP
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Originally Posted by DMadigan
Mine is from a later engine but I think they are the same.
The shaft after the cable hole is 0.282" diam. and 0.740" from the base of the cable square to the shoulder. The shoulder (yours is missing) is 0.437" diam., 0.148" long.. From the pump end of the shoulder to the base of the tang is 1.334" long and 0.250" diam. The sleeve that fits over the shaft is 0.341" O.D., 0.250" I.D., 1.666" long. The tang is 0.236" long and 0.088" thick,.

The O-ring in the adapter is 9/32" I.D. x 0.070" thick. Mine has only one O-ring.
The shoulder on the shaft captures the O-ring in the groove of the adapter. The O.D. of the shoulder fits inside the bore of the inner timing cover.

Thanks very much for the dimensions, they’re extremely helpful as it looks as though I’m going to have to make my own. There doesn’t appear to be any of these tachometer drives in stock anywhere to buy.

Mine only has the one O-Ring too. The photo shows two because I was showing the comparison with the 68-0588 oil pipe union O-Ring. You can see that they are very different in size, so how come they have the same part number in all the A65 parts books? What is the correct part number for the tachometer drive O-Ring?

Cheers,

Kev E

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I should have a spare if you wish to make a copy of it? It was bought as NOS, although I found it was a touch on the tight side which is why I haven’t used it, but we’re talking thou as it fits in fine, just doesn’t want to turn freely. But dimensionally it is otherwise correct.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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Kevin E Offline OP
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Originally Posted by Allan G
I should have a spare if you wish to make a copy of it? It was bought as NOS, although I found it was a touch on the tight side which is why I haven’t used it, but we’re talking thou as it fits in fine, just doesn’t want to turn freely. But dimensionally it is otherwise correct.

Hi Allan,

That would be great, thank you.

How do I go about getting hold of it from you?

What do you know about the larger diameter collar running in the hole in the aluminium inner timing cover? Is the hole supposed to serve as a bush? It seems a bit hit and miss to me, maybe there is something I’m not quite understanding about it, probably something simple? Why do you think that whoever butchered mine ground this collar off?

Also have you any idea about the O-Ring confusion?

Cheers,

Kev E


Moderated by  Allan G, Jon W. Whitley 

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