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Author Topic: 90+ rev counter drive  (Read 1160 times)

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Offline tck

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90+ rev counter drive
« on: 07 Feb 2021 at 11:14 »
Another question from my MK4/90+ rebuild I am afraid. It is fitted with an AC rev counter drive on a plate in line with the standard platform MM2 mag. There was no drive to the rev counter tab when I stripped the engine, so my guess is it was intended to be  driven by a special longer nut with a slotted head to replace the standard 170104 nut on the mag shaft which Joe Lucas seems to have made a 3/8 BSF thread . Did this nut need some sort of pin to stop any tendency to undo? or was there some sort of cage to drive the counter from below the nut?
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Offline Neville Heath

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Re: 90+ rev counter drive
« Reply #1 on: 11 Feb 2021 at 11:12 »
This is the set up I have used on my Pluses for over 40 years with success.  The tab washer is not 100% but it does add the taper grip of the gear wheel to the force needed to undo the nut. I have never had one come undone. 
Neville



Attachments converted to linked images - Dave, 14Feb2021
« Last Edit: 13 Feb 2021 at 20:22 by Dave »
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Offline veikko1

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Re: 90+ rev counter drive
« Reply #2 on: 11 Feb 2021 at 13:07 »
Hello
I am interesting this also. I have only straight plate were tacho"gearbox" coming. Would you give some measures from yours plate?
Regards Veikko
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Offline Neville Heath

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Re: 90+ rev counter drive
« Reply #3 on: 13 Feb 2021 at 11:13 »
These plates were cast and sold by the club many years ago. You can use your flat plate. You must ensure the drive lines up with the mag shaft. If you have a lathe make up a round bar with a 5/16" internal thread bore, screw this on the mag and measure off that. You can then file flats on that and use it to make the special nut with length cut to suit. You can measure with the timing cover on and the plate removed. There is a bit of tolerance in how far the drive plate enters the slot. Once you get it all lined up put a couple of locating pins in the plate/cover joint.
Neville
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Offline tck

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Re: 90+ rev counter drive
« Reply #4 on: 13 Feb 2021 at 15:38 »
My measurement shows about 3/4" from the mag gear face to root of tab on rev counter (plasticene measurement) I will fine tune by turning down drive nut  length to suit, I can see how the tab in the above post works as there are two tapped holes in the face of the mag  gear face my .600" AF hex bar is on order.
Thanks to Neville for his help
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Offline veikko1

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Re: 90+ rev counter drive
« Reply #5 on: 13 Feb 2021 at 18:26 »
Many thanks to both, Neville and Tck. It looks clear now.(I hope so).
Regards Veikko
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