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Tach Drive Fork

brian gains

Well Known and Active Website User
VOC Member
What's the opinion on having a fixed driving fork with no shear key or means of slipping if the cable seized for instance. Please note the set up I have is not the standard ET224 design of fork.
 

erik

Well Known and Active Website User
VOC Member
Mostly in the cable itself there is a gap machined in the lower spade which inserts into the gearbox.This will shear f a Problem occurs.Erik
 

Gordon Ryley

Active Website User
VOC Member
Hi Brian, I have the the same arrangment I do worry what if, the Taco reading can
be a little slow, with not doing a lot of running it should be ok But I do leave the
drive bit out sometimes. At least there is plenty of oil around the drive.
 

brian gains

Well Known and Active Website User
VOC Member
"reading slow" would only happen momentarily before complete failure of one of the tach' drive components (or dropping engine revs).
 

davidd

Well Known and Active Website User
VOC Member
The chronometric tach reads about 300 rpm slow if you are racing and happen to glance at the tach while it is catching up. Coburn Benson had this problem when racing the Flash. He purchased a Smith ATC tach to solve the problem as the chronometric was shifting at 8300 instead of the planned 8000 as an average.

David
 

brian gains

Well Known and Active Website User
VOC Member
just got it buttoned up , but very straight forward.
bolted bridge plate on spacers across steel replacement pinion ET48, ( drives EI) from the centre of the plate is a machined drive fork for the tacho drive,
in my case the tacho was mot working so chased it back and found the drive fork , which initially appeared to just have had its spigot press fitted into the bridging plate was no longer an interferance fit and was only held in place by the male drive on the tacho gear and loosely supported by the plate. I try not to think what mechanical and physical mayhem would have been unleashed had it gone walkies in the timing chest.
On inspection it was meant to have been held in place by a spot weld that did not have any penetration and failed. Fingers X'd my weld is a little more robust.
 

Bill Cannon

Well Known and Active Website User
VOC Member
Hi Brian, it has just occurred to me that you are probably the gent who bought a lightning tacho drive from me on Ebay. But it turned out that it had been modified and was no longer suitable.
I'm glad you managed to find another one.

Cheers Bill
 

brian gains

Well Known and Active Website User
VOC Member
no, not me Bill. On the Shadow it still runs a standard drive, it is the ATD drive pinion that has been modified, No magneto and EI does not require mechanical advance / retard as handled by EI map/circuit board.
The EI is driven by a straight forward pinion, I've not pulled the EI trigger unit so can't comment on drive shaft / taper etc but am intrigued to know how it is supported. The EI trigger unit must have a support bearing and I wonder if the RH case was machined to accept a bearing. Although I find it hard to believe that the case would have been so radically re machined, perhaps there are two bearings contained within the trigger unit?.
Considering the above, what I have picked up anecdotally is the motor was used in a grass track machine, was seen on a bench in a totally beat condition, also the motor has been chopped either when it was grass tracked or rebuilt and I guess when a motor is that knackered boring out a case to take a support bearing for the ignition drive is not that radical a consideration.
 

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