Adjusting the primary

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Jul 18, 2004
Hi Folks,

Another beautiful riding day spent in the basement polishing, painting and fettling.

I am a not sure how to measure the 3/8" freeplay on the primary chain. The shop manual and Clymer say "3/8" movement"
To confuse me, the INOA Tech Digest says:
"freeplay is when there is a slight sag in .... the chain.... not when you can push the chain though a 3/8 arc." This is huge disparity in measurement.
Any thoughts?

While we are on the subject - how does one torque the top transmission nut? - the gearbox blocks the way for a socket! I can barely get a wrench on it.?! :shock:

Yes what they mean is very light tension i.e the weight of the chain pushing down and the same pushing up equals the total freeplay, not trying to stretch a bowstring tight chain to get the measurement.
One important thing to note when adjusting the primary which the factory manual does mention but does not make a big point of is the direction you adjust the adjuster. You should always adjust so the gearbox is being pushed towards the engine by the adjuster and not the gearbox being pulled back by the adjuster. Hope that makes sense but the reason for it is to take up the slack between gearbox shell bolt and adjuster in the right direction, If you do it the wrong way no matter how tight you do up the top bolt the first time you go for a ride the tug of war between the primary chain and rear chain will be won by the rear chain and will pull the gearbox back till all that slacks gone and will result in a loose rear chain and a primary chain like a guitar string.
As for access to the top bolt a thin wall socket should fit ok.
Enjoy your riding

When adjusting the primary chain, I find the widest span in the chain and using a small steel scale, I measure the TOTAL up and down movement of the chain at this point. However, my MK III has no primary chain adjustment. Instead, chain tension is automatically maintained with two quasi-hydraulic adjusters. Don’t you, too, have a MK III.???



It would seem that I could just measure the amount of sag ~ 3/16" - as opposed to pushing and pulling on the chain - as the amount of sag is really what we are after - correct? I find it harder to "read" the primary compared to the final.

Snakehips, there is no way I can get a socket onto that nut!- the corners of the flats are on the same plane as the top of the gearbox... Thanks for the tip on the adjuster nuts, I will have to move one to the front as I had both on the backside... arrgh! Now off to the store to get a 15mm socket.

Jason, I have a May 1970 Roadster that I am converting to a Fastback - so it is really pre MarkII.


I apologize, I somehow had the impression that you were working on a '75 Commando. For adjusting the primary chain on a non '75 Commando, I recommend 3/8 TOTAL movement at mid-point in the chain.

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