Primary Chain Tensioner - A10 Pre Unit

[Brucie64]

Could someone please explain how I should correctly tension or slacken my primary chain as mine doesn't seem to adjust!

I have just changed my engine sprocket and also fitted a new primary chain, unfortuneatly the chain seems a wee bit tight (less than the 1/2" play stipulated), I'm afraid that if I run the bike now it might damage some bearings and/or oil seals etc.

The Haynes manual says just adjust the primary chain using the gearbox tensioner.....what it doesn't tell me is how to use the gearbox tensioner.

The attached photo shows my tensioner and the method I used....i.e. slacken nut 2, then adjust nut 1 to hopefully reduce the tension by moving the gearbox........however nothing seems to happen, the chain stays the same tension whichever way I adjust nut No1.

I am doing somethng wrong?

[BSA_54A10]

Two things
1) you should really slacken the final drive chain first as if it is tight the box can not go foreward.
2) there is a second mounting bolt underneath the gear box which also needs to be loosened.
3 If the bike has run for a substantial amount of time with loose mounting bolt (s) then the cases will get a grove chaffed into them which can effectively lock the box in position . In this case you may need to loosen off the mounting plate bolts a bit and wiggle the box to a new spot. In bad cases you might need to slip a washer in there or even get the cases welded and filed ( Devcon works wonders there)

[Brucie64]

Ah, good point about slackening off the final chain, never thought of that. Does it say in any maintenance literature anywhere that we need to slcken off a bolt underneath the gearbox also... I would have never found that without your help. 

[RichardL]

On that bottom bolt, only just "untight," not loose, if you know what I mean. You don't want the gearbox to twist in its mounts when you do the adjustment. There should still be a little friction left at the bottom for stability, otherwise, when you tighten the mounts the gearbox will twist to a new position.

Richard L.

[Brucie64]

All done, many thanks guys for your help.