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.

[Stephen Foster]

Ive always been a bit unsure of the correct amount of free play in the primary chain on a swinging arm A10 ?
Should this be the total up and down movement or 1/2"from the straight chain top ..cant explain but hope Someone understands ?

[Colsbeeza]

Hi Bruce, This is well covered in the Service Sheets. Cannot remember which number but here is a copy (courtesy of Joules if I remember correctly). I have found that the chain slack noted is correct - 1-1/4" Rear, 1/2" primary. 1-1/2" rear is also OK. The most important thing to note is that the gearbox adjuster ( bit outlined in red ) tensions the gearbox in the forward position to avoid the rear chain pulling on the gearbox and overtightening the primary. I burnt my fingers on an overheated primary doing this wrong when I was a youngster on my A7.
Col

[berger]

the chain also tightens up when it gets hot and burns your fingers doesn't it col so get the tightest spot adjusted correctly 

[Stephen Foster]

Thank You ,
Im still uncertain if the 1/2" is measured 1/4" from level & 1/4" below giving TOTAL movement of 1/2" ?
Ive always been unsure about this erring on too slack rather than overtight ?

[RDfella]

I've yet to see any sprocket that runs anywhere near true, so best bet is to turn engine until the tightest point of chain is found, then adjust to total movement up & down of 1/2"or a fraction less.

[Stephen Foster]

Thank You .
Ive adjusted the chain this way ..I suppose it is obvious but I had never been certain how to measure the tension correctly .

[Swarfcut]

 The real primary chain tensioner is only found on the plunger engine. The item referred to here is better described as an adjustable gearbox anchor, and as in Col's note above is set to prevent the gearbox creeping backwards and tightening the primary chain under the influence of the final drive chain, Newton's Third Law an' all.

 Final drive chain best set with rider seated, feet off ground to mimic usual loaded ride height as chain tension varies with suspension travel. Too tight saps power, too loose gives lash on over run, so aim for something sensible in between rather than trying for ultimate workshop spec.

Swarfy. .

[Stephen Foster]

Many Thhanks Swarfy .
I struggled on My Own yesterday with the final drive chain ..on centre stand I left around 1 3/4" play ..not easy to tell through the rubber bung in the fully enclosed case ..gave it a good dowsing of chain lubricant also .

I dont know what the tension will be like off the stand with riders weight yet ?

[Colsbeeza]

Stephen, I have the same problem, as I usually work alone, so I set the rear chain at 1-1/2". Sofar OK.  Col