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#710205 - 10/01/17 10:56 pm Layshaft end float  
Joined: Mar 2006
Posts: 1,132
htown Online content
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htown  Online Content
BritBike Forum member

Joined: Mar 2006
Posts: 1,132
Magnolia, TX
My 70 Thunderbolt refuses to shift into 3rd and 4th gear. I've turned the eccentric on the spring every way possible with no improvement. I've decided to start taking the gearbox apart to see if I can find the problem. I've looked at a lot of threads on the subject on gearbox issues and most mention checking the layshaft for end float. Can some one please explain how to do this? Also, how much of the primary needs to come apart to pull the gear cluster? I just recently redid the clutch and it isn't giving any problems.


1978 Bonneville T140E
1974 Trident
1970 BSA Thunderbolt
1971 Norton Commando
1972 Norton Commando
2-1974 Norton Commandos
2004 XL 1200R Sportster

Everything will be alright in the end. If its not alright, its not the end.
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#710211 - 10/02/17 12:02 am Re: Layshaft end float [Re: htown]  
Joined: Aug 2001
Posts: 3,291
gavin eisler Online content
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gavin eisler  Online Content
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Joined: Aug 2001
Posts: 3,291
argyll. scotland, uk
The clutch centre must be removed , so all the primary , sorry ,there is no other way..but read on , you might not need to.
Thats the bad news, the good news is that once the clutch is out of the way, the gearbox sprocket and sleeve gear can remain in situ, although it would be a good time to fit a new sprocket if needed. With the clutch centre removed, Undo the 5 x 5/16" nuts on the trap door and the whole lot will slide out, maybe need to waggle the cam plate a bit , it wont pull free if this is at one end of the travel, But hang fire, check some other stuff first before going radge on the primary and stripping the whole lot.

You will need a clock gauge , or lead wire and a micrometer to measure the end float.

Before you strip the primary , pull the inner case TS ( remember to pull the tach drive before destroying the inner and careful when pulling the inner , dont want to disturb the idler pinion and lose the valve timing,), make sure the Main shaft nut which secures the kickstart ratchet is tight, if this has backed off at all ,all sorts of weird things can happen. its also worth checking all the bits of the ratchet mech are present, if a thrust washer is missing from behind the spring, more weird stuff happens even if the nut was tight. DAMHIK. If you are lucky that might be all it is, no need to pull the primary for this .

Once the timing side inner case is removed, the lay shaft end is exposed, push pull axially on this , the optimum end float is 0.003 " , which is "just perceptible", A clock gauge can be mounted to measure the true axial end float, if not available you can use "leads £ and a micrometer, ( now its time to strip the primary) once stripped the lay shaft, thrust washers and gears plus a ring of soft solder or ideally lead wire can be assembled ( no need for the MS at this point, just layshaft with gears and leads on DS end mounted in trapdoor and refitted to case) , crush the solder by nipping up the trap door , strip then mic the solder/lead to get max end float , subtract 4 thou ( 4 because the casing surface will not be even) and thats the thrust washer you need. The thrust washer which sets end float is on the drive side of the Layshaft , this could be left out if using the lead to measure the gap. Compare the two.
Three sizes of thrust washer used to be available, usually the max of the three is needed,alternatively an over thick thrust washer can be lapped to size on a diamond sharpening stone , laborious , but easy low tech for the average joe.
Thats a lot of words,
More simply, the new thrust washer is the total endloat , minus 4 thou, to give about 3 thou when assembled.

Its sometimes easier just to fit a few shims you have lying around, you dont even need to measure anything for this, just trial and error untill "just perceptible", then make a thrust washer the thickness of the test shims.

Hot tip. A scrap piece of corrugated iron ( wriggly tin) makes a very useful parts holder, put MS and parts in one groove, lay shaft bits in the other.

refusing to shift into 3rd, hmm, suspect chipped selector pawls, new pawls and springs are not a budget breaker, of course it could be a few other things,
I have played with these boxes a fair bit, if you cant get 3rd it usually means that the change to 2nd was incomplete ,usually because the gear lever has not returned far enough to reset the selector pawls into the fresh plate windows, if the extreme ends of the spring adjustment bring no change then its probably something further in. The selector pawls are easy to check before pulling the box any further apart.

Badly gouged cam plates and mashed plate index plungers are common, bent selector fork maybe. You wont know until you pull the cluster
One useful feature of this box ( if std ratios are used ) is that the sliding gears are the same, which allows you to swap them from MS to LS , this puts the previously unworn side of the engagement dogs into play, you will need a press to do this. Somewhere out in the ether is a wonderfully detailed write up by Ed V on how to set up and shim an A65 box, but for most folk getting the LS end float set correctly will give very good results.

There are other things that help, polishing around the cam plate selector fork track cheeks gives a slicker shift, likewise polishing the inner faces of the cam plate fork tracks, the cam plate is a pretty crude stamping , its not hard to improve its function.

Last edited by gavin eisler; 10/02/17 12:32 am.

71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
#710227 - 10/02/17 2:54 am Re: Layshaft end float [Re: htown]  
Joined: Mar 2006
Posts: 1,132
htown Online content
BritBike Forum member
htown  Online Content
BritBike Forum member

Joined: Mar 2006
Posts: 1,132
Magnolia, TX
Thanks for the in-depth write up Gavin,
Got a dial indicator(clock) gauge. No tach drive she's a Thunderbolt.
Will post more in a few days when I can get her up on the lift.


1978 Bonneville T140E
1974 Trident
1970 BSA Thunderbolt
1971 Norton Commando
1972 Norton Commando
2-1974 Norton Commandos
2004 XL 1200R Sportster

Everything will be alright in the end. If its not alright, its not the end.
#710256 - 10/02/17 11:04 am Re: Layshaft end float [Re: htown]  
Joined: Aug 2001
Posts: 3,291
gavin eisler Online content
BritBike Forum member
gavin eisler  Online Content
BritBike Forum member

Joined: Aug 2001
Posts: 3,291
argyll. scotland, uk
nae bother, lets hope its the MS nut, the fact that its 3rd 4th thats missing suggests it might be that easy , 1st to 2nd happens on the LS


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
#710333 - 10/02/17 9:32 pm Re: Layshaft end float [Re: htown]  
Joined: Feb 2014
Posts: 827
C.B.S Offline
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C.B.S  Offline
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Lancaster, California
I have some shims here.... if you need a certain thickness call me as they can vary.

#710543 - 10/04/17 10:12 pm Re: Layshaft end float [Re: htown]  
Joined: Mar 2006
Posts: 1,132
htown Online content
BritBike Forum member
htown  Online Content
BritBike Forum member

Joined: Mar 2006
Posts: 1,132
Magnolia, TX
Tried hand shifting the cam plate and it absolutely won't go into 3rd or 4th. Feels like something is completely bound up.
Measured the float on the layshaft, .015, so that will need fixed.
Have everything stripped except removing the clutch hub. The threads on my puller are in bad shape and I don't want to damage the hub.
Have to wait a few days for a new one to come in the mail.


1978 Bonneville T140E
1974 Trident
1970 BSA Thunderbolt
1971 Norton Commando
1972 Norton Commando
2-1974 Norton Commandos
2004 XL 1200R Sportster

Everything will be alright in the end. If its not alright, its not the end.

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