SF2 crankshaft nut

Lyndoneil

Lyndoneil

Full member
Location
Towcester uk
Have just replaced the fiction plates on my clutch, push rods and balls (sounds painful), cant find a torque number for doing up the crankshaft nut on the primary drive side of the engine, anyone have the torque spec?
 
up till it snaps then, back half a turn.

Unless you have a proper crankshaft locking tool you will never get that nut tight enough, use a 3/4" drive socket and tee bar and a length of scaffold pole, the threads internally and externally should be cleaned and wire brushed with a spirit (meths, brake cleaner, thinners etc) a new locktab is a must and threadlock as well, no idea of the torque but in aprenticeship terms it is officialy "FT" has always worked for me.
CLEM
 
Official figure is 10 kg/m.
In practical terms, bloody tight. Use Loctite on the thread and of course the washer which you further block the nut with.

Paul
 
Ah! Got an Aldi pneumatic gun, but only 1/2 drive sockets, digital torque adapter and the other half to stand on the back brake, will have to see what torque I can get up too, have new tab washer and loctite though.
 
I use a half inch drive impact socket and don't need to use the brake or anything, going against compression the crank/sprocket rotates slightly but you can see the nut tightening against it, as has been said, let it really rip into it. The twin leader rear brake is pretty hopeless backwards regardless. Do not be tempted to try putting a wedge between the sprockets.
 
Lyndoneil said:
Ah! Got an Aldi pneumatic gun, but only 1/2 drive sockets, digital torque adapter and the other half to stand on the back brake, will have to see what torque I can get up too, have new tab washer and loctite though.
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I bent a brake stay that way.

Paul
 
Ok, will give the pneumatic impact driver a go, don't like the wedge idea between the sprockets and I wouldn't like to put that sort of force through the oil pump drive.
Thanks for the tips
 
Tippie said:
The twin leader rear brake is pretty hopeless backwards regardless.
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So put the transmission into reverse. 🙄

Seriously though, using the rear brake to lock the crankshaft is a dumb idea. Even in the unlikely scenario where you could get the brake to hold, it puts extreme loads through the entire drive train.

I've seen someone put a lump of wood through the rear wheel because the drum brake wouldn't hold when trying to undo a nut on the crank. It was a Yamaha 125 2-stroke twin from around 1970, but it was a long time ago and I forget the reason why he wanted the crank nut off. The bike's owner was swinging on the crankshaft nut with a long pipe over the socket handle while his mate (me) sat on the bike to hold it down. I did express reservations about the process, but he thought it would be OK (it was his bike so I wasn't going to tell him no). I was expecting a bang to come from somewhere within the drive train but fortunately the clutch started to slip before any damage was done.
 
I used to quite happily put a piece of wood in the rear wheel before I had a rattle gun and when my Laverda had cast wheels.
It may well overload the drive train but I'm not certain that the stress involved is vastly superior to that of coming down through the box at high revs?

Paul
 
Big diff using a rattle gun than a mighty great bar and socket and brute force. If compression is enough to allow the rattle gun to do its thing then maybe locking the back wheel would work with one too - but I wouldn't do it. I've done mine with the steel tab between the sprockets several times and it worked fine - until it didn't ... and broke a tooth off the clutch sprocket. Now the proud owner of a pneumatic rattle gun as recommended by Piet. Not yet christened.
 
piranha-bro2 said:
I've done mine with the steel tab between the sprockets several times and it worked fine - until it didn't ... and broke a tooth off the clutch sprocket.
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have seen around at least 1/2 dozen badly bent gearbox Mainshafts at the clutch end over the years I have been rebuilding these engines, I always check the mainshaft in my lathe to make certain they are straight before rebuilding an engine and if they are bent I straighten them

I have always presumed the bent Mainshaft at the Clutch end was caused by that suggestion to insert a piece of steel between the sprockets to lock the primary drive when doing up the Crankshaft primary drive sprocket
The problem with that piece of advice in the "Green Book" is that they do not provide a recommended length so if an owner goes and cuts a piece of steel that is just long enough to jam in between the teeth and then applies force to the crank and the end result is the piece of steel is forced between the two sprockets you now have a bent gearbox Mainshaft

If the Green book had stated a recommended length for that piece of steel that avoids the piece of steel actually passing between the two sprocket centers then maybe it would have been a half reasonable piece of advice
Your only issue then is that you can break off a few teeth from the cast iron clutch sprocket
 
redax5 said:
have seen around at least 1/2 dozen badly bent gearbox Mainshafts at the clutch end over the years I have been rebuilding these engines, I always check the mainshaft in my lathe to make certain they are straight before rebuilding an engine and if they are bent I straighten them

I have always presumed the bent Mainshaft at the Clutch end was caused by that suggestion to insert a piece of steel between the sprockets to lock the primary drive when doing up the Crankshaft primary drive sprocket
The problem with that piece of advice in the "Green Book" is that they do not provide a recommended length so if an owner goes and cuts a piece of steel that is just long enough to jam in between the teeth and then applies force to the crank and the end result is the piece of steel is forced between the two sprockets you now have a bent gearbox Mainshaft

If the Green book had stated a recommended length for that piece of steel that avoids the piece of steel actually passing between the two sprocket centers then maybe it would have been a half reasonable piece of advice
Your only issue then is that you can break off a few teeth from the cast iron clutch sprocket
Click to expand...
Apparently, the new edition of the Green Book is corrected and doesn't advise that technique any more.

Paul
 
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