crankshaft slack, then binds

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
Phil35p
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Joined: Sat Sep 14, 2019 4:44 pm
Location: Dorset UK

crankshaft slack, then binds

Post by Phil35p »

As a rookie restorer of a Model 8, I hope I'm at least half way up the steep learning curve! My current issue is that on assembling the crankcase halves there is about 0.5mm side to side play in the crankshaft when pulled from the stator side. I have put shims (up to 1mm) on the inside of the left hand side crank bearing but there is still play. If I put on the duplex drive cog and stator and tighten the stator nut, the crank seizes up!
Hopefully someone who has successfully rebuilt one of these can tell me my obvious mistake.
cbranni
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Re: crankshaft slack, then binds

Post by cbranni »

you don't need shims, the manual says a minimum of 20 thou end float without the rotor fitted, when you fit the rotor, sprocket, spacer and tighten the nut this will pull everything towards the drive side and it should revolve okay, you do have a manual?

Kind regards Colin
only dead fish go with the flow
Phil35p
Posts: 18
Joined: Sat Sep 14, 2019 4:44 pm
Location: Dorset UK

Re: crankshaft slack, then binds

Post by Phil35p »

Cheers for the notes Colin, good to know that there is supposed to be a bit of play in the crank before full assembly. The shaft rotates freely when the sprocket and rotor are put on with the retaining nut finger tight. As I tighten the nut the crank gets progressively harder to turn, so something must be binding. You mention a spacer, is there supposed to be one between the drive sprocket and the bearing?
I do have F Neill's manual and will sift through it again for any clues.
cbranni
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Re: crankshaft slack, then binds

Post by cbranni »

I am not sure about the spacer, sprocket first on the right way then the rotor, washer and nut, maybe the spacer was on another engine, old age memory is not a good thing.

Colin
only dead fish go with the flow
Phil35p
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Location: Dorset UK

Re: crankshaft slack, then binds

Post by Phil35p »

There's no spacer shown on the engine diagram, I have the useful A3 size from the spares shop, so that must have been on one of your many other rebuilds? :P
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clanger9
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Re: crankshaft slack, then binds

Post by clanger9 »

You must have endfloat on the crank when the crankcases are assembled. Crank endfloat is controlled by the ball bearing only (and is effectively zero).

Does this diagram help?
lightweight_main_bearings.png
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1989 Moto Morini Dart 350
1993 Ducati 750SS
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clanger9
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Re: crankshaft slack, then binds

Post by clanger9 »

It's worth noting that it is an unusual arrangement. The sprocket runs very close to the backplate (and is often fitted the wrong way round). Sometimes, the shims are necessary to get enough clearance.

Did you fit one roller bearing (nearest the crank web) and one ball bearing (at the drive sprocket end)?
Is the breather rotor missing?
IMG_2246.jpg
Full rebuild thread, complete with main bearing saga: http://www.jampot.com/forum/viewtopic.php?f=159&t=22646
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1989 Moto Morini Dart 350
1993 Ducati 750SS
Phil35p
Posts: 18
Joined: Sat Sep 14, 2019 4:44 pm
Location: Dorset UK

Re: crankshaft slack, then binds

Post by Phil35p »

UPDATE.
Thanks Paul for the very useful photo and diagram, the left hand side of the crank was assembled correctly, but after stripping down and a minor rebuild I have found the cause of the friction. Tightening up the rotor nut on the drive side pulls the crank across (as it should!), causing the timing gear drive cog on the rhs to foul the crankcase. Equally mysterious.
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clanger9
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Re: crankshaft slack, then binds

Post by clanger9 »

Hmm, that's a strange one! Can't think what could cause that.

Timing side bush not fully home?
1989 Moto Morini Dart 350
1993 Ducati 750SS
Phil35p
Posts: 18
Joined: Sat Sep 14, 2019 4:44 pm
Location: Dorset UK

Re: crankshaft slack, then binds

Post by Phil35p »

The timing side bush is flush with the crankcase, which I assume is correct. If I pull the crankshaft to the left, then the shoulder that is on the crank keeps the timing gear clear of the bush and crankcase. This shoulder is pulled just into the crankcase/bush when the rotor nut is tightened :headbang:
I mght have to give it the Basil Fawlty treatment and threaten to bash it with a branch if it doesn't behave!
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