750 & 850 flywheels.

Matchless said:

Does any one have a couple of flywheels lying around to measure & weigh? I need to know the difference between the two.
Thanks.

Martyn.

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Matchless said:

Thanks Bert,
That helps. Could you let me know the width (thickness) across the widest section please.

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Matchless said:

Hi Bert,
That's just what I needed to know. Thanks very much.

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Matchless said:

The gaol is to build up a standard 750 crank. I have in my spares a full three piece crank, but all parts from different bikes. I was trying to figure out which flywheel I have. Once I have the parts assembled & checked for alignment it will be taken for dynamic balancing.

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RoadScholar said:

OK thanks, gives me a better understanding.

Before you get the balancing done I suggest that you check the crank for true. Comnoz (Jim Comstock) posted a very easy/simple way to check, you should be able to get the thread up with a search.

Bolt up the crank with proper torque, don't need to bend the metal lock strips. Install and torque the rods, with the crank/rods on the bench simulating the position they would be in at TDC measure and record the distance from the outer edges of the small rod ends. Repeat this, but at a simulated BDC with the small ends pointed in the same direction as when you took the TDC measurement, ideally there should be no difference. IIRC, and do check, the tolerance shouldn't exceed .005. If you get more, which, depending on the difference, it could be a judgement call or you can swap the big end cheeks and measure again and go from there.

Not meaning to interrupt the conversation, but a diagram or picture would be helpful. I could not find Comnoz's post on this , perhaps someone has a link to it.

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myjota said:

Back when I was a poor apprentice and my daily ride was a 750 Commando I had to use whatever was cheapest to effect repairs. To this end I have used 850 crank to replace a snapped 750 crank and can confirm that it made the engine feel a bit lazier but otherwise worked ok - no rebalancing or anything. Have also used a 750 crank in an 850 (in later life) and quite honestly couldn’t tell the difference no more or less vibration maybe gave the engine more of that “750” feel, a bit freer revving. Road going Commandos are not that fussed about crank balance as long as it’s thereabouts.

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RoadScholar said:

BERT said:
Not meaning to interrupt the conversation, but a diagram or picture would be helpful. I could not find Comnoz's post on this , perhaps someone has a link to it.

Visualize it this way: crank, rods and main bearing are in the engine cases, pull the rods up as high as they will travel (equals TDC), measure the distance between the outside of the small ends. Now rotate the crank so that the rods are as deep as they go (equals BDC), take the same measurement; you are looking for less than .005 difference, obviously .00000 is best.

It doesn't matter much if the crank orientation is off a few degrees, if the big end journals are not in the same plane you'll see it with your measurements. If the measurements indicate more than .005 difference try again with the flywheel in the only other position possible, do mark the flywheel which will insure that you don't end up in endless "do loops"

Best.

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