Flywheel weights (Monza Breakdown)

[Boreham]

On the way back from the Leicester rally yesterday, gave the scoot the beans off a roundabout (9K in 2nd) and pop!, cruised to a stop, quick check and no spark, got the bike home, did all the usual checks and worked backwards....... end result a sheared keyway and a spun flywheel.
A scary few minutes as this is a new built ultimate spec Monza 240 (done 250 miles), on close inspection the angels must have been looking down as no crank damage, tiny screwdriver and a lot of patience and managed to get the sheared part out of the crank.
Some very small scratches which should lap out, so to my question, i was using the AF mid weight (1920gms) flywheel, is this the usual weight people are using on RB's and TS1's etc.., is this too heavy for the spec of the engine or have i just been unlucky.
Before i order a replacement, any comments or recommendations welcome.

Spec is Pro-porting Monza 240, 62x110 PP crank, BGM stator (i tried the AF lightweight whilst building but would not work with the BGM) 29BHP.
Cheers

[garry inglis]

The weight sounds right and if it's ticking over ok it was probably a crap woodruff key it just happened to me on my TS230 I got some off chiggy at lambretta upgrade and all ok now

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[Boreham]

Cheers Gary, yes all fine until yesterday, can't remember where the key came from, ordered a couple of hardened ones from a gear specialist so I know these are right (no doubt some Far East ones made of cheese!)

[mick1]

The keys are only a guide and don't take any of the force.........that's what the taper's for.

Did you torque the flywheel nut ? That's what's stopping the flywheel turning , lapping the crank to the flywheel is also a good idea (which you've done).

I'm not sure if they recommend a higher torque for the higher HP motors ? always a danger of shearing the crank thread if you go too far.....

[Boreham]

Fully torqued, used a nut gun to undo! Which was more baffling as to why it gave in.
I have had this happen on a small frame once and also a high BHP screamer, went to a varitronic 1.2kg flywheel and no problem since (may again not be connected) hence my thoughts on flywheel weights on tuned motors.

[Muppet]

lap ya flywheel onto the end of the crank with fine valve lapping paste

[holty]

im sure someone else will know better than me, but i would suggest a lighter flywheel, a big heavy flywheel like that is going to put some serious strain on your crankshaft, i would get a lighter one or get it in a lathe and get the weight reduced.i would lap the flywheel onto the crank until its a perfect fit, as said before the key is only to line it up, its the clamping force between the tapers appiied by the nut that holds it together, also check the inside of the flywheel hasnt cracked where the key goes, has it been welded for extra strength.many years ago i had a similar problem with a mini flywheel coming off its taper, that was cured by lightening it, and heating it up around the taper before fitting so it shrunk on as it cooled, might be worth a try if all else fails.