Full member of the rt360 club now sheared flywheel key

[Andrew L]

Took the bike out for a test session today to get it ready for the first meeting of the season at dunkeld, whilst riding I backed off the throttle for a corner but when I went to open the throttle to get out of the corner the bike nearly threw me over the bars, some how I pulled the clutch in and coasted to a stop with the engine still running let the clutch out again again the bike went backwards I loaded it on the trailer and went home to investigate found it had sheared the flywheel key which caused it to run backwards does this happen often when riding I thought it only occurred when starting. Still waiting for Rexs to send my pulser less cdi box and stator plate out they have the box but not the plate so its still running the points system.Cheers Andrew

[pancho]

 That was a nasty trick! Must have been a bit loose I think.
cheers

[Andrew L]

Didn't think it was as it was still very tight when I removed it, but its possible, it wont be this time replaced key heading back out for another go.

[Rookie#1]

Geez I think I might buy another pre 75 big bore and race your class, I've gotta at least come 2nd last if someone is going backwards!!

[firko]

This is a common occurence on the RT1-3 with points ignition. The key is only there to locate the flywheel and shouldn't be the thing stopping the flywheel from rotating. Get yourself some valve grinding paste and smear it on the crank taper and lap the flywheel in until there are no high spots showing. Once it's lapped in properly it shouldn't move again.

[Montynut]

Didn't think it was as it was still very tight when I removed it, but its possible, it wont be this time replaced key heading back out for another go.

did you lap the flywheel in ? As we know the key does not actually hold the flywheel in position it is the taper that does the work. The key just makes it easy to install in the correct place. Check the key is not oversize. The key should not bottom in the flywheel keyway.

Lapping the flywheel tells you instantly if the taper is doing its thing

I realise you probably know all this but never hurts to state it again

You beat me too it Firko

[mick25]

I had a suzuki ER 125 BACK in my younger days , the magnets came off in the fly wheel.
so I had a yamaha fly wheel laying around and it fitted , but when I tryed to kick it over it keep kicking back and firing on the kick back.
I got it going it didnt run well but when I put it in gear she had reverse I was 12 or so at the time so it was cool having reverse gear only  

[mustanggrahame]

Lapping tapers is not a good engineering practice. The surfaces should be ground. Lapping might make it look shiny and pretty but it tends to give the taper a barrel shape.
Cheers, Grahame

[Andrew L]

Thanks everyone for the advice I did lap it in ages ago but I will give it another go although the original flywheel will be shelved when my CDI stuff arrives and I fit the dt400 one in its place, will have to lap it in as well. Took the bike out for another run and its been fine since was trying out the jetting in the 36mm mikuni I fitted seems to be a good improvement over the standard carb but only after a lot of rooting around getting the correct jet sizes.

[Lozza]

Lapping tapers is not a good engineering practice. The surfaces should be ground. Lapping might make it look shiny and pretty but it tends to give the taper a barrel shape.
Cheers, Grahame

It's poor technique thats all no squeezing oranges but go round and round one way then the other. The high spots go dull not shine.

[OverTheHill]

Firko's advice is right of course & [is the right way to fix it] but it's super important to 'do it up bloody tight' & use a solid holding tool so as to eliminate any springyness --solid tool resting on solid footrest or something. Half inch breaker bar & swing on it--you won't pull the end off the crank. Never sheared a key yet but every one of my flywheels & cranks are damaged from previous owner mistakes. Have got away with running 'no' key when experimenting with ignition timing but like i say, she was done up fkn tight [thats tighter than tight]. Just remembered my old TT250 is 'minus the key' as ended up with about half a key extra advance & it really is more lively for a standard motor, but, done up super tight. Long story 'longer sorry' but when my yz490k arrived from California years back it came with sheared off flywheel key, un-crated it all excited, gave it a kick & bloody near broke my leg plus took a tooth off the idler gear & didn't improve the other two gears. Guy as it tuned out had changed the stator plate prior to selling & done it up with a compressor running out of air, said it started & ran so must have stuffed the key on the run down to stop unbeknown to him [or me][fairly small narrow key on 490 by memory & wider on 360. So, bottom line 'Tight' Cheers. ps, might be a pic of mine somewhere up here showing the tool engaged in flywheel from a while back. Will post it here if i can find it.

[OverTheHill]

[OverTheHill]

be careful 'not' to get the engaging pins on the arms caught in the coils, i have fairly short pins on this one but they come close. This flywheel is the 'odd ball' advance/retard one--good though for starting the old beasties.

[pancho]

 What I have done to my RT1 is

 1) got a piece of bright mild steel and had it machined to the same taper as the crankshaft.

 2) Lap the flywheel onto the taper. This will remove any high spots 'galling' of the flywheel while not altering the contour of the surface.

 3) Careully remove any ridges or high spots on the 'shaft taper with a warding file.

 4) Lap the flywheel onto the 'shaft with fine grade water mixed lapping compound.

 5) When you see, after cleaning up with water, good surface contact on the shaft, make a new key from ALUMINIUM. If the key does shear again due to a kick back, minimal damage will occur to the taper surfaces.
    I copied that idea off Briggs and Stratton.

 6) Coat the shaft with loctite bearing mount and tighten the nut F'ing tight using 'OverThehills' illustration.

 Works for me so far! cheers pancho.

[Andrew L]

Good advice pancho & overthehill cant be certain it was super tight in the first place but its to use your words shes done up fkn tight [thats tighter than tight].
Gee you've got to be quick typing, my questions about where to get new keys is answered before I finish was going to ask this one

I had a spare key but now need some replacements for just in case where is a good place to source them are they still available from Yamaha or is there another source.

but pancho beat me to it the alloy key seems a good idea to me as my crank has seen some damage from a previous time and my mates one is the same. cheers Andrew

ps Brendan it wouldn't matter if I was going forwards I'd still be near the tail end, you should have a big bore bike somewhere buried in that vast collection of yours.