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G'day Everyone!! Just opened a rebuild set from Motobins for my '81 R100RS. and everything as is as it should be but for one extra o-ring that I can't place. The rings I got were barrel base, top studs, red oil pump, thermostat oil filter (round and square section). The extra is a skinny ring, 30 mm in diameter. Any ideas? I'd hate to rebuild the sucker and realise that I'd neglected something vital. Just can't track it down?!! Cheers, Hools

Dipstick o-ring... or maybe the one between the flywheel and crankshaft? (30mm sounds small for that, though)

I got the same o ring Hools in a gasket set from Munich and I couldn't work out where it went.
I put it on the dipstick but it's not very good for that application, it squeezes out.
Dipstick washer needs to be a flat rubber 'washer'

Edit, 'Alpine Rally' at Yarringobilly, Queens Birthday long weekend in June.
You cummin?

G'day Jeff! No I won't make the Alpine, I'm still in the midst of a rebuild! Getting there mate! That O ring's got me stuffed. I'd thought about the dipstick but you're right, it's too big for that application. Maybe the spares box? Everything else is coming along but!! Do I heat the crank sprocket before knocking it on? Any other tips before the point of no return? New cam, sprockets, mains and big end bearings, oil pump,cam chain kit, gudgeons and small ends. Top end is beautiful, rings and bores good. Let's hope it goes together OK, later, Hools.

Yeah, bang the sprocket in a hot oven for 20 minutes or so, and then quickly get it to the engine and slip it on.
It may need some help with a block of wood and a hammer.
The sprocket will cool quickly when it contacts the crank so, nice and hot sprocket, everything ready to go, and don't !uck about.

Sounds like she'll be a brand new engine! Good luck.

I put the sprocket in the oven for a couple of hours, and one of those gelled freeze packs on the end of the crank.
Slid right on.

Thanks guys! Yes Dougie, like the ice pack routine. That's a goer for sure!! Wish me luck!!! Cheers, Hools.

hools, email and ask Motobins what it's for. They're good for replies etc. Then you can let us all know.

G'day Steve, yep it's the dip stick O-ring alright. Part No. 04471. Theres no way it's going on though!!! The metal ring is fine and I've never had a drama with them!!! Into the parts box with the sucker!!! Cheers, Hools.