Hi all,
I've just done a full rebuild on my MC21 engine - dry clutch model. New crank, pistons, seals, gaskets. etc etc etc. I've used Three Bond 1207B throughout.
Before I put the motor back in the frame, I've done a leak down test.
Left cylinder slowly leaking from 5psi to about 2.5 after 5 minutes. Right cylinder a little faster.
I cannot find the link anywhere. All the gaskets, crank seals and crankcase seams are OK. No bubbles visible at all and I've triple checked with soapy water. I've checked the carb boot/manifold area very closely and that is also good. The oil pump is installed. The only thing i can think of is that there might be something I've not blanked off? An oil channel or similar?
Any help or suggestions would be really appreciated. After dropping nearly $2k into the motor I'm pretty keen to see that I don't ruin it on the first ride.....
Cheers, Simon.

You need to remove and blank off the oil pump when you do the leak down test. You can either buy or make a blanking plate. Use the oil pump o rings.

Are you testing each side at a time without blocking off the cylinder that isn't being pressurized? If not, try to block both sides while testing each cylinder and see if you're losing pressure. If your crank is new, you may still have a leak through the center from uneven mating surfaces on the engine case. I had this problem after my rebuild and had to open it back up, touched up the surface slightly with a grinding stone and applied just a touch more 1207b and then the motor held 8psi for 10 minutes solid with either cylinder open while doing the leak down test on the other end.

I've blanked off the oil pump and re tested. Definite improvement. After pressurizing to 5 psi, both cylinders are now only dropping 1 psi after a full 5 minutes. I'm almost inclined to say I'm happy with that. Would anyone recommend what would be an allowable drop?

I've tested by sealing both cylinders simultaneously, and also individually. The results seems to be the same either way.

Should be good to go.