Hi guys,

I've just done a top end rebuild on my mc21 and am unsure about how to test the centre crank seal.

Ideally i'd have tested the crank seals prior to doing the top ends, but it didn't work out that way, and now i'd like to establish some confidence in them before sticking the motor back in the frame and reassembling the bike.

It ran ok before the rebuild, was just low on compression. I've used the twostrokeracelabs leakdown test kit and the left/right crank seals seem ok (0.5psi loss over 6 minutes after charging to 7psi.

With the boost bottle connected to the carb insulator, am i not pressurising both sides of the crank case at once? Do i need to remove the boost bottle and blank off the hole for it in the insulator and then repeat the test on both sides of the crank to test the centre seal?

What type of seal is the centre seal, and should it seal perfectly?

Any help from the guys who've been three and done it before would be appreciated. This is the first leak down I've done on a twin and I don't understand the procedure 100%.

Thanks,
Scott