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Classically the clearance closes as the motor heats.
I think you'd be safe to set the clearance to 0.5 mm to warm the motor.
With an overhead cam motor, you cannot easily stop every now and again as it warms to check that you still have a clearance, because you will get oil in your face a-plenty as the cam slings it into your visage. With an overhead valve motor, there is very little oil by comparison, and the valve covers can be left off, and checked as the motor heats.
You do no harm until the clearance closes completely. Then the vavle gets a hard time.
A little over-gapping of clearances really does not hurt anything. I was always very wary if valve gear ran very quiet anyway. My old Russky motorcycle dealer always said "a chatty tappet is a happy tappet". I guess years of experience taught him to leave the clearances a little wide.
Let them chatter a bit, within reason. It does no harm, particularly for a low-revving motor. Russkies did not rev much.
Is the MD7 an overhead cam motor? I know not. Personally, the OHC is a performance concept. It isn't needed on a sailboat motor. For me, OHV rules.
Set the clearances to 0.5 mm, heat the motor, and re-set to 0.3 mm. You will be OK. Once set up, and with regular oil changes, over time the clearances really are very slow to change.
Last edited by Rockter; 01-17-2009 at 06:30 PM.