Calculating The Compression

To calculate the compression ratio you need to measure both the volume of the combustion chamber and the cylinder. I must have been very selective about the maths I remembered at school -how to measure volumes of cylinders was obviously deemed important enough not to forget!

I always take Pi to be 3.142, r is the radius and the height is the stroke of the motor. In simple terms, for example on a 350LC with a bore of 64mm, the radius is half the bore, which is 32. The stroke is 54mm.So our little sum is: (3.142 x 32 x 32) x 54 = 173.7cc.

This is the cylinder volume but if the cylinders have been rehored to say 1mm oversize that will alter the sum to (3.142 x 32.5 x 32.5) x 54 = 179.2cc

We now have to measure the other part of the ratio, the combustion chamber volume, this can't be calculated - it has to be measured. To do so you will need a burette or cc measuring tube.

Ideally, the engine will be on the bench not in the bike. Remove tie cylinder head, clean-up the head and barrel faces arid remove the spark plug. Position the piston at top dead centre (TDC) and smear some grease around the edges of the piston to seal it and wipe off the excess. Use a new lead O

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^Mechanics Workshop

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