As supplied the engine is fitted with two compression plates below the cylinder, and the vertical shaft is assembled with a thicker coupling at the top than at the bottom. The thickness (or depth) of the centre circular section of the top coupling is .187-in. between the tongues as against .125-in. in the case of the bottom one.

Should one or both of the compression plates be removed and the cylinder assembled without them, it is necessary to allow for the necessary vertical freedom of the shaft by substitutung for the top coupling a coupling of similar thickness to the bottom one. It is of the utmost importance for this change to be made, otherwise the two vertical bevels would be forced apart and consequently damaged on tightening down the cylinder head bolts. If the compression ratio is raised therefore, do not in any circum¬stances omit to change the coupling.

Compression ratios obtainable are :-

Two compression plates fitted 7.6 to 1 approx.

One compression plate fitted 7 95 to 1 approx.

No compression plates fitted 8.4 to 1 approx.