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... 2. High Compression Ratio Tuning (9.3 to I), and using larger valves The standard engine is raised to 9-3 to I compression ratio by removing |" from the cylinder head face. The finished thickness after machining should be 73-575 mm. This is the absolute maximum to remove. Polish head, ports and manifolds as in Stage I. It will be noticed that a sharp edge is left on the combustion space profile at the end of the sparking plug hole. File this sharp edge back vertically until it is a minimum of ^" thick at the centre. File this only locally at the plug hole (approximately \" wide scoop) and blend into the combustion chamber shape with a radius each side. Do not file back too far. Check that the combustion space edge still extends safely over the gasket edge. Use larger inlet valves (Part No. M.G.862/460), having 36 mm. O.D. heads. In fitting these valves it is necessary to cut away part of the com bustion space wall to clear the valve head. To do this use a 38 mm. diameter (maximum) side and face cutter piloted off the valve guide. The cutter corner should have a I mm. radius. This may alternatively be done by careful grinding of the vertical wall until the valve head has a I mm. working clearance. Then cut or bore out the valve choke in a similar manner to 33 mm. diameter. Feather off by grinding any local ridge left in the valve ...
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