The early barrels as fitted to the Mk3 and Mk3 Sports were of a lighter construction than the ones fitted to Mk4 and 5 machines, and were more prone to cracking just above the base flange when bored out above +.030" (the largest pistons Mr Douglas listed). The early barrels can be identified by the base flange, which is relieved between the base studs. Later barrels have thicker base flanges without any relief between the studs and the cylinder wall is also of a thicker section.
Eddie.
Thanks. I have the earlier version, they are bored out to +0.030", and measure close to 61.5mm at the smallest diameter.
I am measuring +5 thousandths out of round at the base and +9 thousandths tapered out at the top of the bore, before the evident lip, in the thrust plane (top/bottom). Otherwise bores and pistons appear to be in good shape but there's plenty of evidence of oil burning. Coked cylinder head and oil rather black.
+40thou piston sets are available but from a cost and durability perspective a Nikasil coating, with a hone to suit existing pistons (and new rings) seems to be the best solution.
Has anyone experience of Nickel Silicon carbide coating on these cast iron bores? I know it works fantastically well on aluminium engines, high performance 2-strokes etc - but it is also offered for restoration of cast iron bores.