Another question on re-boring Mk engines

[Dougiethenoo]

I have read that piston sizes up to 62mm will work in the Mk III (S) engine, a tad over +40 thousandths.
However I have also read cautionary notes about barrel cracking if +1mm is taken out of the bore.
I have located some +40 thou pistons and rings for a Dragonfly but would appreciate experienced recommendations on whether this will work on a Mk III Sport?
Are the wrist pins the same?
Is the Dragonfly piston crown shape and height compatible?

Many thanks in advance for any advice.

[eddie]

Original Dragonfly pistons were split skirt and when worked hard, the crowns tended to collapse into the split under the ring land, resulting in a wavy form to the ring grooves. This usually resulted in the rings jamming which lead to excessive oil consumption. The usual cure was to revert to using Mark type pistons in the Dragonfly. The crown shape and height were the same for both types - as were the wrist pin diameters. Later repro pistons by JP or Peter Hepworth were solid skirt and based on the Mark type.

  Eddie.

[Dougiethenoo]

Original Dragonfly pistons were split skirt and when worked hard, the crowns tended to collapse into the split under the ring land, resulting in a wavy form to the ring grooves. This usually resulted in the rings jamming which lead to excessive oil consumption. The usual cure was to revert to using Mark type pistons in the Dragonfly. The crown shape and height were the same for both types - as were the wrist pin diameters. Later repro pistons by JP or Peter Hepworth were solid skirt and based on the Mark type.

  Eddie.

Thanks.
FW Thornton offer solid skirt Dragonfly pistons at +40 thou.
Does anyone know if it is un-safe to overbore the MkIII barrel this far?

[eddie]

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.

[Dougiethenoo]

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.