It is assumed that the New Cylinders are Peter Gillespie’s Replicas, bored to 68.0 mm., and that the New Pistons are Industrial H***a type GX160 or GX200.
Axial length of New Cylinder: 108 mm.
Axial length of Original Piston: 44.5 mm.
Compression height: 22 mm.
Skirt length: 22.5 mm.
Top of piston rises to 7 mm. below top of cylinder.
Skirt of piston descends to 2.5 mm. above bottom of cylinder.
Two compression rings near top of piston. One oil-control ring near bottom.
The new piston-types are effectively identical to each other above the gudgeon pins, with a compression height of 24 mm.
Two compression rings near top of piston. One oil-control ring above gudgeon pin.
The effect of the 2 mm. greater compression height is partially nullified by the broader recesses in their crowns, and is not enough to affect the valves.
Below the gudgeons, the skirts vary considerably (see diagram).
In fact, the GX200 could probably be used as-is: its skirt should never protrude beyond the cylinder-bottom.
The GX160 would need its skirt shortening by a few millimetres.
The weights of the new pistons aren’t that much different to the originals.
The biggest difference by far to the originals lies in their much greater gudgeon pin diameters.
Dealing with this is a matter for future study and advice.
New piston prices: there are plenty to be found on eBay or similar, for about £14 each.
Grand Overall Conclusion: both types of replacement piston are nearly-equally suitable, but both have the serious snag of the ‘pins.
Written by Paul Button for the Association of ABC enthusiasts.