I recently fitted a 'split skirt' type piston, a NOS item manufactured originally by Heplex, to an M20.
These pistons run a far tighter skirt clearance than the .004"~.006" of the standard 'solid skirt ' type.
(.0007"~.0027").
Implications of running a tighter tolerance are obvious from the point of view of reduced mechanical noise and potential reductions in oil consumption and piston 'blow by', at least during the 'warm up' period. I'm guessing with both types ultimate piston clearance at normal running temperatures will be broadly similar.
The 'split' takes the form of an angled slot through the piston skirt, on the front thrust face, with a small hole drilled at the end of the slot to prevent it 'spreading'.
When the piston warms up expansion is absorbed by the slot closing up a little rather than an increase in overall piston diameter..thus the tighter bore clearance.
I have looked at standard type solid skirt pistons and can see no reason why they could not be altered to the 'split skirt' type..However, I would be interested in anyones view on this before I try it as I may have missed something!...Ian
email (option): ian@wright52.plus.com
Sounds reasonable to me Ian. I can easily clamp them in my workmate and run the old angle grinder cutting disc up them for you.
Seriously, would you not have to try one and then do some heat test measurements? Ron
email (option): ronpier@talk21.com