Triumph Repair: Piston sizes, cylinder bores, thousandths

Question
Hi Howard,

I'm buying a new set of pistons for my 1974 1500 Spitfire and in the catalogs they have piston set discriptions. IE: STD, .020, .030, and .040. If I look at my old piston what are those sizes refering to so I can order the right ones?

Answer
Hi Bill,
The size of a piston is marked on the top with either a "STD" meaning it is standard and each other size is the amount of oversize it is.  .010" is 10 thousandths over standard and .020" is 20 thousandths over. etc, etc.

This brings up a question. Why are you buying a new set of pistons? The best mechanic on earth can't tell if an engine needs pistons until he tares it down and measures the piston and cylinder bores.

Before you do anything, you need to read my tech tip on rebuilding an engine.
http://mg-tri-jag.net/Preamble.htm

Be sure to read all five pages of the rebuild before doing anything.

Howard