[MiCk B.]

Here:

A - Losi pistons are in 'numbers' as well as colours they are as follows
#60 (Natural)
#57 (Black)
#56 (Red)
#55 (Orange)
#54 (Blue)
This number also relates to a drill size.
Whever people talk about a standard piston in number they would just use the 2 digits or the colour.
If they talk about a drilled piston they will use a 3 digit number, something like 556 or 566.
What it is the last number of the 'standard' piston numbers, notice they are all 5something so that does not matter
so a 556 piston would have 2 holes in #55 and one hole in #56
a 566 piston would have 1 hole in #55 and 2 holes in #56

**Tip From Richard Cree**
The number as Chris says relates to an imperial drill size, bear in mind the bigger the drill number, the smaller the hole.
Another thing to consider is a drilled 555 piston will have a slightly different size to a moulded 555 piston, for this reason I always used drilled pistons, even if they are standard versions available.
This really is very fine tuning, but once you get the hang of it you will find the jump between std pistons to be immense.

Measurements
#60 - 1.02mm
#59 - 1.05
#58 - 1.07
#57 - 1.10
#56 - 3/64"
#55 - 1.30mm
#54 - 1.40mm


Or this: http://www.oople.com/forums/showpost...47&postcount=2

MiCk B. :-)