HT = (r + c) - (r cos (a)) - SQRT(c2 - (r sin (a))2)

r = s/2
dtor = PI/180
a = d x dtor

HT = The height of piston
r = The stroke divided by 2
c = The rod length
a = The crank angle in radians
d = The crank angle in degrees
dtor = Degrees to Radians

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http://www.coolmath.com/graphit/
For a stock 2.0 with an 83mm stroke and a 139mm rod this is the calculation:
(41.5 + 139) - (41.5 cos(x*pi/180)) - sqrt(139^2 - (41.5 sin(x*pi/180))^2)

Xmin = 0
Xmax = 360
Ymin = 0
Ymax = stroke (in this case, 83)

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When comparing the results, think of the difference a degree of cam duratation or degree of timing (cam or spark). Think of what 1mm difference piston height means.


-=Whittey=-

{edit} Do 420A pistons have a wrist pin offset? That will offset the entire graph above by a slight amount. I'd have to sit down and work it out (or break out what little google-fu I have left) but really this is just for comparison. If the crank center isn't centered to the bore, that will offset it as well.