Piston protusion

[Ben]

I was about to crack on and fit the head onto my block today, but I've just noticed that the 4th piston doesn't protrude from the block surface as much as the other three.

Pistons 1,2 and 3 all stick out about 1mm. Piston 4 does not stick out, it remains about flush with the block surface.

Am I right in thinking this is going to cause a big difference in cylinder compression and cause a rough running engine?

Can't think how this has happened because the pistons were all removed from the same block and the HDI rods were also a matching set.

Really hope its nothing to worry about! Any thoughts appreciated..