wonder if they are designed for the shorter stroke of the f20.

 

Piston height above deck using a dial gauge
#1 : 1.3mm
#2 : 1.12mm
#3 : 1.22mm
#4 : 1.12mm

Top of pin to top of piston crown using digital vernier caliper
#1 : 18.52mm
#2 : 18.44mm
#3 : 18.49mm
#4 : 18.47mm

Does anyone have measurements for stock pistons, I no longer have mine

 

Work out if its hitting the whole squish platform or just the edge of the piston. If its just the corner of the crown, machine that for clearance as I suggested and bolt the head on.

Use your verniers to measure the depth of the squish area in the head. Alternatively if you have the pistons out, put a head gasket on the head using the dowels to locate it, and see where the piston hits as you try to push it through the head gasket into the combustion chamber.

 

Has anyone got a piston they can measure the top of the pin to the crown please as I don't have mine any longer

@Chris I am concerned about putting a chamfer on the piston as I'd have thought that will direct the combustion more towards the rings and put more pressure on them?

Pistons are out and on their way to Wossner to make sure they are correct

If they are correct then I will put the original head gasket on and see where the piston htis

 

Pistons are normally smaller OD above the rings to allow some pressure to get down to the rings. Its actually the pressure on the top and inside edge of the rings that help them seat. Some race pistons even have "gas ports" down to the ring to help with seating pressures. The pressure waves behind the squish area are much lower than in the centre of the combustion chamber.

If the chamfer approximately the edge of the combustion chamber you will not have any gaping gaps.

I normally setup my race engines so the pistons will not hit the head without a head gasket. This allows enough running clearance once the piston is moving around and everything has expanded. I've found 0.5-0.8mm the minimum you want to run. Rods actually stretch during their cycle, especially on an over rev.

 

the stock pistons sit 1mm above the head.

In the case when we use bigger bore pistons we chamfer the top of the piston or mill the head to take the "step" out of the combustion chamber then use a custom headgasket to get proper piston-head clearance.

We had to add a chamfer to even the .25mm oversize mahle pistons to deal with this. Stock pistons are this way as well. The diameter of the chambers is usually around 86mm.

The issue I see with your setup is the variance.. they should be all the same.