Originally Posted by Chris W

I think that it was on the Parra web site that claimed that a dead flat or a 0.3mm combustion chamber step to the contra gave the best runs.

The late and great Pe Rievers was also on record as stating the same.

A point of history here, wasnt it Ken Bedford of ETA fame that first used a "squish band" type of head successfully in racing?
(If you can confirm the answer to this then you are showing your age like me! Looking at you Ray and Chris.)

P.S.
http://www.control-line.eu/modern-diesels.htm

l"Your engine has been factory set with a clearance of approximately 0,28 - 0,30 millimetres between the cylinder head and piston at the top of its stroke. This is optimum for operation at sea level with the recommended propeller and fuel mixture given for running-in. Adjustment of the contra-piston via the compression adjusting screw will give a wide range of operating speeds. If you need more compression than is available, remove a shim, back off the contra-piston a little (to allow for fine tuning) and reassemble. The engine will run satisfactorily with the contra-piston recessed back in the head. However, the best combustion chamber shape is when the contra-piston is flush with the head, or recessed slightly (about 0,3 millimetres), which allows for some fine-tuning. Once you have settled on fuel composition and propeller andestablished the in-flight compression setting, then remove the head and inspect the contra-piston position. If it is significantly recessed, add a shim and adjust the contra-piston down to restore the desired compression ratio. It may require several such adjustments to arrive at the ideal position."