After further research:

Tune for performance, then add/remove foil to get temps around 200-250f.

Pre-ignition can be caused by an exposed thread on the glow plug or a piece of carbon in the combustion chamber, a sharp edge or particle that heats up and ignites the compessed fuel mix before the plug would normally ignite the mix.

So, tear down the motor and check to see how much of the plug is sticking out into the combustion chamber. Instead of a long reach plug you may want a std length plug. To reduce or eliminate any edge (thread) of the glow plug exposed in the combustion chamber.

Clean the combustion chamber of any carbon residue.

Check the top of the piston and chamber for 'pitting' a sign of detonation.

The previous post made a good point about shims. Adding a shim will reduce compression. Too much compression is the cause of detonation. Detionation will cause severe damage.

Changing to a 'cooler' plug will retard ignition timing (if no pre-ignition).

Hope this helps, GOOD LUCK