Well I must say this is turning into a very interesting discussion!

@gcb;
I'm not abusing my engines to that extent. Experience learns that most engines we use hold up for at least three seasons before they need a new sleeve / piston.

As for why Davis doesn't make a head for my engine, obviously I can't say for sure. But I can imagine that it has to do with the fact that this is an unknown / little used engine. This also leads me to think that Davis never tested these engines (I bought my engines from someone who bought them on a trip to Eastern-Europe and I saw one for sale once on this website, but that's it)

We do allow our engines to warm up before we do a pull, so I don't think that would be a problem?

@Jim Thomerson;
The contra-piston in my head cannot protrude from it's bore, because the set-screw isn't long enough (unless it should fall out)
I also think that there is a direct relationship between making more power (i.e. burning more fuel and / or using a higher cr) and the engine running hot. The trick is to find out how far you can take this safely.

For more info on our hobby, here's a few websites:
http://www.micropulling.nl
http://www.rc-tractorpulling.com
http://www.micropopeye.nl
http://www.micropulling.com
http://home.wanadoo.nl/harry.vink
http://people.zeelandnet.nl/virtua
Most of the are in Dutch or German, but it should give you an idea of what we're doing.

@Motorboy;
I intend to make a new head with more / bigger cooling fins. Hopefully I'll have it ready before next weekend (we have a pull on Saturday), so I get a chance to test it.