I have heard of a few cases recently of cracked heads in Limbachs usually from plug to exhaust valve. In talking to Nigel Baker he correctly pointed out to me it is caused by shock cooling of engine. I can say air cooled GA aviation engines should not cool at rate of more than 40degF/min (22degC/min) but ideally you should aim for lower than this like 30-35degF/min BUT DAMAGE IS ONLY DONE FROM HIGH TEMP TO MEDIUM TEMP and not from a medium temps to a low temp. Thus if you can get that maximum temperature really low then you will have a lot less problems infact I believe head problems should be totally a thing of the past in aircooled motorgliders as they are with Pawnee Tugs these days (I can remember in 1976 when we started using C180s for towing and then 18months all the cracks started and $4000 to $6000 repair bills)
Limbach tech bull 53 (or is 54??) point 4 clearly states CHT should not go above 180degC in climb or words to that effect. Those temps in Limbach manuals of 230degC or what ever is, is total CRAP. Nigel is correct but I say get the CHT <170deg C AND cool down slowly from 170deg c to 150deg c and the heads WILL LAST the life of the engine no iffs or buts. It is very difficult to cool from say 195degC to 170degC without doing serious damage. Another way to check is a Lamba meter OR some means of measuring the CO (needs to be about 6-7%) I think I am right but I am open to thoughts on this one. It is not like a car as we are using high power settings and huge changes in density altitude. Ian M
