I think Jim is correct. If the engine is not running sloppy rich, it will probably overheat in the early break in stage. Remember, there's no airflow except from the prop, which may not be enough. A very unscientific way to tell if it's over heating is to wet your finger and tap the head after it stops running. If the moisture from your finger sizzles, it's too hot. You should be able to tap your dry finger on the head without burning your finger. Hot, but not too hot to touch.

Paul

Basicly, the iron piston expands more than the steel cylinder. This can be caused by friction due to parts that do not yet fit (needs break-in), insufficient lubrication caused by not enough oil content in the fuel, or too lean a needle setting, or a combination of these things. Iron/Steel engines, if run too hot, can experience thermal runaway which can wear out an engine very quickly, or sometimes cause it to stop. Many (most?) Fox .35 users use at least half castor oil in their lube for added protection.

How much explanation do you need about care and feeding of a Fox .35? They are a little different from most engines in that they need more lube (25%-28%) is common. If it is an old one that has never been run you may need to give it an extensive break-in.

George

Edit: Oops, forgot about vibration. Fox .35's are shakers and need a very firm fuselage mount. Otherwise you can experience fuel foaming which causes a lean run. I think you mentioned that. I'd check there first. You might consider bench running to check out the engine.

Well guy's thanks for your thoughts, but was doing some more runs today on the 35 and noticed that while it was running i could see bubbling around a section of the head.So i have replaced the 35 with a fox .40 rocket that the piston and liner were shot but the case was in great condition, so i took the piston and liner and head from a Fox .36,( which the case bushing was shot), I had to use the rod from the .40 and installed all of it into the Fox .40 Rocket case, i had to notch the bottom part of the liner so the rod would clear, and shave the head and baffle on the top of the piston.I had done this sometime back but never got around to running this put together.Well today i ran several tanks thru it very rich and sloopy, last tank i lean her out man she seems to run strong.Will see how this plane fly's this weekend, wind permitting.

Question.In regards to performance,is the slant plug head better than the straightup plug design ?Does that make sense? Or what is the reason for the slant plug design and the straightup plug design?
Thanks again guy's.