Hi I have an old never used OS 50 max but it's missing one head bolt. There are suppose to be 3-long head bolts and 3-short head bolts with one of the short ones missing. Is there any substitution bolt for this engine or am I going to have to hunt one down. Or should I just drill the hole out bigger and used a different bolt. Here's some pics of the engine. I believe its from the early 70's.

You have an old O.S. Max .50 RC. It's not even the .50 FSR. The parts for your engine are no longer available. You would have to purchase a screw set for the .46 or .50-size engines of current-production and then carefully trim the bolts as needed.

Hello Bax,
I have an older OS .50 FSR I bought new and put 1-1/2 seasons on. Great engine. I misjudged power lines and flew into one. Plane divided and pancaked to the ground. Runs OK on new plane but if I do high alpha, it breaks into a raspy ring-ding sound that I can't figure out. Sounds like a dirtbike winding down after reving. Quits from there. Compression weak when warm. Was in a cowl. Overheat? Bad ring? I like this engine and would like to find parts.
Thanks, Dan

The sound may just be a result of the throttle setting, propeller, and exhaust system. Many an R/C engine would have such a sound when the throttle was at a particular point with a particular prop and muffler. If the engine runs well otherwise, we'd not be particularly concerned.