I tried for the first time last night what I thought was a great tip I picked up off a forum here. To use a rope laced into the sparkplug hole for a piston stop. I found a barely run Chinese JF (MTD-McCulloch) 50cc chainsaw. The saw displayed very little use - that's because the magneto was loose and apparently they threw it away rather than bothering to take the cover off to see what made the ugly screeching noise. Saw ran excellent. I began to dismantle for conversion to airplane engine - decided to try the rope in the sparkplug hole - got the reverse thread nut off OK but it was tight - tried the flywheel side and it was very tight. Had about 8" long 3/8 ratchet wrench on the nut - braced the base with my elbow/arm - pushed down on rachet handle with left hand and rapped with small claw hammer. Busted the piston into 4 pieces. I don't have an impact wrench and have used the wrapping hammer technique successfully before many times with no hazard when I have had the flywheel clamped or braced. I am not a gorilla - force was maybe ill advised but not excessive - any body else have this happen or was I just unlucky. I heated the nut to loosen it - wished I would have the first time. Engine looks great otherwise - anybody got a piston for sale? One interesting thing - the screws and bolts on the engine proper were all "excessively" tight - like impact wrench tightened - screws and bolts on the housing were all loose or barely tightened and I don't think anyone had loosened to inspect - they were never tight.

Did they by any chance have Loctite, or some variation thereof, on the tight threads?

I've used the hammer on the wrench but with no rope in the combustion chamber. The suuden impact caused by the hammer seems to loosen them up.

The bolts into the aluminum casting are usually self tapping and have some blue or green Loctite on them. If you heat the bolts up with your Monokote gun or a propane torch and remove them hot, there is less chance of pulling the threads out with the screw.

Yes it does look like there was some type of loctite - kind of reddish tan residue - I never thought of that - will next time - it came loose better after heating with torch but was still very tight. I have never had this much trouble loosening something before.

That, combined with too little rope.
If the piston is allowed to stop near top dead centre, the forces on the piston are HUGE just to obtain a moderate torque.
More rope makes the piston stop sooner. Ideal is the position where the crank is at 90° to the connecting rod. That way, the parts are loaded as lightly as possible when you apply the hammer shocks. Will save you bearings and pistons.

Thanks - that makes sense - I probably had all of the above - may have had locktite and I only shoved in a little bit of string- probably torqued on the piston if it was to one side or something - I wasn't expecting the nut to be so tight. I have worked on many engines before but not this new or small of one and usually lawnmowers or old chainsaws. This piston really is quite fragile in design but that makes it light - cheap too I suppose. Probably wasn't smart but I decided to order a piston - rest of engine is so clean, light and it really is a compact design and looks like an easy conversion - I hope. What happend to me is apparently a rare occurance - perfect storm of mistakes.

Does anybody make a prop adapter for these engines?

I think that if you send the engine to C & H Ignitions they can do about anything including setting the timing and running the engine.
They are on the net, and work out of Riverton, Wyoming. Flathead 4

what's the model of the saw? I have a McCulloch Titan 57 that I'm going to convert in the near future, if it's the same thing I could double up on the prop hubs...

RangerFredBob - I am going to try to contact CH Ignitions this afternoon - I will see if they can make? I was going to find out if it is better to reverse this engine (if possible) and use the pto (sprocket end) or the short tapered shaft on flywheel side and if porting etc is worth pursuing - I'm pretty green on all this stuff. I intend to use either gas/glow conversion or CH - ignition and not the magneto. Will post back what I find out.

Reread your post - are you going to make a prop hub yourself and offering to tool another? I would be interested. These are different engines though. Mine is 50cc with short tapered-keyed shaft on the flywheel end and 3 stepped on the pto end with left hand threads. Yours would be a 57cc. This model was actually called a Timber Bear but it is not the older 3.7ci version - only made a short time - they were total plastic junk and apparently not much of a saw but the engine sure looks good - although maybe as weak as the rest of it. Guess I will find out.