The main problem is of course the very brittle iron barrel, too much force in any place is going to crack it. Some methods I’ve used sometimes to free pistons stuck in bores is to do one or more of the following:
1. If the pistons are stuck above the ports you can fill above them with thick oil (50W or more) to the bottom of the plug thread. Now screw in a piece of bar with the M18 thread and it in effect creates a closed pressure cylinder. You can exert many tonnes of pressure equally across the piston crown doing this, but you risk bursting the iron barrel. You can acheive a similar effect by filling with water, plugging the holes and putting it in the freezer, taking advantage of the fact the alu piston will contract more than the bore. Again you risk bursting it though but it is a safer method than the oil one. If inlet and ex ports are open you can make a 1/4″ plate blanking plate to close them off. Ice is your freind and your foe with engines!
2. If pistons are below the ports you can screw a profiled bar down the plug hole to load the crown with a force then use a CO2 fire extinguisher up from the crankcase door to freeze the piston, again hoping the contraction is enough to free it. You can also buy freeze spray from DIY places that does the job, it is used by plumbers to temporarily seal off pipes.
3. I was out of options on the last engine I had with siezed pistons (20 years sat outside with no carbs on!) and in the end it got a profiled nylon bar down the bore and struck with a lump hammer while someone held the crankcase to prevent the shock going through the case. It worked, much to my surprise, but is a last resort.
As a side note I’ve got complete engine Y8286 here.
Good luck, let us know how you get on.
Cheers,
Richard
