If the thing has been standing for any length of time, it’s quite possible that the piston rings have rusted to the bore so an “oil” soak is the first thing to try. (I’d use PlusGas or similar)
Even then a touch of persuasion can be necessary. I recommend a 16oz ball peen hammer and a wooden drift. But not if it’s at top dead centre.
You might also try heating up the barrel with a hot air blower. Annular expansion being what it is, the barrel will expand more than the piston.
That don’t sound good, it maybe that whoever put the piston in the bore managed to break a ring :w00t:
Similar to above, try this …
Place barrel in bowl and give the bore a good soaking of Carb cleaner working in both directions from above and below, as the fluid gathers in the bowl re-use it to soak the bore, keep soaking for 30 minutes or so. Apply a squirt of engine oil to the bore and allow to soak. Supporting the barrel lightly tap the piston with a wooden drift, best done in a criss cross pattern over the piston crown, and alternate between tapping the crown and the gudgeon pin journal (so you’ll be tapping the piston in both directions).
If still no movement, apply more engine oil to the bore and lightly play a blow torch all around the outside of the barrel to heat it up. Repeat the light tapping technique described above.
If still no movement, apply more oil, more heat and heavier taps, eventually it will give
It’s actually a brand new piston stuck in a new barrel. The lot came as a kit.**
And there’s me thinking you’d left a bike standing. If this is a bought kit, can’t you take it back and ask them to do the work.
If the answer’s no, the advice you’ve got is still good, it’s just a whole lot easier to deal with on a bench. (A.k.a. Kitchen table, when the missus is out.)
Oh. Wait your chance, you can pop it in the oven to roast it up. Just got to have a decent reason handy for why the oven stinks of hot oil.
That was my mistake dropping the bearings out of a pair of crank cases.