To be safe, take the head off the engine, and put in the plug. If the plug extends past the top of the combustion chamber, it's probibly too long, you usually want the plug flush with the inside of the head. Sticking in a very small bit is ok, as is being recessed a little.

You can stick an extra washer under the plug to raise it up if you need to run a longer plug in an engine designed for short plugs.

Most (but not all) .15 size engines take long plugs just fine. The OS LA, Mag .15, OS CV-A, Norvel, and Megatech .15's all take long plugs, they all have a thick enough head that it's not a problem.

But that doesn't mean the AP can handle a long plug with out getting in to trouble. Even if the piston doesn't contact the plug, an overly long plug increases compression which can make the engine tempermental. I don't know what kind of plug it likes, honestly.

However, a "heavy duty" plug is probibly a long plug, not a short. I know that's true with the K&B plugs I use, their heavy duty is the same length as their long.

From what I've seen, all the OS plugs are shorter than typical plugs. Why, I have no idea, but they are. So if you can't get an actual short plug, an OS is going to be the next best thing. If that's still too long, try adding an extra washer.