Although the NGK BPMR6A gasket'ed seat is more or less the standard spark plug used in larger chainsaws for many years, Chainsaw derived model airplane engines used the NGK BPMR6F or 7F tapered seat spark plug in almost the same engines.

Both spark plugs are the same reach and are both 14mm threads. They will actually interchange back and forth in the cylinder head. The "A" plug has 3/4 wrench flats and the "F" plug has 5/8" wrench flats.

But the "F" plug is smaller and shorter on the outside. It is used so as to fit under the cowling. However for shielded applications, the shield spark plug caps will not interchange on the different spark plugs. Your ignition spark plug cap has to be set up for one or the other.