RE: Norvel Big Mig .061 vs AP .061 Wasp
The AP will not interchange without an adapter. The base of the NORVEL is oblong; the base of the AP is round. The passage area of the NORVEL is larger, but this is partially compensated for since the mount screw of the NORVEL passes thru the base and the AP screws do not.
The QC with AP is hit and miss -- sometimes you get a decent one and sometimes not. Average performance is considered to be generally below the NORVEL.
I see that Tower has raised the price back to the $45 quoted by skaliwag. They had been $40, then raised to $45, then dropped to $40 again, and now back up to $45. [link=http://www.selecthobbies.com]Select Hobbies[/link] also lists the engine.