Hallo everyone,
                      I managed to drop the liner from my FP 40 and now its way out of round! Fortunately, Jim Thomerson came to the rescue and sent me a Tower 40 piston liner and conrod (Thanks Jim, what a kind gentleman you are). As I got to assembling the FP 40 with Jim's new bits, I got to wondering.
  Perceived knowledge says that the FP 40 is almost certain to runaway after the first manoeuvre. Most posts that I have read seem to go along with this opinion, unless you use the Philly Fliers tune up. There are a few folk who say that the FP 40 is good out of the box. Apart from the Philly Fliers tune up, most other people resort to sending off the motor to a man who can fix it!
  On the other hand, everyone including Jim, says that the Tower 40 is a real pussycat stunt motor, good straight out the box with no runaway! This is a bit odd as the Tower is an excellent clone of the FP 40, you can mix and match the parts and they all go together, no problem. The only tangible difference is that the Tower is ABC and the OS is ABN. is the difference in behaviour down to this? If it is then why does the ABC liner work and the ABN liner most times doesn't?
  It may be that the difference in liners is a red herring. I know that the fabled OS quality is somewhat lacking in the later FP 40s. For example the crankshafts are not up to the Tower item (reported in some threads), also the deck height of the OS has varied surprisingly in the samples I have measured. I don't know about the Tower 40 as I have never had any to measure! Could the unpredictable behaviour of the OS be linked to this rather than the ABC / ABN difference?
  The reason that I ask is I am wondering if my OS 40 with Tower 40 innards will behave like a typical OS or a typical Tower! The engine is now assembled but its too cold to go fly it and find out. I am getting a wimp, no flying with snow on the ground!

Regards,

Andrew.

The trick is getting the right propeller on your engine.  The best prop I have found for the Tower 40 is the Supercool petal blade 11 x 5.  Mine would wind up on an APC 11 x 5 enough to be unusable.  A Zinger wide 11 x 5 worked OK.  People recommend the APC 10 1/2 x 4 1/2 for the Tower (and many other engines as well) but I have not tried one.  I've flown a Tower 40 with boost port blocked and could not see any difference. 

The T.40 has been out of production for about 5 years! There may be some parts left, but not backplates...and finding them with search is pretty nigh impossible, unless you have PN's, and maybe not then.

The trick with most these engines seems to be to find the right prop/venturi/muffler/fuel combination. I think the idea is to put enough fuel through it to keep the engine well lubricated (so the venturi can't be too small), and cool (so the muffler can't be too restrictive, the prop too large, or the oil too sparce). The right combo seems to give a launch rpm about 1k below peak, with the lap times you're seeking.

Lots of folks like the 10.5 x 4.5 APC for various engines...but I've had such good results with the Thunder Tiger Cyclone 11 x 4.5, I just have to brag on it...and I hate to admit it, but the quality is better.  Steve