I have been using a Tower 40 with an RJL diesel head. I broke it in as an glow plug engine according to the RJL instructions and put perhaps an hour's running time on it on the bench as a diesel (over a period of several years). Finally built an airplane for it. Put about 15 flights on it and the contrapiston in the RJL head came apart without damage to the engine. Visited the local engine Guru and got another bulletproof contrapiston made. Put another 15 or so flight on it and the crank broke, right at the crank web. Perhaps this was because of running it as a diesel with a two-oz Harry Higley heavy hub; which many will tell you neither is a good idea.

I ordered two new cranks from Tower and was most impressed with their service. I also picked up a NIB Tower 40 off ebay. I started to break it in as a glow and it would not start. It was obviously way overcompressed. Not having extra head gaskets, I took the head and went after it with my Dremel. I cut a bunch of triangular notches in the bubble of the combustion chamber, producing my new invention, the swirl-quench combustion chamber. This lowered the compression ratio sufficiently, and I broke it in. I took my old engine and lapped one of the new cranks in. The bronze bushing had gouges in it from removing the broken front part of the crank. Ran it once with the glow head to seat the crank and set it aside as a spare.

I made two flights today with the dieselized new engine and it ran excellent. Needless to say the spare engine and the extra crank will go along when I go flying, and I may pick up another Tower 40 off ebay. I guess I could be called bull-headed.

Jim

Well, I broke another crank, just like the first one. Found that the case of the NIB Tower 40 off ebay is cracked. Put an engine back together and hope I can make it through the Vintage Stunt Contest in Tuscon this coming week before another crank goes. Just bought another NIB Tower 40 off ebay. Other than breaking, the engine runs great. I am not as sold on conversion engines as I had expected to be.

Jim

G'day Jim...I gather you're using a plain bearing 40? In general, plain bearing cranks are smaller in diameter than the ball raced cranks so they've got less resistance to the bending loads from ignition. Maybe that's why they keep breaking. I doubt the Higley hub contributed to it because it mainly loads up the front end of the bush, especially in hard turns, from gyroscopic precession plus of course the extra weight.

Unlike a glow engine where the flame propogates from one location and gives a reasonably smooth rise in combustion pressures, in theory with a diesel it's possible for the complete mixture to start burning at the same time. This would give an incredibly fast rise in pressure and to a much higher maximum than a glow could ever do. This won't happen in practise of course because no mixture is completely perfect but it can still ignite at any point in the combustion chamber and maybe at several points which still gives a very fast rise in pressure. I know a lot of guys have had success with conversions but maybe the Tower is a bit on the weak side mechanically.

Most glow conversions are simply not as strongly built as diesels and slight overcompression may often lead to a broken connecting rod or crank. They also don't idle as well as purpose built diesels because the carb throat is too big (although the Tower has a very small carb). In a .40 size you are better off getting a PAW or Aurora diesel instead of a conversion. Both excellent performers. Sadly the best.40 diesel (by Irvine) is no longer available. If you see one on Ebay, snap it up. Mine has given outstanding performance for 15 years and still runs strong.

Max