Greg,

To yours of June 26, I know that it is possible to measure in-air RPM, as CL speed and racing fliers do. It is not simple, nor usual in the gear most of us haul to the field... Last I saw it done (in CL Speed), it involved a fairly high-fidelity tape recording, from the flier's center location (to reduce Doppler frequency shifts) and a program in a laptop computer to convert the recorded sound frequencies to RPM.

Now, that's too technical, for me at least... [8D]

Andy, to yours of July 6:

You wrote: "...most times a reduction in bore helps..." Am I correct that the 'bore' you refer to is the choke ID?

Also, agreed, some mass-produced glow engines may not be sturdy enough for dependable conversion, without needing to be nursed along like a feeble old maid... THIS is probably where the charge that DDD heads break engines started. Bob D's first commercial product was the Cox .049 conversion head - teflon seal, loose "contra-piston," large thumb-screw comp adjuster... It wasn't Bob's head that broke shafts. It was the users who expected the "usual" 16,000 to 18,000 RPM on 5" props, instead of the more suitable 10,000 to 12,000 on 7" props. Bob's stronger shafts have about ended that problem, no?

However, most of the schneurle ported, taper-cut ABC/AAC/ABN engines that are almost universal now, are much more sturdy than the reed valve Cox .049s of the 1970's, when Bob first released his head for them. Even so, one of our regulars in this forum has managed to suffer shaft breakage on a highly reputable glow .40 converted to diesel. I know JT didn't intentionally set up any conditions that would cause the breakage, so I'll write that off as an "off label use" (as they say about medicines) that the engine could not handle.

Exceptions prove???

To some others in here, allow me to paraphrase Jack Nicholson in a movie: "The TECHNICAL? You can't handle the TECHNICAL!"