It's been a while to get back on this...

I haven't had opportunity to bench run this engine further. Some thoughts on restoring the intended flow patterns to a restricted OEM configuration:

(Sport and everyday) diesels seem to be much less fussy than glows RE: so many things, so this simple butchery may work quite well.

In most schneurle engines, the third bypass, often called the "boost" bypass, opens after the mains. It may - probably does - complement the charge flow, but the basic pattern and timing is established by the two which open earliest and close latest. Again, the simple full-block may provide enough restriction to simplify a very satisfactory conversion.

The specimen engine, Glow Fox 15BB Schneurle, is capable of quite high RPM on enough nitro and a small-enough prop. That suggests to me that optimal crankcase interior flows were developed for RPM well above, say, 17,500. Are flow patterns in those conditions suitable for a diesel conversion for sport use?

In this instance, I did not want a permanently attached 'block,' for several reasons. One, of course, would be further shape and area variations in all three bypass channels. I shaved a balsa segment for the boost channel because it would be least likely to press on the exterior of the cylinder harmfully. Fox sleeves, like most, are susceptible to distortion. At least, this one is steel; brass is likely more so. You've inspired a thought or two more. Two pieces of 1/32 balsa, sandwiched, might allow some boost flow by shaping channels in the one against the sleeve. The boost port 'window' has ample height for a continuous piece across the top to fill the gap... Might try this, time and occasion permitting.

Of course, repeated disassembly and reassembly will likely disturb fits. That means several additional "re-seating" runs after each, with additional wear against the compression sealing surfaces...

Of course, a lot of these speculations cannot be answered without more rigorous experimentation and comparison. I have flown converted Fox15BB-Sch's with this simple mod, and without, at times over the past few decades (the engine first appeared in about 1972/73...) Any problems I had can easily be called fuel related. It is only in the past few years I've settled on a home-brew method that does work well and dependably. (Not knocking DDD, Aero, or Clutton fuels - they are very good - but HAZMAT charges being what they are here, we might pay more for the shipping than the contents.)

Don't know if you have anything similar - sounds like it in recent comments on difficulty obaining fuels and aircraft dope. Our HAZMAT fee is a shipping surcharge for flammable liquids, and other "HAZardous MATerials." There have been a few 'incidents' involving such. The fee supposedly insures the carriers against future 'incidents.'

Edited to add: The engine's main bypass ports are a bit harder to mod evenly. The backplate extends up to the bottom cooling fins, and that bypass channel is primarily IN the backplate. Shaping, fitting and securing a filler volume in it would be much less certain. If front and back fill is uneven, results would be unreliable and harder to identify...