which, one is true ???

I recall seeing an old advertisement somewhere for a Tee Dee muffler kit which instructed the installer to use timing shims under the cylinder to get rid of the SPI. These shims are sold by Cox International for this purpose.

Also, I'm pretty certain that Den's Model Supplies is an agent for Cox International, so the Cox products they sell should be identical.

I should amplify my comment above-a genuine STANDARD Medallion cylinder has only one transfer-some of the later R/C ones have two-but single flute, not with the additional bypass grooves on either side that the TD has. then later on you went from open exhaust to slit exhaust-which further complicates matters-in terms of variables. AFAIK nobody has the ability to remanufacture Cox cylinders in the way that Cox did it-to the original specs-and Cox International are doing their best to meet the needs of the customers by modifying existing inventory cylinders to meet the specs-as closely as can be done-of the original types.  Here is a typical example: https://coxengines.ca/cox-.049-tee-dee-cylinder-and-piston.html

I suspect this is largely because the original equipment used for manufacture has long since been superseded in general light engineering manufacture

So what you get is a 'best we can do now' version of the original using late OEM production cylinders as the starting point-so now there is a whole raft of 'retro' cylinder porting variations possible when compared with the originals....the net result being that some combinations if retrofitted will perform better than the original Cox OEM engine, others about the same-and others somewhat worse.

The original TD 049 used a tapered bore-the 'retro' cylinder does not-the original Killer Bee used both a tapered bore AND a lightened piston-thus accounting for the amazingly good-for a reed valve-performance. The differences may be subtle-but they stack up.  Essentially now we have a situation where-depending on the age of the engine-and what if any retro parts have been fitted-there is now an entire spectrum of performance possible-rather than the fairly clear division of power in steps that existed in the past  ie. Babe Bee - Golden Bee- QZ- Black Widow -Medallion TeeDee -KillerBee.....throw the various product engines plus the Texaco, the R/C Bee/Dragonfly into the mix-allow for a bit of tolerance variation and it becomes virtually impossible to create a meaningful ranking in the 049 category-other than to say the Babe Bee is at one end and the TD at the other...

I have never seen-anywhere-what the Cox factory figure for SPI was-people have measured their own engines and found anything from zero to about 0.015"  -but what was the factory specification for it-and did it intentionally vary from cylinder type to cylinder type-excluding of course the ones intentionally intended-such as the QZ 049-to have none.

ChrisM
'ffkiwi'