I have been cleaning out a fellow modeler's building room that passed away . In the engine drawer were two Cox engines that look like they could be Medallion .074 or .090 . There were no boxes for them and they are not marked, How do I know what size they are ? easily larger than the .049/.051s that I have had but not as big as the .15 Cox engine. The plastic venturi body is red and the prop hub is silver. I think it is a later production since it has a large diameter rotating barrel around the base of the cyl.  I would post a picture but not sure I can (tech challenged at 69)

Ok so that makes this an .090 then? It looks like a beam mount TeeDee but integral plastic venturi as part of the red portion of the case.≈

There is a .09, and a .15. Post your head diameters, or wait until I measure mine. I have a couple of each.

Here is some reading material in case you want to brush up. Look to the left for Medallion and Tee Dee:
https://www.mh-aerotools.de/airfoils/cox_frameset.htm

Are you thinking of selling?

Fins on cyl head/glow plug measure .925"