A 7-4" is kind of the standard for .09 and .10 engines. Normally the older steel liner motors like a somewhat bigger load than ABC stuff. I think the fiberglass F2D combat props may be the ticket for that size motor. IMHO they are too small for most .15s. I measured one of mine, and it was 6 1/4 - 5. Very thin blades so it should rev up good, if that is what you want. Fairly good deals for $5 or so. 049 flyer, was your .15 the one with the blue head, or one of the older ones? Mine is getting better every time I run it. Of course I have changed a few things, The turbo head I made seems to make the biggest gain with the 7-5 MA that I tested with. It seems hard on plugs, even on the stock head. Mine does not leak at the liner, must have been a Wednesday motor. I have an AP .15 that seemed a bit loose with the liner fit in the case, but it runs up even better than the last one. They are pretty light and somewhat powerful, although not in the same class as the blue head one.