Tapered cylinder bore

Is it possible that the cylinder taper is because of the porting? Do tapered cylinders have boost ports? Maybe that's not the question being asked here.. I imagine that Phil is correct about honing being the method of forming the taper. I wouldn't be surprised if Cox had tiny little specially-designed angle-hones for just such a purpose. Tolerances within .0001" which Cox was known for.. would definitely have to apply for tapering a cylinder/piston.

Some interesting reading here.. http://www.flyfreeflight.com/Site/Cyl_Piston_Rework.html

All stamped TD cylinders Nos. 4&5 were tapered
http://en.m.wikipedia.org/wiki/Cox_model_engine
These were thin wall cylinders with stepped outside diameters and 2 bypass ports plus 2X2 booster ports.

Later TD cylinders with no stamping were not tapered to save costs,but had the same porting with thick wall.

Tapering had to do with compression better retained when hot.

This is how I see it: The cylinder bores were probably milled to a certain dimension, at or very close to the minimum dimension of the final product. The taper was then made with a tapered hone and plenty of fluid. Every now and then they would measure a cylinder to make sure that the hone is still in spec. After a while they learned that a single hone can do so-and-so many cylinders before it is rebuilt or replaced, reducing the need of measurements. The cylinder was probably measured with a precision "pin" inserted into the cylinder, if it goes through the cylinder is OK, if it sticks, the cylinder is too tight. This is still a very common way to QC high precision stuff.

Of course, because of the honing tool wear, some cylinders are tighter than others. The ones that were too tight, and were not caught by QC, became warranty cases or "rare clean barely run with box" eBay scores later on The "loose" ones most likely worked ok, maybe with a top speed of a few hundred less than the best ones.

All of the above is of course just another "educated guess"

Yes, this is the way I also imagine it must have worked...I value the tapered ones higher than non-tapered based on my experience with a tapered one that can simply not be "killed" no matter how many hours I have run it, the compression just remains as-new. Added, of course, is the beneficial effect of castor the varnish of which is a self-building sealant on any tiny scratches and wear spots inside the cylinder.

When I flip over a hot TD the new, non-tapered shows lower compression , than the tapered when hot, after so many run-hours...Too bad that COX gave up tapering for cost reasons....not that the non-tapered and not good, but the tapered ones are the best.