To the best of my knowledge, there is no other piston engine on the planet that is built with an intentional taper in the cylinder bore (choke). If you believe the story "cylinders get hotter on top and swell to a straight bore" you likely still believe in Santa, too. For the last ten years, we have refused to traumatize the poor cylinder with "CHOKE" in the bore, and for a simple reason explained here.
Using 5th grade geometry, the circumference of a circle is found by multiplying its diameter by pi (3.142...). Since an engine's piston rings follow the circumference of the cylinder, this means that the ring gap must change by pi times the cylinder diameter change due to choke. Using a standard aircraft cylinder with a .010 inch choke will require at least a .032 inch ring gap just to keep the ring ends from touching, or "bottoming out". This additional gap is just more space to lose compression through and increase blow-by.
While talking about compression, the top ring can not properly seat since it must flex .032 inch for every time the piston goes up and back. The top ring has now become a broaching tool, trying to scrape the taper out of the top of the cylinder, damaging both ring and the top of cylinder bore. My experience, measured both in cylinders (over 100,000) and years (over 30), has always found the same wear pattern: a ledge just above the top ring travel. In some cases this ledge exceeds .020 inch in height!
We now use only good 1st run, crack-fee cylinders, grind the barrel STRAIGHT, and install new valve guides and seats. These cylinders are then finished to the most reasonable oversize (.005,.010,.or 015) and given new rings and matching pistons. This results in a bullet-proof cylinder that makes 7 to 10% more measurable horsepower with a 15 to 45 degrees cooler CHT. Reliability is improved as more than 99% of these choke-free cylinders achieving factory TBO - and beyond. This simple fact is used by fleet operators who have received extensions to the FAA required TBO limits for their commercial operations. Additional advantages include reduced vibration and up to 25 hrs operation per quart of oil consumption.
This is real-world proof of 5th grade math. Would you now rather run your piston in a funnel or a true cylinder like the rest of the internal combustion engine world?
We use these cylinders in all engines rebuilt here. We also have been machining and line-boring crankcases since 1969. Although unique in how we handle cylinders, if plagiarism is the most sincere form of flattery, we are nearly flattered to death. Nearly every crankcase overhaul shop, including the factories use the techniques and approved procedures I pioneered several years ago. Why bother going to these copy-cats? Come to the original thinker, Chuck Ney, to have your next engine rebuilt. Join our hundreds of satisfied customers.