[bluejets]

In R/C 2 stroke engines it is a well known fact that the bore is tapered towards the top. So much so, that say a K&B front exhaust rear induction marine engine for example, will "seize" when cold going over TDC and will actually "squeak" when doing so.

Many R/C engines with the tapered bore are referred to as ABC engines (Aluminium piston Brass liner Chromed) Also others using an aluminium bore.

Material selection both for the cylinder liner and the piston is such that the two calculations allow for running temperature and the clearance required.

Difficult building a 2 stroke successfully which is why many prefer 4 stroke. Pop a 1 thou (bottom) 2 thou (top) undersize aluminium piston in a cast iron bore with a heat-treated cast iron ring and it'll run successfully for many years.

[RICHARD ZASTROW]

In my experience I've not encountered tapered cylinder bores. Round and straight was the goal. These were liquid cooled 2-stroke engines.

However, For reasons stated above, The pistons were slightly oval and tapered to the bottom of the skirts.

The top of the piston heats up more than the skirt; the wrist pin support area has more material and holds more heat therefore expands more; incoming charge cools the skirt area (crankcase induction). Results in a straight, round piston when the engine is up to temp.

The air cooled engines might react differently, so they might expand more or less in different areas than an optimized liquid cooled engine.

Dick Z

[wendtmk]

If you're talking ringed pistons, the bore is straight. Non-ringed 2 stroke engines, like the R/C engines mentioned above, do have tapered cylinders.

Mark

[lkcl]

If you're talking ringed pistons, the bore is straight. Non-ringed 2 stroke engines, like the R/C engines mentioned above, do have tapered cylinders.

Mark

i'd heard about piston rings that are angled so that when it all goes "bang" the pressures push them harder against the side-wall. the tapered cylinders (no rings) had me pause for a second though. don't believe i need that (whew).

l.