On Mon, 06 Jun 2005 22:05:00 -0700, Richard Riley
wrote:

I'm looking for an engine that may not exist. If anyone has a notion
of what it might be, I'd be grateful.

It's for a 103 legal ultralight.

60 lbs or so all up, including re-drive and cooling
35-40 hp.
4 stroke.
More reliable than a 2 stroke Rotax

Any ideas?

Colin Chapman of Lotus fame was a pilot and into Ultralights. He had
his engine group design a small four stroke intended for this type of
flying that had an interesting prop drive: They simply beefed up the
camshaft and bolted the prop to it. Since all camshafts spin at 1/2
crankshaft speed, they had themselves a PSRU without all the belts or
gears.

Alas, with the death of Chapman, the engine died with him, or shortly
thereafter.

Finding a decent, reliable small fourstroke engine has been going on
for a long long time. Motorcycle engines are intriguing, but attain
their high output via extremely high rpms. In addition, nearly all of
them come with the transmission case cast along with the engine
crankcase. You don't need the transmission, unless you want to use it
as the PSRU, and then you don't need all the gears which add to the
weight you have to carry. People have tried to use the transmissions
as PSRU's with mixed success.

For one thing, the transmissions aren't designed to spin a prop so
some kind of beefed up power takeoff has to be added and it just
becomes a bit of an engineering nightmare. It might actually be
easier to have one of the local machine shops machine a case out of a
block of aluminum using their computerized milling machines.

It's the old story of liability issues, plus the relatively limited
market for such an engine. But I watched Jesse James order a couple
of cylinderheads for a custom chopper he was building, and the camera
guys went to the machine shop where this CNC milling machine cut the
cylinders out of a block of aluminum. If they can do that, they can
cut out a crankcase, it should be a more simple job.

Or, go whole hog and recreate an Offenhauser engine in miniature; no
cylinderhead so no cylinderhead gasket to worry about. Valve jobs are
a bitch though...

Corky Scott