Preheat is far more important on Lycoming engines because the camshaft is at
the top. Oil is supplied to the valve lifters only by being sprayed from
the camshaft and crankshaft bearings. Colder oil is thicker and does not
spray very well.

On oddity about the dynamics of the valvetrain is that the pressure of the
camshaft lobes on the lifter is highest at low RPM and least and high RPM.
The metal pressures on the lifters are very high anyway. Oil also cushions
and distributes the load of the cam lobes against the lifters.

Everything is therefore working against the lifters on startup. The oil has
drained off while the engine was not running, the oil is thick so doesn't
flow easily through the oil passages and bearings, the thick oil doesn't
spray well, the pressure of the cam against the dry lifters is highest due
to low RPM.

The high cam pressures bend the surface of the lifters microscopically and
the metal fatigues. Small pieces come out in exactly the same mechanism as
potholes forming in a road. Then the edges of the holes breakdown and the
"potholes" become larger. The lifters then start to wear away the camshaft.

The faster a good oil film gets established on the cam, the less chance
there is of lifter damage. Anything under 60 degrees carries some risk. We
preheat below 40. A single cold start when it is really cold can damage a
lifter.

--

Roger Long



"PaulaJay1" wrote in message
...
Winter is comming to Ohio ( thou the 60 deg day today disagrees) and the
subject of engine preheat arrises. I have a Tannis for the cylinders and
a pad
to heat the oil. I can call the FBO and have them plug it in an hour
before I
arrive and the engine is, at least, not cold to the touch. Still there is
the
time when I fly somewhere for dinner or overnight and park in the cold,
cold
outside.

The question, why is preheat necessary?

If it is OK to start the engine cold when the temp is 60deg F and let the
cyl
head temp go up to 500 or 600 deg, why is preheat recommended when the
cold
temp is 10 or 20 deg? The percentage difference in the delta T is less
than
10%. Are the engine parts that critical that this small percentage change
makes that difference?

I use 15/50 oil so the "cold" oil should be a good at the lower temp.

Chuck