A friend who is into motorcycles gave the following explanation for the
rationale behind not revving the engine too high after a rebuild:
The piston wears itself into the cylinder bore. If you rev the engine high
soon after a rebuild, it will tend to oval out the cylinder bore. If you run
the engine for a while without allowing the engine to rev too high, it will
seat without ovaling the bore. You can then rev higher without ovaling out
the bore. The ovaling is slight, of course, and the symptom of having this
ovaling is engine smoothness at high revs, not reliability or anything like
that.
At first, I thought this sounded kind of hokey, but each part of it does
kind of make sense. For instance, we know that the piston does wear the bore
down some, otherwise there wouldn't be a ridge in old engines. The heat and
pressure of combustion almost certainly causes the metal on the wall of the
cylinder to become hardened (empirically, because if it didn't fancier metal
than steel would have to be used), so it makes sense that you could cause some
kind of damage by not letting the metal harden before subjecting it to maximum
lateral force. The failure mode also makes sense, because I've never actually
heard of someone breaking a new engine because they reved it too high.
My friend broke in his motorcycle engine extremely carefully, going through
several stages of limiting the revs, and he claimed that his motorcycle ran
much more smoothly than others of the same type that he had driven because of
this. I have no basis for either believing or doubting him.
I don't intend to rebuild an engine in the near future, but based on my
friend's explanation, and what we used to do after a major repair in the
shop, if I was to rebuild an engine I would do the following:
1. Pressure test the cooling system, look for puddles of coolant or oil under
the car, triple check that there is enough oil in the crankcase, etc,
and if everything checks out, start the car.
2. Run it for about 10-15 seconds, looking the oil pressure guage (or light
if that's all you have). If you don't get oil pressure within 10-15
seconds, I would turn it off.
3. If there was oil pressure, run it for about 30 seconds to a minute. Stop
the engine, top up the oil. It is surprising how much oil you have
to add to an engine that is completely dry.
4. After fixing the inevitable oil, coolant, and exhause leaks, run the
engine up to operating temperature, making sure that the choke is
turned off as soon as possible (the extra fuel washes the oil off
the cylinder walls causing extra wear).
5. Drive the car keeping revs as low as possible for the first 500 miles.
Change the oil at the end of 500 miles. Avoid short trips.
6. Allow somewhat higher revs during the next 500 miles. Change the oil
at the end of this period.
7. Rev the hell out of it from now on and have fun.
The only part that I feel is crucial is looking for oil pressure when you
first start the engine, and topping up the oil after running the engine
for less than a minute. The rest probably falls somewhere between religion
and a good idea.
Joe
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