The only thing that is breaking in ,except for the cam and lifters, are the
piston rings. Rings do need loaded engine to push themselves against the
cylinder walls, then a coast at high vacuum to draw oil to the walls to wash
metal
particles away.
High RPM or excessive load is not desirable, they do nothing to promote ring
seal. Spirited stop and go city driving with no more than 80% throttle and a
couple of shifts to 40 MPH is the procedure I use for the first 10 miles.
After that it is normal driving with varying throttle and no sustained high
speed
for the next 100 or so.
On my 1/2 mile asphalt oval track race car ( 307 Olds motor ) I turn laps
with the motor no higher than 3500 for two 5 minute sessions and I consider the
motor broken in. After that it is normal race 4500 to 5200 RPM .
This ring breakin must be done no later than the first 30 min of operation,
if a engine is not loaded past that time the rings will take a very long time (
3,000 + miles) to seat if ever. Over heating a engine reduces ring tension
further extending the breakin time. In other words, the 3 hour no load running
on a fresh rebuild makes breakin academic.
Harold
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