Breaking in a newly rebuilt engine is something that, like the proper choice
of Traveling Music, underwear color and beer temperature can be argued about
for some time, with little or no persuasion from one viewpoint to another.
I think one should not be overly fussy about it all, but keep a few things
in mind. What you usually have with a rebuilt engine is a unit with NO wear
on the cylinder walls, rings, possibly cam lobes and lifters, maybe the
rockers, etc. One needs to keep this in mind. What I would recommend, with
absolutely no provable basis in facts is the following.
1. Once installed and ready to go, start the engine and let it run at a
fast idle, say maybe around 2000 rpm, no load, for 20 or 30 minutes.
If it starts leaking water, try to keep it running and add water to the
cooling system. If it starts leaking oil, you may be able to try the
same thing, but with the oil added to the proper place. If it leaks
badly enough to lose pressure, shut it off and fix it, repeating as
necessary. Shut it off, let it cool a bit then drain and replace the
oil and filter. Fix the water leaks also.
This first step does a few things. It lets the engine run long enough to
get all the parts up to operating temperature, long enough to put enough
stress on various gaskets and such so that any poorly assembled joints will
start leaking and any poorly assembled parts may fly off at this time. It
is a chance to make sure the thing will work once again. By letting it cool
a bit and then changing the oil and filter, you increase the chance of any
metal shavings and machining residue to actually have gotten into the bottom
of the sump or the filter before changing. 30 minutes at a fast idle should
have certainly gotten enough oil flowing to flush out any residue in the
rod and crank bearings, cam bearing surfaces, etc.
2. Assuming the engine didn't spray any vital fluids all over you and did
not toss any of its vital parts through the garage wall, pretend that
it is now ready to use as normally.
You can just drive like you normally would, though it may be prudent to not
be too rough on it. Don't take it to redline under full load, but do let it
rev. If you don't get the rings seated through the full range of stretch up
to (and including) redline, then you could have problems later on. I have
read of the possibility of engines developing a ridge at height X in the
cylinder wall while never running over Y RPM, and then the first time that
the thing is taken to Y+500 rpm the rods stretch enough to put the rings at
X+delta. Well, the engine tries to put the rings there, but the aforementioned
ridge sees to it there will be none of that. At which time, the rebuild
procedure is reiterated. Never has happened to anyone I know, so such a
scenario may be all myth.
Still, I have to wonder why one would have to be so cautious as to never exceed
such and such a percentage of redline during the first N miles, then another
percentage for the next M miles, and so on. If the engine is going to be run
in a range from idle to redline during normal use, one should make some sort of
effort early on to see if it really will run to redline without blowing up.
But of course there are folks who think that the redline is some unobtainable
magic point, and never rev the engine that fast anyway, no matter how many
miles may be on it. Poor folks.
Oh well, enough unsubstantiated ramblings for now.
mjb.
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