At 01:13 AM 8/7/02 -0700, Larry Colen wrote:
>Somewhere along the line I was told that it's bad to idle a car for too
>long while the engine is being broken in. Why is this?
>
>Also, does anything besides the rings get broken in when an engine is new?
>I've been told that there's no need to break in new bearings, and nobody
>has ever mentioned breaking in new valves.
Break in time for the crankshaft bearings is about twice as long as it
takes to bring up the oil pressure (zilch). Reason is that ideally the
bearings never touch the crankshaft, always ride on a thin film of oil. If
that were truly the case, then the crankshaft bearings would last
forever. The primary reason the crankshaft bearings wear is lack of oil
pressure on startup after sitting for a long time. For a new engine,
assembly lube (or lots of oil) is important.
For a new valve job, assuming new valve guides, and the valves and seats
are precision ground, or they are properly lapped in, then break in time
for the valves is about as long as it takes to bring the coolant up to
running temperature (also zilch).
Break in time for cylinders and piston rings can vary considerably
depending on how the cylinder walls were treated. If the cylinders are
worn somewhat oval and you just installed new rings on the old pistons,
honed cylinders or not, all bets are off and the rings may never seat to
the walls. If the cylinders are still round within spec, and they are
properly honed (not perfectly smooth), then the ring break in time might be
anything from a few hours to a few thousand miles, depending on how rough
the honed surface finish and exactly how round the cylinders. If the
cylinders were rebored and are perfectly round, and the honing has a
moderately fine finish, then break in time might be as little as a few
hours, but 500 miles is a safer generalization. If cylinders were rebored
and perfectly round, and the shop opted to "soft hone" to finish the
cylinders, then break in time for the rings could be less than 30
minutes. Soft honing is like a polishing process that gives a near mirror
finish to the walls. This is absolutely not a good thing to do if the
cylinders are not perfectly round, as the rings might then take a very long
time to seat (if ever).
The greatest concern for break in time and technique is for a new camshaft,
which will of necessity also include new tappets. The new cam should be
generously greased during assembly (or use commercial assembly lube) to
provide good lubrication on startup until normal running oil circulation
can be established throughout the engine (which can take a few minutes of
running time). On startup the engine should be run at 2000 to 2500 rpm for
at least 20 minutes to get the tappets seated properly with the cam
lobes. This is usually done with no load while sitting in the garage, but
you could drive it as long as you keep the engine speed to a minimum of
2000 rpm. Reason for this is that the new tappets are slightly crowned on
the bottom end and are mounted slightly off center from the cam lobes to
make the tappets rotate when running so they do not develop flat spots on
the bottom. If the engine is run at slow idle for a while on initial
startup, it is very likely that the new tappets will not rotate and will
immediately develope flat spots or small indentations that will prevent
them from rotating in the future, which can lead to very premature camshaft
failure (like as early as 500 miles).
There is an idea that one should not run an engine at a constant speed
during the break in period. This is more of a concern with an engine block
which is a "green" casting (brand new engine block), and not so much a
concern with an engine rebuilt with an old engine block. The concern there
is that a new engine block with residual internal stress in the casting may
distort slightly from thermal stress. Running at constant speed and
constant load has a tendency to concentrate the thermal stress in local
areas of the casting, while running at variable speed and load will
distribute the heat load better throughout the part to minimize such
distortion. The instruction is generally stated something like, "Run
engine at various speeds and load during the first 500 miles of
operation". But for a rebuilt engine, that particular first 500 miles
occured many years earlier and should be of no concern now.
Another common admonition is that you should not "lug" an engine (run at
very low speed and high throttle) during the break in period. There are
two concerns here. Keep the engine speed high enough to maintain good oil
pressure, and minimize the side load on the pistons while the newly honed
cylinder walls are still slightly rough.
Bottom line is, after the initial camshaft break in run you can go ahead
and drive it a normal manner, just take it easy on the throttle and keep it
under the yellow line on the tach for a few hundred miles.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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