Dan Ray has asked about rebuilding his engine. Most of the list has
recommended doing the whole thing. I will offer my opinion on this as
well.
Almost everyone who spend time around the A asks what I do to keep it
running so cool. We have had a fairly wicked heat wave here lately with
temps in the 90's F and high humidity. During that time the A has run
around 185-190 in normal driving and up to 200 after 40 at 65-70 mph on
the freeway. It cools down right away once I slow down. I have done
nothing special to the car. I did replace the water pump this year as PM
but it didn't run any hotter or cooler with the Moss aluminum unit than it did
with the old cast iron pump that came out.
I believe that the reason it runs cool is that the motor is "loose." When I
replaced the rod bearings they Plastigaged towards the top of the
acceptable range. The bores had about .003" of wear and the standard
rings went in with the gap just over the maximum listed in the book. I
expect that the pistons are also a little looser than on a newly rebuilt
motor.
If you talk to racers, building an engine for power means setting the
clearances as loose as possible and still have the reliability to finish the
race. Sprint engines are looser than endurance engines, for instance.
This is because the tighter the clearance at the rod bearing, for instance,
the more friction and heat are generated there. Also, higher volumes of
oil through the bearings cool them more effectively. The down side is you
have less time to run the engine before it is too loose and lunches
something. In the A I also have lower oil pressure than typical although it
is high enough in my opinion.
So, the reasons that the A runs cools are 1) the engine is nice and loose
so the bearings and rings don't generate as much heat as they would in a
tighter motor. 2) I keep the timing as far advanced as I can without
causing detonation or kick back against the starter. 3) I run the A a little
rich. This make it idle funny when warm but otherwise it runs great and
keeps cool. Don't overdo this however since you can start washing the
cylinders and cause accelerated wear.
So, in answer to the original question, I think that the best way to figure
out whether a rebuild is needed is to see what the clearances are. As
long as they are within reason (Remember that the specs listed are for
acceptable range for a new engine. You can probably run a little looser
with no problems), then just replace the wear parts and go for it. For the
rings, you can figure out how much wear you have by comparing ring
gap just below the ridge and the bottom of the bore. Lacking a decent
micrometer, the best indication of main and rod bearing fit is the condition
of the journals. If they look in pretty good shape then they will probably
be OK. A little frosting in the center can usually be buffed out but if any
scratches will catch a fingernail then you should consider grinding the
crank. For the cam bearings, if you can see copper you need to replace
them. These are about the only thing you can't get to with the engine in
the car so you may want to do it anyway since they are not that
expensive.
I expect that the A's motor will last another 30k miles before it needs
attention to the bottom end. A full rebuild may have doubled that time but
then again 30k is 5 to 10 years based on how much I drive the A so why
worry. If all of the clearances had been brought down to minimum the
engine would have a longer theoretical life but it would also be more
prone to overheating. I believe that overheating has killed more MG
motors than bearing wear so I think I made the correct decision.
BTW- I believe that John Twist agrees with this approach (as far as the
rebuild goes) so I'm not the only crazy person out there.
I ran seven autocross runs this weekend plus 50 miles each way to and
from the meet. The A was the only British car out the that never belched
smoke or some vital fluid (other than a couple of 50 cent piece sized oil
spots). I made the last two runs back to back then shut down
immediately and the heat soak caused some fuel percolation but five
minutes with the (bent) bonnet up solved that. This is with 82k on the car
and new rings, rod and main bearings, cam, lifter and cam bearings, and
umbrella style valve seals with new exhaust valves/ original intakes
running in the original guides. I put in a new rocker shaft too. Nothing
was boiled out although I did my best to clean everything. What more
could you ask for?
Regards,
Bill Eastman
61 MGA still the daily driver.
PS- if I was to do it again I would have replaced the oil pump too.
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