Many thanks to Bert Palte and everyone else for all the info that
helped me decide to have it balanced. I took my crank, rods, new
pistons, flywheel, pressure plate, and front crank pulley to Dennis
Wells Racing Engines in Duncanville, TX (a south suburb of Dallas). I
looked his shop over, was impressed with the cleanliness and the number
of engines they were working on. Left the parts there on Wednesday,
picked them up on Friday. Cost was $120, that included pressing out the
wrist (gudgeon) pins and re-installing the new pistons on the con rods.
Being curious, I asked and they showed me how the balance was done.
1. All pistons were weighed. Metal was removed from the inside
surface, under the top, of the heavier ones to get all weights equal.
2. The rods were weighed with the big end on the scales and the small
end (wrist pin end) supported on a pivot above the scales so that the
rod was in a horizontal plane. Metal was shaved from the very bottom of
the rod caps to make the big ends match in weight. Then the rods were
weighed again, this time fully on the scales, and the weights were
matched by removing mass from the small end. This gives equal weight
and equal mass distribution.
3. The flywheel and pressure plate were balanced separately, then
together, then mounted to the crank. This was then placed on a computer
balancer, much like a computer wheel balancing rig. The assembly was
spun at 500 rpm. The computer told the machinist the angle of rotation
and which end of the crank to remove (or add) mass. Removal is by
drilling the counterweights. Adding mass (didn't have to on mine) would
have been done by depositing a welding bead with a MIG welder. Since
the rod journals are diametrically opposed on a 4 cylinder, the spinning
can be done without hanging weights where the rods would connect to the
crank. They tell me it is quite different on a V-6 or -8 engine,
becomes a lot more complex.
Bert had mentioned that MG cranks were some of the most imbalanced that
he worked on. I got lucky, the machinist said the crank was actually
pretty close to right, didn't have to remove much mass. The rods were a
different story - were several ounces in variation. The new pistons
were fairly close.
I don't expect this to prevent any failures; maybe it wasn't necessary
to have it done. John Twist said he has never worried about balancing
and that with hundreds of MGA and MGB rebuilds, he never had a failure
that could be attributed to imbalance. I would have to agree, after
all, my engine had over 84,000 miles with original pistons and bearings.
My goal is to have an engine that idles a little smoother and revs a
little higher without shaking bits and pieces off. Other than the money
spent, it can't hurt.
Again, thanks to all for their responses to my original question. It's
done, and I'll let you know in a few weeks how the final result turns
out.
Wayne Kube
'79B
Plano, Texas
|