Balancing Fixture allows you to do all of the time-consuming and
therefore expensive work all yourself.
You can now take your crank, harmonic balancer and flywheel or
flex plate, to the machine shop, along with the proper weights
obtained by using the fixture …. And then all the machinist has to
do is attach the proper bob weight, spin the crank, and add or remove
weight. This saves him
time and you money. Another
big advantage to using the PMS Balancing Fixture is the fact that you
can now duplicate any part in the engine without having to re-balance
the entire assembly – now you will have a record of all the weights.
necessary to briefly explain the balancing procedure in order for you
to correctly use your fixture. When
balancing an engine, there are two categories of weight to contend
with. These are referred
to as “Rotating” and “Reciprocating Weight”.
Rotating weight is the weight that spins in a circle.
Reciprocating weight is the weight that goes up and down.
This is pretty simple until you get to the connecting rod which
does both. The big end of
the rod spins in a circle, while the small end goes up and down.
It is essential to determine how much rod weight rotates (big
end), and how much reciprocates (small end).
The PMS Balancing Fixture allows you to do this.
the weights you must provide the machinist in order for him to balance
your motor (along with the crank harmonic balancer, flywheel or flex
plate if externally balanced).
Small end of the rod
Piston weight (in
Piston pin weight (in
Piston pin locks (if
floated pins are used)
Ring weight for one
There is one other weight involved.;
This is the weight of the oil that clings to the components
when the engine is running. The
machinist will estimate this weight.
decision you will have to make is whether to “Over Balance” which
is the addition of more than the customary .50% of the bob weight.
Over balancing is an often argued subject.
Some feel it adds to bearing life and high RPM smoothness.
Personally, I don’t feel it provides anything but good bench
racing material. But you
will have to decide.
Use of the Scale and Fixture:
Weighing Piston, Rings, Pin &
you can weigh anything, you must “zero the scale”.
This is done in two ways: For
micro adjustments, use the thumbwheel on the side of the scale.
For large increment adjustments, use the screwdriver slot on
the bottom of the scale. The
scale can be run on 110V with the adaptor or with a 9V battery.
We have found that the scale needs clean electricity when using
the adaptor. If you are
having trouble getting the scale to zero, try using a battery.
weighing the above components, you need only the scale.
Simply place the scale in the gram mode, zero the scale, weight
the components, and record the weights.
Weighing Rod End for End
what the fixture is for. Place
the fixture over the scale platform (the roll pins will secure it).
Record the weight of the fixture platform.
Note: This scale does
not have a TARE feature which would allow the scale to be zeroed with
the fixture platform in place.
So, you must ALWAYS subtract the fixture platform weight from
the total weight. Example:
If the fixture weighs 279 grams and you get a reading of 520
grams with the small end over the scale, your small lend weight would
be 520 minus 279 which would be 241 grams.
Also, this weight will naturally increase as you place the
bushing over the platform fixture to weigh the big end of the rod.
Weighing the Big End of the Rod
scale with nothing on it. Place
the fixture platform over the scale platform and place the appropriate
bushing (the one that best fits your rod bore) over the two bearings
on the scale platform. The
ball plunger in the bushing will secure it to the fixture.
Record the weight of the fixture platform with the bushing
attached – this will be the number you subtract to get actual big
important for repeatability:
Align the rod so it is
level. (Center of both
platform fixture bearings and hanging bearings are the same
Align center to center
distance so that the chain hangs straight and is not in a bind
either for or aft. (This
alignment is done by sliding the scale in the milled tracks.)
Align the two bearings
of the platform and hanging fixture so they are in a straight
plane when viewed from the top.
(This is done by loosening the large clamp and sliding the
5/8” bar with the chains on it either in or out.
Align the hanging
fixture with the threaded chain attachments to get it level side
fixture is properly aligned, place the big end over the bushing and
the small end on the hanging portion of the fixture.
Record the total weight on the scale and subtract your fixture
and bushing weight from this number to arrive at the big end weight.
Repeat on all eight rods – be sure to record and identify
each rod weight.
Weighing the Small End
Remove bushing from
fixture platform and place it on the hanging portions bearings
just as you did with the platform.
Record the weight of
the platform without the bushing (this will be the weight you
subtract to get small end weight).
Check fixture alignment
(it should need NO adjustment).
Weigh and record weight
of all small ends (again, be sure to identify the correct weight
with the correct rod.
Balancing Rods End for End
BALANCE THE BIG END FIRST!
balance the big end before even weighing the small end.
the lightest rod and remove weight from the balance pads to bring all
of the rods down to the weight of the lightest.
(Use a belt sander for this.)
Balancing the Small End
Pick the lightest small end weight
and remove weight from the others.
Balance pad to bring them all down to this weight.
(Just as you did the big end.
Again, a sand belter works very well for this.)
are usually pretty close. Place
the piston in a piston vise on a mill table and remove weight with an
end mill from the balancing pads under the pin bosses.
Note: Leave pin
bosses AT LEAST .125” thick, or distortion will occur.