Precision Measurement Supply - Fixtures, Tooling And Measuring Devices For Professional Engine Builders

Balancing Fixture Instructions

The PMS 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.  

It is 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.  

Below are 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).  

Rotating Weights:

Reciprocating Weights:

NOTE: 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.  

Another 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 & Bearings  

Before 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.  Note:  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.

For 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

This is 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

Zero the 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 end weight.

Rod Alignment

This is important for repeatability:

  1. Align the rod so it is level.  (Center of both platform fixture bearings and hanging bearings are the same height).

  2. 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.)

  3. 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.

  4. Align the hanging fixture with the threaded chain attachments to get it level side to side.

When the 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

  1. Remove bushing from fixture platform and place it on the hanging portions bearings just as you did with the platform.

  2. Record the weight of the platform without the bushing (this will be the weight you subtract to get small end weight).

  3. Check fixture alignment (it should need NO adjustment).

  4. 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


Completely balance the big end before even weighing the small end.

Pick out 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.)

Piston Balancing

Pistons 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.  


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