Connecting Rod Balancing Tool

I'm preparing to build an engine from scratch and one of the steps will be balancing the connecting rods.

There are professional tools available on market, but the design is not complicated, so I've tried to build one.

It's not super-perfect, but it should get the job done.

But why?

You have set of connecting rods for an engine and for the smooth idle and high RPM it's best if they are all within ~0,5g of each other. When you weight them all, you select the lightest one as a baseline for the rest.

Now the question is where to remove some material to mach rest of the connecting rods to the lightest one?

Some might be top-heavy, some might be bottom-heavy and again it would the best if they all have the same balance - this contraption allows you to check it. Depending on the results, you grind off some of the material from the bottom rod cap, or from the top of the small end.

Connecting rods are usually pretty closely matched straight from the factory nowadays, but this is an old Russian engine so it's not the case :)

It's only part of the process. Pistons with pins have to be weight-matched too. When you know the weights (including piston rings, circleclips, bearings...) then you can send the crankshaft, flywheel and pulley to the specialized machine shop to balance them too.

Tools used:

• Angle grinder
• Dremel
• Drill
• Belt sander
• Wood saw
• Hole saw

Parts:

• Cheep-o precise (2000g/0,1g) scale
• Six 608 ball bearings
• Piece 50mmx20mm of poliamid
• Sheet of duraluminium
• Plywood
• Wooden pegs
• Wooden plank
• Screws, bolts, nuts & washer

I've made this tool for one specific engine, so I didn't have to worry about making it universal.

When weighted, the connecting rod should hang horizontally, so some measurements were required for the location of the scale and the supporting column.

To keep the measurements repeatable, I've fixed the location of the scale with four wooden pegs.

I had to grind off some of the outer race of the bearings, so it will fit inside the connecting rod's small end.

A washer acts as a stop for the connecting rod placement.

I'll probably use double sided tape to keep this column fixed to the scale when times comes.

Slice of poliamid bar is used as a spacer for the big end.

I've used a dremel and a belt sander to make it fit.

Again, some measurements were required to make sure the connecting rod will hang horizontally.

Bearings are used so the arm of the support column can swing freely, otherwise side load might disrupt the weighting results. Two small wood screws hold the bearings fixed in the hole.

Remember to tare the scale after placing the support :)

Seems to work just fine, I'll practice on sam scraps rods first - it's easier to remove some metal, than to add some...

Next - piston ring grinder!