Connecting Rod Balancing Tool

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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.

Step 1: Parts & Tools

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

Step 2: Base

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.

Step 3: Scale Column

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.

Step 4: Spacer

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.

Step 5: Support Column

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.

Step 6: Testing

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!

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    3 Discussions

    0
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    tytower

    4 weeks ago

    So this looks pretty good . I do not understand what you are doing however. If the rod hangs perpendicular then x amount of weight is on the scale?
    Then you compare each rod in the same position it should weigh the same ?
    Why not just weigh the whole thing and compare each?
    I see that you get different weights for either end but does that mean that you remove metal on the weighed end to get them all the same ? Won't the harmonics be affected by where you remove metal?

    2 replies
    0
    None
    Rzooqtytower

    Reply 4 weeks ago

    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, then you can send the crankshaft, flywheel and pulley to the specialized machine shop to balance them too.

    0
    None
    tytowerRzooq

    Reply 4 weeks ago

    Good stuff but you must be building a 10,000 RPM machine for those weight tolerences., Good luck with it . I hope it roars!