Balancing Conrods

[andyhodg]

Hi All

I've been attempting to balance the conrods out of a ford engine. The conrods are125E 4.8 inch rods.

The conrods in question are set that came out of an engine and weigh as follows:

Number 1 538 grams
Number 2 542 grams
Number 3 531 grams
Number 4 539 grams

The spread on three of them is probably workable 538 to 542 i.e. 4 grams but the one in question is the 531 gram light weight?

When I put the rods with the gudgeon pin on a V block and measure big end weights they are:

Number 1 305 grams
Number 2 307 grams
Number 3 304 grams
Number 4 306 grams

A spread of 3 grams and this is where the motion is.

I can adjust number two at the big end by two grams which will bring it in line at the big end weight on all four rods within 2 grams but it will still leave spread on the overall rod weight chart of 9 grams. I have read that the big end is more important on balance but I have no idea as to what is an acceptable spread?

Any advice would be appreciated.

All the best

Andy

[AHM]

you an only do what you can do with the parts available.

Two comments on balancing:
1. It is a process of optimising rather than perfecting.
2. don't forget that you are balancing a system

Make sure your measurement method is repeatable - friction in the v block? placement of the big end on the scales?

Now weigh each component so that you know what you have and what is achievable.

You should be able to get all of your big ends to 304 grams (but 305 may be optimum you choose) now put the heavy piston with the light conrod etc

It is a lot of hard work.... but worth it!

[john.p.clegg]

Andy

When I did mine i measured the overall weight,big end and little end and in your case it looks like you may need to take some weight off the little end or the overall length....

You can balance the pistons also to match..
Don't forget to spend some time polishing,not only do they look good but helps prevent stress crax..

John

[andyhodg]

Hi John

From which area does one remove material from the little end? In my case I need to remove up to 8 grams of the little ends. Is this possible?

Best regards

Andy

[john.p.clegg]

First weigh the little ends just as you did the big ends to determine if it is from the little end or the shank,the stress on the con-rod is compression and so it is safe to "round off" the little end eye,the bush will keep the width..

John

[rcraven]

Do a few grams here and there matter? The forces on a crankshaft vary all the time in quite unmatched ways at any given time at different points along its length, the crankshaft itself twists, and it has the transmission etc at one end and the quite different forces of the timing mechanism at the other. Aren't all these differing forces going to swamp the effect of one conrod having a few grams more mass than another?

[rgh0]

With a 4 cylinder engine you have 2 rods and pistons going up versus 2 going down. If they weigh the same the primary forces and couples balance out and vibration is minimised

regards
Rohan

[rcraven]

But one of the pistons going down is being pushed by the exploding fuel/air mixture whilst the other is sucking in the mixture, and one of the pistons going up is compressing the mixture and the other is forcing out the exhaust, so won't each of the pair have greatly different forces being applied to it at any given time? So greatly different that a few grams makes no appreciable difference? And the rods are at constantly varying angles and distances from the centre of the crank..
I've heard it said a crankshaft really behaves like a piece of rubber, albeit fairly stiff rubber.

[john.p.clegg]

O.K. that seems logical.....so why not add a few kilograms to one or two con-rods at random...and maybe a few ounces to the pistons..say 2 ounces to piston No 3 and 6 ounces to piston No 1 and see how it behaves at 6000rpm??

John

Please let us know how you get on...

[AHM]

But one of the pistons going down is being pushed by the exploding fuel/air mixture whilst the other is sucking in the mixture, and one of the pistons going up is compressing the mixture and the other is forcing out the exhaust, so won't each of the pair have greatly different forces being applied to it at any given time?

Pistons conrods and crank are all attached so you may have an explosion on one and a vacuum on another, but the one pulling the vacuum is accelerating at the same speed as the one with the explosion. Force = Mass x Acceleration. So if the acceleration is equal but the masses are different the forces will be different and you will feel a vibration.


To simplify
In balancing you are only considering the masses within the system and the force required to accelerate them.
1 consider the components that only go round and round ie crank and clutch - balance them as you would a wheel
2 consider the components that only go up and down ie pistons if the mass of what is going up equals the mass of what is coming down they balance.

3 Now consider the conrods the big end is going round and round as much as the crank, and up and down as much as the piston. But the centre of the little end doesn't go round and round with the crank at all and goes up and down as much as the piston. So we need to consider them differently.
We can measure the weights of the big end and the little end.
The big end is attached to the crank so to keep it in balance we want to add 4 equal weights.
Now the little end weight is the result of balancing the big end and we want to even them up

4.The little end is also attached to the piston so at the centre of the little end we can take the weight of the Little end, pin, and piston. Take the conrod that is one gram under and put it with the piston that is one gram over.

The method of balancing is well defined in text, the reason for balancing is well defined in physics.

It takes a lot of effort to get every measurement within one gram, but every time you rev your engine you will be pleased that you put the effort in, rather than wishing that you had!

[john.p.clegg]

Yep,just like I said....

John

[rgh0]

The pressure forces on the engine are always balanced and thus don't cause vibration

e.g. the pressure force down on the piston is balanced by a pressure force up on the cylinder head

You need to match the weights and balance the moving bits so that the acceleration forces are also balanced.

cheers
Rohan

[brettengelaz]

Correct balancing is a very worthwhile task. Decreased stress, less vibration and possibly some extra cushion beyond the redline.. I vaguely remember a square boss on the top of the small end of the 125E rods that could be ground down for balancing.. (I have billet rods now ) The pistons may also be milled or taper bore the gudgeon (wrist) pin for fine tuning.. Oh, you may want to have the rods crack-checked or "MagnaFluxed", and replace the cap bolts with higher strength bolts like ARP or equiv..

Brett.

[AHM]

Yep,just like I said....

John

Yes! But you described the empirical method!

[rcraven]

A crankshaft twists and untwists as it rotates and the twisting varies along its length. Presumably this means that the pairs of cylinders moving in the same direction are not moving precisely in unison. Unless you know that the effect of the twisting is negligble compared to the gram or so you're removing from the rod and piston you can't know that this is doing any good. In fact if the twisting varies along the length of the crankshaft it may be beneficial for some rods and pistons to differ from others. Presumably dynamic balancing claims to deal with this.