Balancing the rods

[Ed]

I'm rebuilding a F54. I took it to the local machine shop to get it hot tanked and all cleaned up (looks good). Anyway I asked about getting the crank, rods, and pistons balanced. He told me that he couldn't do it because he couldn't separate the piston from the rod without risking damage to the piston (because the wrist pins are pressed into the small end of the rod). Has anyone balanced the piston and rod assembled?

[2ManyZs]

I think you need to talk to another machinist. Unless there is some kind of damage that makes them unuseable, there is no reason he can't remove the pistons from the rods. If he is afraid of these, he probably is not the person you want doing the work.

You can possibly balance both at the same time, but it is going to be a bit more difficult and time consuming for the machinist that is doing the work. I guess it would depend on whether they have a scale that will go high enough to give the weight of the combo instead of each piece individually.

[hmsports]

I agree with 2Many that the wrist pins are an easy press out. Any machine shop the deals with performance engines should be able to do this no problem. Sounds like the local shop used that as an excuse.

...and I don't think you could get a valid balance without taking the pistons off the rods.

[Ed]

The machinist I took it to has over 20 years experiance on american engines. He said that the boss on the side of the piston is too small to properly support the piston and you might take a chance at cracking piston.

[2ManyZs]

Ed, you just answered the question yourself. He may have 20 years experience on American engines, and know everything there is to know about them. What you need is someone that knows imports. Any competant import shop will be able to do this with no problem at all.

[Phred]

All info above is true. However, even with the proper fixture, the possibility of damaging a piston exsists. If your willing to spend the money to balance your engine, consider replacing the pistons too. After all, at minimum, it needs to be honed, and rings replaced. Pistons wear out like everthing else. they might look ok,

but the skirt can wear down slightly, increasing pist/bore clr.

Also, more important to horsepower, are the ring lands. If they have any significant wear, which is common with a used piston, performance goes down. Also the piston pin bore will wear, especially with a pressed in type. If your budget will allow, replace the pistons too.

Balancing Rods: (never when connected to a piston)

Rods are balanced rotationally (big end) and reciprocating(small end) The combination being total weight.

First, the caps are torqued on without bearings. They are hung on a fixture which supports the small end off to the side of the scale. The big ends are all weighed and recorded. Then The heavy rods are ground or machined on the cap till they weigh the same as the lightest one. Then the complete rod is weighed. Again the heavy rods are ground or machined to match the lightest one, but this time all weight is removed from the small end, usually around the top of the pin bore. Presto! a balanced set of rods. A good bal. job will have a +/- .5 gram tolerance.

The pistons are weighed individually with the pin. As with the rods, the heavy ones are machined till they weigh the same as the lightest. This is usually done as an assembly, which would include balancing the crank, flywheel, clutch, and damper. Good luck!

Phred

[Ed]

Thanks for the good info. I don't think I can afford new pistons right now. And I guess I can't really get the engine balanced because the Flywheel that I plan on using is on my car still (long story).

I am replacing the rings, bearings, seals, gaskets etc...(boy it really adds up). Would you guys happen to know the proper ring gap for this engine?

I was planning on getting a digital scale and balancing the piston/rod assembly myself. Theoretically it does seem possible. I mean the small end, pin and piston all rotate as an assembly. Why not weigh them as such.

[hmsports]

I think the problem with balancing them all together would be that you would be guessing which component to remove material from. You may be removing from the piston to get things balanced when it is actually the rod that is causing the difference in weight.

There is a really good book on building performance nissan engines (I'd have to check tonight the exact name) that explains the balance process for stock, mildly modified and killer engines. It explains where and where not to remove material from each component so as not to weaken things.

[Ed]

When you put it that way it does make more sense.

[Sleeper78Z]

I have a question I have never seen addressed regarding engine balancing. If you can make each rod weigh the same, big and small end, and make the pistons all weigh the same how can you consider this balanced?

For a boxer motor, you would have equal weights opposite each other, but an inline motor has all this weight countered by the crank counterweights.

So in order to balance an inline motor, is it not necessary to match the piston/rod assembly to it's respective counterweights on the crank?

Edit: just writing and thinking about this, I suppose if the crank throws are balanced by the counterweights, and your piston/rod assemblies are equal weight, that comprises a balanced rotating assembly? Hmmmm, cool....

[2ManyZs]

Actually, all crankshafts may not be balanced as well as they should be. It should also be balanced separately. Once it is balanced and the rods and pistons are balanced, only then is it truly a balanced assembly.