Piston/Pin balancing
 

I was going to try Wayne's poor man's approach to balancing the pistons by mixing and matching the pins with the pistons.
Anyone got any good ideas on what scales I can use, buy or borrow?
We have an inspections department but bringing contaminated parts into a medical device company, well the FDA would $hit.

I don't think digital postage meters go out enough digits. So the one in the mail room is out.
Short of a trip to the Police Departments' evidence cage I can't think of anything free to use.
There must be something cheap out there.

Does your company ever upgrade? perhaps they have an old set of scales you can get hold of?

Try your local machine shop?

-Wayne

I used a postal scale and found one rod 2 grams heavy. I lightend it down one gram, got it magnafluxed. I paired it with a piston that was 1 gram lighter. Not what I call aerospace accuracy but much better than 9 grams tolerance.
The heavy rods went more towards the CG of the car. : )

Those parts are so good that the pins and piston swapping was the easy way to do it.
I've heard High School science beam scales are awesome.

I got my scale from The Variety Shoppe (http://www.varietyshoppe.com/). It's a "My Weigh i1200". It's good to .1 grams and weighs up to 1200gr. About $120.
-Chris

Pistons and pins are about the only pieces you can measure this way. The con rods must be weighed with a jig to measure big vs. small end (not difficult to fabricate, btw), and the crank and other rotating pieces must be ... rotated at speed. I guess if you amortize the cost of the scale it'd be worth it, but what is the current labor charge for all the pieces at a proper balancing shop?

This sounds like a Car Talk, Stump-the-Chumps question. "Can you find the one heavy piece out of 21 pieces with only three shots at a scale?" (or something like that).

Something to consider,
Sherwood

Two scales would probably work...

-Wayne

Two scales would probably work, but then you would have to have access to them, or purchase 2 x $120.

Sherwood

Just one scale and a jig should work. Here is one from ProformParts
-Chris

All you need is one wrist pin and a couple of vee blocks tall enough so when you weigh the small end the big end doesn't touch the bench surface.

Any scale that can weigh in one gram increments is good enough to balance an engine.

The best proceedure is to first balance the big ends, ie make all the big ends weigh the same. To do this the rod must be exactly horizontal, the small end supported with a near zero friction pin and the big end supported by the scale. All material is removed only from the big end.

Once the big ends have been matched, the small end is used to make the total weight of the rods equal, ie only remove material from the small end to finish the balance. The small end does not need to be put in a horizontal position like the big end was, just the total weight of the rod measured. The reason you can do it this way is clear if you do the math.

instead of or in additon to machining the small ends for balance you can use the wrist pins to acheive the desired result. This is done by removing material from the insides of the wrist pins and or mix and match. In any case the wrist pins must be included in the balance.

The piston weights are simply matched. Again the wrist pins could be used to help achieve the overall balance.

TO summerize:

All rotating (big ends) weights must be equal. All reciprocating weights (piston plus wrist pin plus rings plus small end) must be equal. All total rod plus piston weights must be equal (which it is by definition if you do the first two).

Unlike a V type engine there is no need to come up with bob weights to balance the crankshaft. Opposed type engines and in line engines can be balanced independantly of the other components. SO if your balancer says he needs your pistons to balance the crank, say to him " Come again?"
YOu do need a special machine to balance the crank and flywheel. The cost is so low that it is a no brainer, just pay someone to do it.

That jig looks more expensive than the 2nd scale.

-Wayne

Oh probably. I think you could make a jig easily though.
-Chris

THe jig could be as simple as a 6 penny nail thru a small block of wood and some leftover magazines to adjust the height.

I'm in the process of doing this now using a postal scale which works great. Does anyone have a feel for the greatest amount of difference you sholud allow between piston/pin/rod
conbinations??

PS: how do you remove wrist pin material - that is pretty tough metal!

I do believe you should put the heaviest towards the flywheel.

RE wrist pin material removal.. Die grinder and small stone. From the inside only.

I don't know of a reason to worry about which end of the wrist pin metal is removed from. The mfgs do it from one end. Look for a step in inside diameter. Also be careful with thin wrist pins, try to reduce the inside dia just a little and go deep for more.