Question: Balance pistons and rods - how close?

[OrangPeEL]

OK- "zero grams variance"...but in my DIY real-world how close should be enough (or, what's too much) for daily/spirited road driving?

Overall weights of my forged connecting rods vary about 2 grams; 1/2 gram difference in each. What would be a good target for end-to-end weight?

The 2.0L pistons are within 1 gram (3 are within 1/4 gram & 4th is about 3/4 gram heavy). Maybe weight pairing of new pins will offer opportunity to close gap, but where is best place to take <1 gram weight from stock Opel pistons for balance?

[opelnut10]

Originally Posted by OrangPeEL OK- "zero grams variance"...but in my DIY real-world how close should be enough (or, what's too much) for daily/spirited road driving? Overall weights of my forged connecting rods vary about 2 grams; 1/2 gram difference in each. What would be a good target for end-to-end weight? The 2.0L pistons are within 1 gram (3 are within 1/4 gram & 4th is about 3/4 gram heavy). Maybe weight pairing of new pins will offer opportunity to close gap, but where is best place to take <1 gram weight from stock Opel pistons for balance?

In-line 4 cyl. engines do not require bob weights when balancing the rotating assembly. CIH crankshafts are very close from the factory, in fact of 20 that I have seen balanced, only two would even light up the numbers on the balancer. Pistons and pins can be weighted together and if you get them within 1/2 gram (the wieght of a one dollar bill ) that is close enough for an engine that is going to be ran below 8000RPM, same holds true for the rods. If you are running the same flywheel that came on that crankshaft it was balanced at the factory, if not you can have it balanced also.

[opelwasp]

Just get the engine balanced at a shop. It only cost me $125. It is well worth the reassurance it was done right. They will spin the whole rotating assembly and balance it the right way. Makes for a super smooth revving engine.

[wrench459]

The biggest question is how good are your scales?
From the info provided you can pair the heavy(piston/rod) with the lightest and be good to go. Where going to assume the crank is ok.

But oh what the beep while its in pieces why not go all the way.
First you'll need a scale that measures into one tenth of a gram.
CLEAN and blow dry all parts even a small amount of dirt can be worth a 1/10.
Stack your rings into pairs measure and record. My experience there normally dead on. Next the pins..now they can be off by a few tenths.
Pistons are next.. only they can come up way out..even as much as 8 grams!!
If you get a set thats has one that out oh lets say 3 grams. Get another piston.
Ok now lets mix and match the rings,pins and pistons again lightest/heaviest and record. Take the light guy..thats our target to match the other three to.
One of the places on the piston to get her light is the pin bosses or the skirt ribs. That should take care of the top half of the reciprocating mass.
Next is the rods. Without a special fixture to measure the bigend and small.
Your limited to total rod weight.
Be consistent with how your placing parts on the scales.
And last the crank that of course will require a balancer to measure.
Don't even ask about race engines!

[OrangPeEL]

Originally Posted by opelnut10 ...Pistons and pins can be weighted together and if you get them within 1/2 gram (the wieght of a one dollar bill ) that is close enough for an engine that is going to be ran below 8000RPM, same holds true for the rods. ...

Thanks! .5 gram for below 8000RPM (below 7K is good for me). The donor is from a '71 auto- no flywheel options in-hand yet, but I'll take your experienced odds & go as-is on the crank.

[OrangPeEL]

Originally Posted by wrench459 ...how good are your scales? ...One of the places on the piston to get her light is the pin bosses or the skirt ribs ... Without a special fixture to measure the bigend and small. Your limited to total rod weight...

Thanks for reply! From your experience regarding lightening pistons, is .75 gram very much material to remove from an Opel piston (as in maybe just equally take it from the pin bosses at both sides?)

Scales are old-skool triple beam... 10th gram yes, but accuracy = as properly used (& I'm pretty OK there). I made a "thing" yesterday for my scale to weigh rods end-to-end...I'm reproducing measurements within .0-.3 gram now...I think it'll do better with different "big-end" pivot and lengthening the hang chains (probably still has some side load). Really just wasting time playing with it because rod bolts are not new; but it's definitely indicating which ends should be lightened & by about how much (without I probably would have just gone after all the cap pads in error)!

[wrench459]

I got some numbers for ya.
BE 493
SE 147
Total 640 grams
That should help you get into the ball park.
.................................................. .................................................. ...............................................
The above was from a stock forged (unmodified) rod. Yes I rounded the numbers off for example purposes.
You'll notice that the BE+SE=Total.
With that in mind..here's how I balance rods.
First get your total measurement I write the number on the beam with a permanent marker.
Next is the BE and this is where it becomes a pain in the butt.
Duck tape your fixture MAKE sure it don't move!
The BE and SE center line must be level, the BE adaptor needs to be a snug fit and the SE needs to float.
So we got the BE on the scales,level, and the wieght centered right?
Lift the small end to center the floating arm of the fixture several times and take a reading.
Don't be surprized if you get three different readings. Now might be a good time to average!
Remove and remount the rod several more times and you should start to see a pattern.
Crud forgive me I'm no technical writer and it shows.
After you get the BE the rest is down hill. I skip the SE and go straight to total.
Remember BE+SE=Total wink wink
Hope this helps