TheSamba.com :: View topic - Engine builders, why not balance rods w/bearings?

Mon Apr 26, 2010 11:06 am

Hello,

I always wondered why when balancing a connecting rod, the bearings shells are never in the lower (big) end. Why is this? The shells most definitely contribute to rotating mass.

Mon Apr 26, 2010 11:52 am

Probably because every bearing shell is virtually identical to the next one of the same stock #.

If there is a weight difference it would be so extremely small.

I am assuming that you are talking about balancing by weight.

dave

Mon Apr 26, 2010 12:17 pm

Right, I mean weight balance. I have a 1000 gram scale I an going to use for the pistons and connecting rods.

Mon Apr 26, 2010 5:23 pm

If you really want to be super accurate, just install the con-rod bearings then weigh each rod and balance accordingly.

But I too think the difference will be negligable. And, say you balance them now with the bearings in, and rebuild the engine at a later date using new bearings, you will have to balance again since you are installing different bearings.

Mon Apr 26, 2010 5:30 pm

I'd be very surprised if the bearings were off by any more than 1 gram.

Mon Apr 26, 2010 8:09 pm

03OrangeSVT wrote: I'd be very surprised if the bearings were off by any more than 1 gram.

i'd be shocked silly if they were off by a gram.... the tolerances on bearings are tight enough that if you have a gram difference from one to the other, you have a different size... :lol:

Mon Apr 26, 2010 8:15 pm

While it's a legitimate question, I think we can get carried away with perfecting balancing, in theory. What about when oil is being flung onto and off the rods (especially with H-beams)? Doesn't that add a few grams at certain stages of rotation?

Mon Apr 26, 2010 8:54 pm

neil68 wrote: While it's a legitimate question, I think we can get carried away with perfecting balancing, in theory. What about when oil is being flung onto and off the rods (especially with H-beams)? Doesn't that add a few grams at certain stages of rotation?

an excellent point, however getting things as close as humanly possible before assembly certainly doesn't hurt. It depends on who you ask. Builders like Jake Raby and Nick Sison insist on balancing and indexing every component that goes into an engine. If nothing else its peace of mind and knowing that all was done to ensure a smooth running, long lasting engine.

Mon Apr 26, 2010 9:13 pm

what about the rod bearings wearing and changing weight?

Tue Apr 27, 2010 12:56 am

and piston carbon...

Tue Apr 27, 2010 5:31 am

Just a thought on parts that wear and parts that are reused.

For parts that do not wear: pistons & rods. Once you have them balanced they shouldn't change. So balance them once and keep them together as a set. Check them before you re-use them.

For parts that wear: bearings. Balance them separately against each other and install them. When you next need to replace them, balance the new replacements.

Why you don't want to balance with bearings:
If you had a set of unbalanced bearings installed in a perfectly balanced set of piston & rods, and you wanted to balance them... You would see one (or more) rods as heavy (because that bearing is heavy). So you grind some metal off the rod or piston. Now they are all balanced.
During the next rebuild you install new bearings and find a different rod needs to be ground down to balance the set.
If you keep this up, you will grind down all your rods until they break! (just exaggerating!)

Better to just balance the wear items separate from the non-wear items.

Tue Apr 27, 2010 11:25 pm

ashman40 wrote:

For parts that wear: bearings. Balance them separately against each other and install them. When you next need to replace them, balance the new replacements.

Why you don't want to balance with bearings:
If you had a set of unbalanced bearings installed in a perfectly balanced set of piston & rods, and you wanted to balance them... You would see one (or more) rods as heavy (because that bearing is heavy). So you grind some metal off the rod or piston. Now they are all balanced.
During the next rebuild you install new bearings and find a different rod needs to be ground down to balance the set.
If you keep this up, you will grind down all your rods until they break! (just exaggerating!)

Better to just balance the wear items separate from the non-wear items.
That is a complete waste of time.

Your rod bearing shells will be IDENTICAL in weight straight from the manufacturer. That's because every dimension on them is controled to tight tolerances. If you think one shell will weigh more, at least one dimension must be way off. They aren't.

This is why nobody includes the piston rings in the balancing recipe. Again, every dimension is controled to a tight tolerance. Thus all the top rings will weigh exactly the same. And the second ring, and the oil rings....

Same goes for the wrist pins.

Wrist pin circlips too. Or your teflon buttons.

There's a reason why the engine balancing shops don't ask for all that JUNK! It doesn't make a damn bit of difference.

Wed Apr 28, 2010 6:33 am

Bruce wrote: ashman40 wrote:

For parts that wear: bearings. Balance them separately against each other and install them. When you next need to replace them, balance the new replacements.

Why you don't want to balance with bearings:
If you had a set of unbalanced bearings installed in a perfectly balanced set of piston & rods, and you wanted to balance them... You would see one (or more) rods as heavy (because that bearing is heavy). So you grind some metal off the rod or piston. Now they are all balanced.
During the next rebuild you install new bearings and find a different rod needs to be ground down to balance the set.
If you keep this up, you will grind down all your rods until they break! (just exaggerating!)

Better to just balance the wear items separate from the non-wear items.
That is a complete waste of time.

Your rod bearing shells will be IDENTICAL in weight straight from the manufacturer. That's because every dimension on them is controled to tight tolerances. If you think one shell will weigh more, at least one dimension must be way off. They aren't.

This is why nobody includes the piston rings in the balancing recipe. Again, every dimension is controled to a tight tolerance. Thus all the top rings will weigh exactly the same. And the second ring, and the oil rings....

Same goes for the wrist pins.

Wrist pin circlips too. Or your teflon buttons.

There's a reason why the engine balancing shops don't ask for all that JUNK! It doesn't make a damn bit of difference.

For once we agree...besides,the closer the item to be balanced is to the axis of rotation,the more tolerant it is of deviation. Balanced pistons,then rods,then crank,in order of importance.Rod bearings are captured between to tightly controlled tolerances...the rod bore and the crank journal and by virtue of design almost impossible to get out of balance with any other rod bearing,along with its proximity to the axis point.

Wed Apr 28, 2010 6:46 am

johnnypan wrote: .besides,the closer the item to be balanced is to the axis of rotation,the more tolerant it is of deviation. Balanced pistons,then rods,then crank,in order of importance. Rod bearings are captured between to tightly controlled tolerances...the rod bore and the crank journal and by virtue of design almost impossible to get out of balance with any other rod bearing,along with its proximity to the axis point.

Johnny described this accurately, and very well. Like torque, or balancing a tire, the distance from the center point makes a world of difference.