BBC engine Balance

[55chevygasser]

Hi, this is my first post so I hope I do it right. I am building a 572 BBC. New Dart block, Wiseco pistons, Eagle rods and Eagle crank. Everything is new except the crank, I ran the crank in a 511 combo that was a balanced kit from Eagle. My question is do I need to rebalance the crank? I compared the weights of the piston,pin,locks,rings and rods from the 511 and the new stuff and the are exactly the same 1623.0, I did not weigh the big and small ends separately. The new rods are 6.535 and the old ones are 6.385. The new pistons are Wiesco vs The old Mahle which were heavier and the new rings are lighter .043 vs 1/16. So the total weight worked out the same but I don't know if the big end small end is a big deal or not and if Eagle does a good job on their balance kits. This is a race motor and I was turning the 511 to 7200. Thanks for any advise..

[MileHighMan]

Hi, this is my first post so I hope I do it right. I am building a 572 BBC. New Dart block, Wiseco pistons, Eagle rods and Eagle crank. Everything is new except the crank, I ran the crank in a 511 combo that was a balanced kit from Eagle. My question is do I need to rebalance the crank? I compared the weights of the piston,pin,locks,rings and rods from the 511 and the new stuff and the are exactly the same 1623.0, I did not weigh the big and small ends separately. The new rods are 6.535 and the old ones are 6.385. The new pistons are Wiesco vs The old Mahle which were heavier and the new rings are lighter .043 vs 1/16. So the total weight worked out the same but I don't know if the big end small end is a big deal or not and if Eagle does a good job on their balance kits. This is a race motor and I was turning the 511 to 7200. Thanks for any advise..

If it were mine I'd have it rebalanced.......Dan.

[SchmidtMotorWorks]

The big and small ends matter, try harder to weigh them.

[MadBill]

The rod length does not affect the balance. I'd go half way on the balance job: Have the rods weighed total and end plus check all piston/ring/etc. weights. Equalize them if necessary, but If the resulting calculated bob weights truly are within tolerance [or perhaps a percent or so higher (do a search for "overbalance" for discussion)] of the previous BWs, there's nothing to be gained by spinning the crank again.

[robert1]

If you correctly calculated the bob weight it doesn't matter what component weighed what. The bob weight is the bob weight. If it was 1623 and your new weight is 1623 nothing would change.

[Hanzeng]

If you correctly calculated the bob weight it doesn't matter what component weighed what. The bob weight is the bob weight. If it was 1623 and your new weight is 1623 nothing would change.

Yes but he didn't calculate the bobweight- he weighed the pieces flat on the scale

[55chevygasser]

Thanks for the replies. I got to thinking about it, since both set ups are using the same rods( except for length ) is there really any difference in end to end? The big ends are the same, I realize that as you suspend the rod further away from the end the weight will change somewhat but at only .150 extra length will it be enough to matter? With the rods being the same (except length) it makes sense that the small end with all it's components weigh the same as the old set up. When I ordered the new pistons and rods the weights they gave me added up the same as the old set up, but when I got the pistons they weighed several grams more than what they said. But I guess with the pins and thinner rings it all averaged out. I have an appointment to have it balanced in the morning but 200 dollars is 200 dollars. I don't know if my machine shop is going to be any better than Eagle. I live in an area that is somewhat performance challenged LOL. I had Dart do all the block work.

[rustbucket79]

Trust me, the "balanced" kits from the manufacturers are a very rough balance. Unless it isn't practical, you should have it balanced. I've rebalanced those balanced kits before, and found the difference at 20 grams or more out.

[MileHighMan]

Thanks for the replies. I got to thinking about it, since both set ups are using the same rods( except for length ) is there really any difference in end to end? The big ends are the same, I realize that as you suspend the rod further away from the end the weight will change somewhat but at only .150 extra length will it be enough to matter? With the rods being the same (except length) it makes sense that the small end with all it's components weigh the same as the old set up. When I ordered the new pistons and rods the weights they gave me added up the same as the old set up, but when I got the pistons they weighed several grams more than what they said. But I guess with the pins and thinner rings it all averaged out. I have an appointment to have it balanced in the morning but 200 dollars is 200 dollars. I don't know if my machine shop is going to be any better than Eagle. I live in an area that is somewhat performance challenged LOL. I had Dart do all the block work.

My opion is if you can't trust your machinest,,,,you need to look for one you can.....Dan.

[55chevygasser]

Trust me, the "balanced" kits from the manufacturers are a very rough balance. Unless it isn't practical, you should have it balanced. I've rebalanced those balanced kits before, and found the difference at 20 grams or more out.

Thanks that's what I wanted to hear.. I will keep my appointment. I am more concerned about the quality of the Eagle balance than the new components. www.dthotrods.com

[robert1]

If someone specifies a bob weight I give them credit for figuring it correctly. If he knew the weights of the old rods and the new rods come with a spec sheet he could calculate the weight. As Jon said the big end is going to matter more than anything else in the caculation. Get it balanced and you wont have to worry about it. I recently finished a Briggs style flat head. Nobody balances these motors so I just went together with it. I called several of the guys doing JR dragster motors and they all said you are wasting your time they don't need it. Now I'm going to pull the new one apart and another one I built to balance them. These turn just over 10,000. By the way JR race cars quoted me $400 ea to balance 2 one cylinder Briggs assemblies. I would rather they just told me we don't want to mess with it.

[justahoby]

not to hijack the thread, but i have a related question. Over a year ago was trying to weigh a big and small end of a rod without a jig. Now I would get totally different readings even at the slightest different angle of what appeared to be even. Now was just curious if there was a trick to checking rod end to pin end without one, on just a scale? BTW i took it in anyway to be balanced as was only $180

[Hanzeng]

not to hijack the thread, but i have a related question. Over a year ago was trying to weigh a big and small end of a rod without a jig. Now I would get totally different readings even at the slightest different angle of what appeared to be even. Now was just curious if there was a trick to checking rod end to pin end without one, on just a scale? BTW i took it in anyway to be balanced as was only $180

There's not really a 'trick,' it's just that the rod need to be held as level as possible with frictionless holding points. Friction and out of level rod results in inconsistent readings as you found. A fixture is quite easy to make or jig up if you look at a commercial fixture. Hanging the outboard end of the rod from some type of string or chain removes the friction, as does a small roller bearing in that end.

[SchmidtMotorWorks]

Nobody balances these motors so I just went together with it. I called several of the guys doing JR dragster motors and they all said you are wasting your time they don't need it.

You received good advice, balancing a single cylinder engine is a waste of time for any standard engine, the reason for this is there is no bob weight for a single cylinder engine that will eliminate vibration or bearing loads, it can only change the direction that it vibrates.

[MadBill]

I suppose with a long, flexy chassis (assuming JDs use same), 100 percent balance would result in less vertical shake that might be more noticeable than the fore/aft result from zero %, but wouldn't 50% be the best compromise to minimize bearing loads? Also, I wouldn't think it could be assumed that 'as received' the crank was in dynamic as well as static balance, i.e. counterweights not disposed equally relative to the con rod, leading to uneven bearing loads and perhaps increased crank flex? (Of course, it's probably not that simple...)