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New BBC pistons 60g lighter need rebalancing?

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New BBC pistons 60g lighter need rebalancing?

Postby Harleyoz » Wed Nov 24, 2010 4:46 pm

I'm just in the process of rebuilding my 427 & with the freshen up. the engine is being overbored from 0.030 to 0.060 & fitted with new pistons. The old 0.030 TRW's are 60g heavier than the new JE 0.060 pistons (with pins). The same rods are being used (scat H-Beam) as is the steel crank, fluid damper & flexplate.
The entire rotating assembly was balanced last time & is still in mint condition, so other than the rebore it's all the same. Obviously I will have the pistons checked for piston to piston weight varience
My question is will the whole assembly need rebalancing with a 60g piston weight change? would save a lot of hassle & $$$ if not
thanks!
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Re: New BBC pistons 60g lighter need rebalancing?

Postby Alan Roehrich » Wed Nov 24, 2010 5:13 pm

Either re-balance, or get 60 gram heavier pins. Check the weight of the rings as well.
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Re: New BBC pistons 60g lighter need rebalancing?

Postby dfree383 » Wed Nov 24, 2010 6:04 pm

rebalance the stuff... its not that expensive.
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Re: New BBC pistons 60g lighter need rebalancing?

Postby bigjoe1 » Wed Nov 24, 2010 6:12 pm

You will NEVER know the difference


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Re: New BBC pistons 60g lighter need rebalancing?

Postby Harleyoz » Wed Nov 24, 2010 6:19 pm

Joe, does that mean a re balance is not necessary? & wont hurt the engine to leave as is?
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Re: New BBC pistons 60g lighter need rebalancing?

Postby WDCreech » Thu Nov 25, 2010 5:15 am

I think what Joe is saying is 60 grams @ 50% of 2 pistons(reciprocating wieght) is only 60 grams of bobwieght 60/2000%(est bobwieght)is only 3% diference in bobwieght. If you were changing from steel to aluminium rods and the bottom end was 200 grams 2x200/2000% a 20% difference you would need to rebalance.


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Re: New BBC pistons 60g lighter need rebalancing?

Postby Alan Roehrich » Thu Nov 25, 2010 8:19 am

If 60 grams was "close enough", why did you bother to balance it in the first place? [-X
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Re: New BBC pistons 60g lighter need rebalancing?

Postby dwilliams » Thu Nov 25, 2010 9:59 am

Alan Roehrich wrote:If 60 grams was "close enough", why did you bother to balance it in the first place? [-X


A V8 balances as four 90 degree V-twins. With 50% reciprocating and 100% rotating mass on the bobweight, the shake vector for the piston pair shifts into another vector at 2x the frequency and 1/2 the amplitude.

If you change the bobweight, the vector shifts from perpendicular to a slight angle; the result isn't really noticeable. If the bobweight is off a lot, the internal stresses across the mains go up since the counterweights aren't symmetrical across the rod throws, but you'd have to be way off before it became a real problem.

That's why you can fire up a 350 with a set of narrow 305 rods, or one with old heavy TRW domed pistons, and it runs just fine.

As another example, most modern 90 degree V6s are balanced at 50%/100%, but the old Buick V6s were balanced at 33%/100%, to shift the vibration angle to something easier for the motor mounts to take care of.

Balancing doesn't have much to do with power. It's about keeping the vibration down so parts don't shake off, to reduce main bearing loads, and to reduce stresses in the block. If it's too far out of balance you might start hammering mains, cracking main webs, or breaking caps... but it won't make any difference to the power output.

The same basic principles apply to counterweighting. A V8 crank isn't balanced along its own length anyway.
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Re: New BBC pistons 60g lighter need rebalancing?

Postby rskrause » Thu Nov 25, 2010 10:22 am

I once installed pistons that were ~30g lighter in a BBC w/o problems. It just made the RA 1-2% overbalanced. According to my balancing guy, that how's some people like them anyway. It was fine, actually felt smoother at high rpm, though that's subjective. Certainly no problems at subsequent teardown. He said much more than 2% and rebalancing would be in order. 60g sound like a lot. What is the bobweight? Based on my limited knowledge of the subject, if it's more than say 2% of the bobweight you should consider rebalancing. In any case, if you decide to have it reblanced, it would involve removing weight, so you wouldn't to buy any Mallory.

Grammar lesson: the M in Mallory is capitalized because it is a proper name for the alloy produced by Mallory Alloys Group. Of course, other companies make similar tungsten alloys.

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Re: New BBC pistons 60g lighter need rebalancing?

Postby Alan Roehrich » Thu Nov 25, 2010 12:27 pm

dwilliams wrote:
Alan Roehrich wrote:If 60 grams was "close enough", why did you bother to balance it in the first place? [-X


A V8 balances as four 90 degree V-twins. With 50% reciprocating and 100% rotating mass on the bobweight, the shake vector for the piston pair shifts into another vector at 2x the frequency and 1/2 the amplitude.

If you change the bobweight, the vector shifts from perpendicular to a slight angle; the result isn't really noticeable. If the bobweight is off a lot, the internal stresses across the mains go up since the counterweights aren't symmetrical across the rod throws, but you'd have to be way off before it became a real problem.

That's why you can fire up a 350 with a set of narrow 305 rods, or one with old heavy TRW domed pistons, and it runs just fine.

As another example, most modern 90 degree V6s are balanced at 50%/100%, but the old Buick V6s were balanced at 33%/100%, to shift the vibration angle to something easier for the motor mounts to take care of.

Balancing doesn't have much to do with power. It's about keeping the vibration down so parts don't shake off, to reduce main bearing loads, and to reduce stresses in the block. If it's too far out of balance you might start hammering mains, cracking main webs, or breaking caps... but it won't make any difference to the power output.

The same basic principles apply to counterweighting. A V8 crank isn't balanced along its own length anyway.


I don't recall saying I was concerned about power. I didn't say that if he ran it without balancing it, it would be 50HP short. I said "If 60 grams is "close enough", why did you ever bother to balance it in the first place?"

Yes, we used to do this sort of thing all the time. I used to go buy a 454 oval port out of a car or station wagon. Put a set of rod bolts in the rods and recondition them. Grind the crank. Put a set of LS-7 pistons in it. Put big stainless valves in the heads in it, a Comp 310B10 cam in it, with the correct springs, a set of roller rockers, a stud girdle, and 3/8" pushrods. They ran great, and in a car with motor plates, who knew if the thing was anywhere near balanced. We spun them to 7500, and ran them for 1000 passes or so. That was common 20 years ago. But there's no need to do things like that now. We'd probably have had a lot fewer problems, and they'd have lasted longer.

You bought a new set of $600-$800 pistons, and had the block bored, but it's too much trouble and expense to spend $200 to balance it again? It's not like you have to pay for 2-4 slugs of heavy metal. Why not do it right?
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Re: New BBC pistons 60g lighter need rebalancing?

Postby Dave Koehler » Thu Nov 25, 2010 12:29 pm

Alan Roehrich wrote:If 60 grams was "close enough", why did you bother to balance it in the first place? [-X

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Re: New BBC pistons 60g lighter need rebalancing?

Postby Harleyoz » Thu Nov 25, 2010 7:37 pm

Well I thought that the orignal balance would have all the piston/ rod assemblies weighing the same & the rotating mass balanced with the new damper & flexplate. The new pistons are all the same weight (within 1/2 gram out of the box) so i thought the 2.4% over balance with the new pistons might be ok with a relatively short stroke 427 turning to 7000 rpm.
Seems there are opinions here that agree with both sides of the argument! Balancing over here in Oz costs about $470 so I guess I'll think about both options and do a little more research, Though I must say I'm leaning to saving the money.
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Re: New BBC pistons 60g lighter need rebalancing?

Postby rskrause » Thu Nov 25, 2010 7:49 pm

For $470 I'd try it.

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Re: New BBC pistons 60g lighter need rebalancing?

Postby In-Tech » Thu Nov 25, 2010 8:03 pm

Wow, $470 just to re-spin the crank with some simulated bobweights on it? The rest of it is already balanced. I ran my 427 based bbc to 11,000, over-balanced all season, with no problems or anything unusual upon teardowns. I wasn't as over-balanced as you are by ~30 grams but even my balance guy told me spending the $100 bucks to re-spin the crank would be a waste of money since it was a drag race deal.
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Re: New BBC pistons 60g lighter need rebalancing?

Postby hodge » Thu Nov 25, 2010 8:42 pm

Here's the question to ask yourself how close was the balance first of all. Next if there is a vibe to you want to chase it later. I'm doing an engine now a customer has been chasing a vibe for years in a crate engine we spent 30 hrs to cancel out all other vibes and it is internal he is an engineer has been involved the whole time checking no squabble over time, now I am rebuilding and balancing it. It's your choice but to me cheap investment.



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