crank rod piston balance Q&A

Posted By: kilroy

crank rod piston balance Q&A - 01/27/09 05:51 PM

Street strip 440 w/ iron heads, stock rod arp bolts, stock forged crank, and kb184 hyper piston which weigh in a 765g (piston weight only), machine shop balance job (I dont have any of the balance spec with me). Got curious the other day about compression ratios so I tore my motor down and did some measureing and calcing and figured I was around 8.5 to 1 , yuck. So I decided to order a new set of 10 to 1 flat tops and install my new eddy heads! More vroom vroom for my new clutch install which might lead to my new cam going in also but back to the issues. KB237 pistons (770g) fit the bill and are within 5 grams each on the pistons.

Question is will 5 grams a piston heavier throw off the balance in my motor severly? Or am I within the a balance margin of error? Can I just assemble the pistons and go or do I need to weigh and remove weight to keep balance?

Posted By: MR_P_BODY

Re: crank rod piston balance Q&A - 01/27/09 05:57 PM

Sure its not perfect but the factory balance was FAR
worse than 5g
EDIT
I should say also that if you intend to take the
revs higher than stock rpm's I would balance it
again..... but I'm a firm believer in balanceing

Posted By: QWK_ENUF

Re: crank rod piston balance Q&A - 01/27/09 06:07 PM

first let me say balanced is the way to go

then let me say this

i had a stock 383
replaced the stock cast pistons with TRW forged dome pistons and ran 11 teens without any problems

had a stock 76 440 short block
took out stock rods and cast pistons
replaced with eagle h beams and wiseco forged flat tops
ran 10.60's with no problems

my new 511 is balanced because i wanted it perfect

balance it if you can but if not

Posted By: kilroy

Re: crank rod piston balance Q&A - 01/27/09 06:10 PM

Have been know to take it to 7k but generally only blast it to 6500. Ill agree, once you balance youll never go back, but just wondering if I had any "saftey" margin.

Posted By: hemiiroc

Re: crank rod piston balance Q&A - 01/27/09 06:55 PM

That should be close enough to use,but you could balance the new pistons to the old weight.

Posted By: kilroy

Re: crank rod piston balance Q&A - 01/27/09 07:15 PM

yep, I can and probably will. Anybody have any idea of typical balance variations stock and machine shop wise?

Posted By: MR_P_BODY

Re: crank rod piston balance Q&A - 01/27/09 07:21 PM

Quote:

yep, I can and probably will. Anybody have any idea of typical balance variations stock and machine shop wise?

Most any machine shop will get it within a 1/2 gram

Posted By: kilroy

Re: crank rod piston balance Q&A - 01/27/09 07:26 PM

thanks, where is the best place to trim weight on pistons. On the pin pad, or the pad under the dome, and is it possible to lose 5g off a piston?

Posted By: MR_P_BODY

Re: crank rod piston balance Q&A - 01/27/09 07:36 PM

Quote:

thanks, where is the best place to trim weight on pistons. On the pin pad, or the pad under the dome, and is it possible to lose 5g off a piston?

You may need all the points you stated, dont take it
from the flat on the bottom of the wrist pin take
it at a 45* angle from the side to the bottom of the
wrist pin. See how thick the under side of the dome/top
is, if you can take it from there

Posted By: Cab_Burge

Re: crank rod piston balance Q&A - 01/27/09 08:16 PM

Quote:

yep, I can and probably will. Anybody have any idea of typical balance variations stock and machine shop wise?

I have never weighed any of the stock rods or pistons before having them balanced, I have weighed rods and shot for + or - 3 grams in picking the rods and made sure that they where within + or -.003 on the center to center length before starting. Most of the good automotive balnancing shops will shoot for + or - 1/2 to one gram on the rods and pistons . You can take those pistons to a good machine shop and have the 5 grams removed and use them that way. Five grams is a lot of aluminum to remove from a piston, take that from me. I have done that and I will take all of the pistons I need rebalanced to a shop from now on

Posted By: dthemi

Re: crank rod piston balance Q&A - 01/27/09 08:19 PM

Personally, 5 grams isn't that much, but if you're going for a neutral balance, then take 5 out of each piston. It's not hard to do with a heavy piston.

Just for thought though. We rarely balance neutral on an all out race engine. Depending on what it is, what RPM it's going to operated at most of the time, and what we think windage could be we'll either go for an over balance, or under balance, meaning we're either adding to, or taking away from what the actual bob weight is.

Posted By: aarcuda

Re: crank rod piston balance Q&A - 01/27/09 08:27 PM

Quote:

Quote:

yep, I can and probably will. Anybody have any idea of typical balance variations stock and machine shop wise?

Most any machine shop will get it within a 1/2 gram

MAKE SURE YOU ASK!!!

I bought a crank, pistons and rods with rings and bearings from Scat last summer. I asked them if I should have them balance it or have my own balance done.

They went on to tell me how great their expensive balancing machine was, how experienced they were, how they balanced crate motors for mopar and all these big named drag racers etc.

So i said GO AHEAD.

When I got my kit, the pistons (KB) were STILL wrapped in factory plastic. No attempt was made to make them all the same weight.

Same with the rods. No grinding on big or small ends to match them.

So I got curious and weighed them. They were off +/- 5 grams on the pistons alone. I was pissed and called scat. got into a big argument as they were telling me they did it right and that 5grams doesnt matter and what do i expect for $150. I said, I didnt sset the price- you did. I expected it to be done right.

I did a charge back on my credit card for the cost of the balance cause they would NOT refund my money.

I got it balanced at a local shop. I too am a big fan of balanced motors

Posted By: kilroy

Re: crank rod piston balance Q&A - 01/27/09 08:32 PM

Well as I said it a street strip motor in a 4000lb! charger w/ a neutral balance stock 11" flywheel (ya I know but I run a scatter can, cant afford flywheel yet) would be curious what you think about over under balancing as you were talking about. Also aprox ,just because you brought it up, what are some of the over under balances youve seen.

Just to reiterate, it was a machine shop netural balanced.

Posted By: hemiiroc

Re: crank rod piston balance Q&A - 01/27/09 09:34 PM

Quote:

Quote:

Quote:

yep, I can and probably will. Anybody have any idea of typical balance variations stock and machine shop wise?

Most any machine shop will get it within a 1/2 gram

MAKE SURE YOU ASK!!!

I bought a crank, pistons and rods with rings and bearings from Scat last summer. I asked them if I should have them balance it or have my own balance done.

They went on to tell me how great their expensive balancing machine was, how experienced they were, how they balanced crate motors for mopar and all these big named drag racers etc.

So i said GO AHEAD.

When I got my kit, the pistons (KB) were STILL wrapped in factory plastic. No attempt was made to make them all the same weight.

Same with the rods. No grinding on big or small ends to match them.

So I got curious and weighed them. They were off +/- 5 grams on the pistons alone. I was pissed and called scat. got into a big argument as they were telling me they did it right and that 5grams doesnt matter and what do i expect for $150. I said, I didnt sset the price- you did. I expected it to be done right.

I did a charge back on my credit card for the cost of the balance cause they would NOT refund my money.

I got it balanced at a local shop. I too am a big fan of balanced motors

Yep that's what you get from some of the mail order type engines & rotating assemblies.
Makes it impossible for the small shops that do it right to to make any money because those big shops cut quality to show a low price.

Posted By: 440Jim

Re: crank rod piston balance Q&A - 01/27/09 10:16 PM

Quote:

MAKE SURE YOU ASK!!!

I bought a crank, pistons and rods with rings and bearings from Scat last summer. ...
So I got curious and weighed them. They were off +/- 5 grams on the pistons alone.

Wow, that is pretty far off, IMO. Most (but not all) of the reputable stuff is within +/- 1 gram or so from each other, that means the lightest and heaviest in a set could 2 grams different. And that is what a precision balance job will fix.

I have seen factory connecting rods be 20 grams different from the same engine! That is way too much for even street performance, IMO.

Posted By: 69Cuda340S

Re: crank rod piston balance Q&A - 01/27/09 10:59 PM

Quote:

MAKE SURE YOU ASK!!!

I bought a crank, pistons and rods with rings and bearings from Scat last summer. I asked them if I should have them balance it or have my own balance done.

They went on to tell me how great their expensive balancing machine was, how experienced they were, how they balanced crate motors for mopar and all these big named drag racers etc.

So i said GO AHEAD.

When I got my kit, the pistons (KB) were STILL wrapped in factory plastic. No attempt was made to make them all the same weight.

Same with the rods. No grinding on big or small ends to match them.

So I got curious and weighed them. They were off +/- 5 grams on the pistons alone. I was pissed and called scat. got into a big argument as they were telling me they did it right and that 5grams doesnt matter and what do i expect for $150. I said, I didnt sset the price- you did. I expected it to be done right.

I did a charge back on my credit card for the cost of the balance cause they would NOT refund my money.

I got it balanced at a local shop. I too am a big fan of balanced motors

And AARs was not an isolated case. I called Scat and asked about their balance and they said they "guaranteed" it would be balanced. The local shop I took my Scat Stroker Kit to "required" that I have it rebalanced anyway and it was WAY off. Just like AARs, none of the pistons or rods were equaled out and the crank was like 17 grams off. My engine is now running in the car and its silky smooth going down the road probably due to the precise balance job done by the local shop.

If you can afford to get it balanced that would be the way I would go.

Posted By: Anonymous

Re: crank rod piston balance Q&A - 01/27/09 11:27 PM

Balancing, decking and honing is the most important things that you an do to get max performance from the short block..

Posted By: dthemi

Re: crank rod piston balance Q&A - 01/28/09 12:30 AM

Quote:

Well as I said it a street strip motor in a 4000lb! charger w/ a neutral balance stock 11" flywheel (ya I know but I run a scatter can, cant afford flywheel yet) would be curious what you think about over under balancing as you were talking about. Also aprox ,just because you brought it up, what are some of the over under balances youve seen.

Just to reiterate, it was a machine shop neutral balanced.

I've seen 15 grams over or under. For me it's more about them being the same cylinder to cylinder. I'd rather be 10 grams heavy on every piston than to have just one heavy. When you think about what's going on in there just the weight of oil that's being slung around, the drag from the rings, the load differences through the RPM range, and a host of other things including side loading, 5 grams isn't the end of the world.

When we do a balance job for a customer we get no more accurate than a half gram, but more importantly trying to keep all 8 the same. With aluminum rods you'll find some variance too, but I won't go cutting on an rod without a balance pad like a factory rod has to correct a gram or two, it's not worth it, and it'll do more harm than good. I like things to perfect as much as anyone in the world. We've done balance jobs that cost $1500 with mallory and time when you have heavy parts, so I understand your feelings, but I wouldn't worry myself over what you've described.

There are also lots of arguments about how much oil weight to account for where 5 grams would be on both sides of the fence.

Posted By: hemi-itis

Re: crank rod piston balance Q&A - 01/28/09 02:34 AM

I think I know the answer but must ask.A friend bought a crank from a vender and that vender is claiming that he balanced the crank.

How can a crank get balanced without the rods,pistons etc??

Posted By: Anonymous

Re: crank rod piston balance Q&A - 01/28/09 03:34 AM

Quote:

I think I know the answer but must ask.A friend bought a crank from a vender and that vender is claiming that he balanced the crank. How can a crank get balanced without the rods,pistons etc??

it's done all the time. all you need is the weights from the other components.

obviously, without those weights it's an exercise in futility at best.

Posted By: Kelob_pie

Re: crank rod piston balance Q&A - 01/28/09 03:42 PM

A while back I balanced a set of stock 400 rods, from heaviest to lightest there was a difference of 40 grams.

Posted By: Anonymous

Re: crank rod piston balance Q&A - 01/28/09 08:51 PM

Quote:

Quote:

I think I know the answer but must ask.A friend bought a crank from a vender and that vender is claiming that he balanced the crank. How can a crank get balanced without the rods,pistons etc??

it's done all the time. all you need is the weights from the other components. obviously, without those weights it's an exercise in futility at best.

Posted By: kilroy

Re: crank rod piston balance Q&A - 01/28/09 10:06 PM

40 grams thats amazing! I think superstock said it best about the "balanced" crank shaft w/o other speed parts

Well I hate to admit it but I ll check my weight between pistons, make sure they work together and go ahead and strap it together. The 1.5 point gain in compression and the ability to use my new heads is going to be worth the risk of beating the bearings out and having harmonics. Anybody deny me that?

Besides, gearheads live in a world of broken parts!