Tell me about "balancing" a crankshaft. I want to refresh a '92 Camaro motor that has about 120K on it. The motor was running OK when I it was pulled from the donor car. I'm just looking at rebuilding the donor motor in the exact same configuration as it currently stands. If I buy an OEM-spec crankshaft, do I need to worry about "balancing" issues if I'm not changing the type of pistons or rods? The website I'm looking at says "you know it is capable of going from the box to your block without worries... guarantees a target bobweight +/-2%" My theory is that even if the existing crankshaft looks serviceable, its bound to have some wear. OEM-type crankshafts are so inexpensive for these motors, it seems advisable to replace it unless there is some compelling reason to not replace it. The price of a replacement is probably less than the price of paying a competent machinist to do anything with journal-surfaces of the existing crank - let alone paying to have a crank balanced. Interested in opinions and experience on this subject.

 

Balancing means matching the crankshaft weights to the weights of the

pistons
rings
wrist pin (and retaining clips if full floating)
con rod (balanced on both ends)
rod bolts/nuts
rod bearing

This is done by adding or removing weight from the crankshaft weights to match the bob weight.

Chances are if you're simply doing an OEM type replacement and it's just a daily driver then getting everything rebalanced isn't worth the extra cost however even something as simple as a 0.030" overbore means different weight pistons. Once you start changing pistons, rods etc then the crankshaft should always be rebalanced.

So many things are different weights. I went with thicker wall wrist pins in my current engine and if I ever need to replace one, I need to know the weight because another wrist pin with the same wall thickness won't weigh the same.

When I switched from a 540 to 588 engine, I needed bigger pistons. The ones I bought for the 588 were lighter than the ones used in the 540 even though the bore is now larger.

I have no idea how many times I've had a rotating assembly balanced because I've changed something. Probably at least 4 times now. The guy that does my engine balancing gets all the balance weights down to 0.0 grams. I have a balance sheet showing that he spun up the crank 12 times before finally getting it down to zero.

 

You haven't explained why you need a new crank. So it's hard to advise the most economical part. Most owners will have the stock crank polished or ground down for oversized bearings (I think they call them undersized for some reason). This little more than half the cost of a replacement crank.

BTW I have to mention the stock cast cranks are very strong and acceptable for street performance use under 7,000rpm. The advantage of using the original crank is you normally don't need to alignhone/alignbored the block - $200 or more. Just see if the stock crank turns easily by hand with the pistons off.

If you must replace the crank there are usually crankshaft specialty shops in most large towns that get you the correct crank reconditioned for less than a new crank. Or you can buy new cast replacement cranks from the parts vendors but need an exact replacement so double/triple check your specs.

As far as balancing the street motors can run well and have a long life without high accurate balance. Engine reman shops won't do any balancing for stock replacements - just use same part numbers or cross referenced parts. Motors run quite well with overbored replacement piston on only one or two cylinders. If the crank applications are the same they get used not rebalanced.

IIWM and I needed a new crank and balancing I would consider a stroker. You're already there with the cost of machine work.

Hope this helps.

 

I'm not sure I need a new crank.

More thinking-out-loud, and pondering at this point.

Much appreciate the information and insights. Very helpful info.

I'm not completely green on motor assembly. But, my prior experience with cranks only extends to a few occasions of watching others do some of it. My advantage - if it counts as such - is time and budget on my side. Not in any sort of rush, and not having to decide whether to pay for diapers vs. car parts.