Rebalance crank or Not?

[Xnke]

I'm building a bored L28, using a (somewhat) lightened L28 crank (2.4lbs lighter than a completely stock L28 crank), and I need to know if it's imperative that I re-balance the crankshaft.

 

 

Originally, the crank was used with stock 86mm flattop pistons and 130mm rods. I am using 88mm Z22S pistons and 133mm rods. The total weight difference in the {rod, rod bolts+nuts, wrist pin, piston, rings} is 40g each-my setup is lighter.

 

Do I need to go ahead and re-balance the crank, or is this insignificant?

[nismopick]

Who did the "lightening" of the crank in the first place? If you did it yourself... absolutely. If it was a machine shop, I would think they would have done it.

 

Either way... it's out, DO IT.

[Xnke]

The crankshaft was lightened and balanced by a professional shop for the original L28 with the 40g heavier rod/piston assemblies. I haven't touched the crank, except to clean and inspect it.

It's in perfect shape, which is why it's being used instead of lightening the other crank up and having it rebalanced...

[Tony D]

Unless the bobweight is identical...

It has to be rebalanced or you're setting yourself up for stressus maximus at even pedestrian rpms...

[Xnke]

Thanks, TonyD. I figured it would, but also thought that it may have been "close 'nuff". Now I know.

[BRAAP]

As I am aware, inline engines do not use bob weights when balancing the crankshaft like V-engines.

As such, an inline 6 crank that was neutral balanced is fine so long as it was not reground, carved on, or altered in any fashion, after the "balance" job was performed and is useable with any piston rod combo.

[Xnke]

then why, pray tell, would an stroker motor need to get the LD28 crank rebalanced? the crank isn't altered in any way, just the weights on it.

 

Either way, 40g lighter piston/rod assemblies means the metal comes out of the crank, and no mallory metal needs to go in. (less $$$$)

[BRAAP]

Cranks should be rebalanced if they have had work performed on them such as lightening or being reground. Just that inline cranks don’t utilize "Bob Weights" when they are being balanced.

It doesn’t matter how much heavier or lighter the reciprocating parts are for an inline or flat/opposed engine. When these cranks are balanced they are balanced to itself only, not balanced to a particular piston/rod mass. i.e. NO bob weights used when balancing an inline crankshaft.

 

 

http://www.eatonbalancing.com/ebalance6.php

In getting to the point in which the crankshaft from a V8 or other V style engine can be spin balanced' date=' several different operations had to be already completed. [b']Had this been an inline or opposed cylinder engine, then the crankshaft could have been balanced at any point in the operation due to not requiring any bobweight fixtures to be installed on it.[/b] However, when the cylinders are orientated in a V style, bobweights are required due to the rotating and reciprocating masses not being equally opposing. Throughout the previous articles in this series, the pistons have been matched to each other by weight, the connecting rod ends have also been appropriately lightened and matched by weight on each end as a set, and the miscellaneous components such as the bearings, rings, and piston locks have also been weighed. With all these values known and the estimated oil along with any heavy balancing values also added, the calculation for the amount of weight required for the bobweight fixture is ready to be put to use....

 

 

http://www.rpmmachine.com/engine-balancing.shtml

Balancing Act
The process of balancing begins by equalizing the reciprocating mass in each of the engine’s cylinders. This is done by weighing each piston on a sensitive digital scale to determine the lightest one in a set. The other pistons are then lightened to match that weight by milling or grinding metal off a non-stressed area such as the wrist pin boss. The degree of precision to which the pistons are balanced will vary from one engine builder to another' date=' and depends to some extent on the application. But generally speaking pistons are balanced to within plus or minus 0.5 grams of one another.

 

Next the rods are weighed, but only one end at a time. A special support is used so that the big ends of all the rods can be weighed and compared, then the little ends. As with the pistons, weights are equalized by grinding away metal to within 0.5 grams. It’s important to note that the direction of grinding is important. Rods should always be ground in a direction perpendicular to the crankshaft and wrist pin, never parallel. If the grinding scratches are parallel to the crank, they may concentrate stress causing hairline cracks to form.

 

[b']On V6 and V8 engines, the 60 or 90 degree angle between the cylinder banks requires the use of "bobweights" on the rod journals to simulate the reciprocating mass of the piston and rod assemblies. Inline four and six cylinder crankshafts do not require bobweights.[/b] To determine the correct weight for the bobweights, the full weight of a pair of rod bearings and the big end of the connecting rod, plus half the weight of the little end of the rod, piston, rings, wrist pin (and locks if full floating) plus a little oil are added together (100 percent of the rotating weight plus 50 percent of the reciprocating weight). The correct bobweights are then assembled and mounted on the crankshaft rod journals.

 

The crankshaft is then placed on the balancer and spun to determine the points where metal needs to be added or removed. The balancer indexes the crank and shows the exact position and weight to be added or subtracted. The electronic brain inside the balancer head does the calculations and displays the results. The machines we have use graphical displays that make it easy to see exactly where the corrections are needed.

If the crank is heavy, metal is removed by drilling or grinding the counterweights. Drilling is usually the preferred means of lightening counterweights, and a balancer that allows the crank to be drilled while still on the machine can be a real time saver.


If the crank is too light, which is usually the case on engines with stroker cranks or those that are being converted from externally balanced to internally balanced, heavy metal (a tungsten alloy that is 1.5 times as heavy as lead) is added to the counterweights. This is usually done by drilling the counterweights, then press fitting and welding the heavy metal plugs in place. An alternate technique is to tap the hole and thread a plug into place. Drilling the holes sideways through the counterweights parallel to the crank rather than perpendicular to the crank is a technique many prefer because it prevents the metal from being flung out at high rpm.

After drilling, the crankshaft is again spun on the balancer to determine if additional corrections are required. If the crank is for an externally balanced engine (such as a big block Chevy), the balancing will be done with the flywheel and damper installed. On internally balanced engines, the flywheel and damper can be balanced separately, or installed on the crank and balanced as an assembly once the crank itself has been balanced....

 

[Xnke]

So really, it's a non-issue and I do not need a rebalance? I know the crank was balanced when it was lightened, as the shop that did my block also did that crank 4 years ago. (They keep records and also stamp an ID number into the end of the crank snout, on the flat that the crank pulley bolt mates to.)

 

 

Thanks guys, I'll go put my bottom end back together tonight.

[BRAAP]

So really, it's a non-issue and I do not need a rebalance? ...

Thanks guys, I'll go put my bottom end back together tonight.

 

Exactly.

I added another reference in my post above.

[Tony D]

I may have been semantically incorrect. Our balancer guy called our for 'bobweights' of our original L28 when we had another crankshaft done up to replace the one with the buggered snout.

 

This may have been because the pistons we had were a different weight than stock. But those gold things on the crankshaft were what was on ours when it was balanced. Maybe he's just being superflous.

 

BRAAP, I PM'd you wit some more particulars, maybe it's 'our application' that calls for this methodology. The same shop uses them on my VW and Corvair Crankshafts, even though much of the literature of the Corvair Guys says the crankshaft is 'inherently balanced'...

 

Hmmmm. When I'm back in country, I will have to enquire with the guy and see what is the deal. I didn't get into it too far, what he did worked and we weren't having any issues, so I never went further into what exactly he was doing. I'll ask him what the deal is with the weights if they are not needed, why is he using them. And was it because our piston/rod is a different weight from stock. This may take a while, it's not high on my priority list, though! Forewarned!

[Xnke]

Well, it looks like i'll be waiting on the rebuild anyway, the machine shop buggered my rods up.

 

I was having them assemble the pistons and rods, and install 8mm ARP rod bolts. Instead, they drilled the rods for 9mm ARP rod bolts, ruining them in the process. It has become obvious to me that the rods can't simply be drilled out, you'll run the rod bolt into the back of the bearing shells. I got the correct 8mm rod bolts for free, and luckily they had not installed the pistons yet, but now I'm out a set of 133mm long-8mm bolt rods.