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Old 08-18-2010, 02:53 PM
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Crankshaft Balancing (a tough one)

Howdy All,
I swapped a crankshaft from a LT1 (truck) engine into another LT1 and now have a vibration. Research reveals the probility for the problem is a balancing issue.

Donor engine was a LT1 truck engine that had the long skirt (heavy duty) pistons and used powdered metal rods, but had cast iron "normal" heads. (Year unknown).

My engine is also an LT1 truck engine, (1997), w/the powdered rods, but has vortec heads and pistons. My research reveals that both engines used the same crank (indeed, the casting numbers are the same), but had different balancing specs because of the vortec pistons.

The only way to confirm the proper crank is to obtain the bob-weight specs, attach the weights to my crank, spin it up and see if the balance is correct or not.

To that end, I am seeking those bob-weight specs for the two engines mentioned above. Anybody that knowledgeable out there?

Many thanks ahead of time, Waldo

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Old 08-18-2010, 03:59 PM
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Easiest way to figure this out is to spin it up with bob's that match the weight of the parts you are currently using, rather than some random published spec. If it has problems with your current bob weight, you can then correct it right then.
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Old 08-18-2010, 04:19 PM
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Are you using an externally balanced flexplate?

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Old 08-18-2010, 07:54 PM
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Quote:
Originally Posted by waldoweirdo
Howdy All,
I swapped a crankshaft from a LT1 (truck) engine into another LT1 and now have a vibration. Research reveals the probility for the problem is a balancing issue.

Donor engine was a LT1 truck engine that had the long skirt (heavy duty) pistons and used powdered metal rods, but had cast iron "normal" heads. (Year unknown).

My engine is also an LT1 truck engine, (1997), w/the powdered rods, but has vortec heads and pistons. My research reveals that both engines used the same crank (indeed, the casting numbers are the same), but had different balancing specs because of the vortec pistons.

The only way to confirm the proper crank is to obtain the bob-weight specs, attach the weights to my crank, spin it up and see if the balance is correct or not.

To that end, I am seeking those bob-weight specs for the two engines mentioned above. Anybody that knowledgeable out there?

Many thanks ahead of time, Waldo
Trucks didn't have LT1 engines and LT1 engines can't use Vortec heads. So what is it we're really working with here? Start with casting numbers.

Bogie
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Old 08-18-2010, 09:01 PM
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Thanks to all for the input so far. I'll check on the numbers tomorrow and post 'em. This whole deal is for a 75 year old pal of mine who is not internet savvy, so I volunteered to help out.
The devil is in the details.
Waldo
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Old 08-19-2010, 06:14 PM
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Quote:
Originally Posted by waldoweirdo
Thanks to all for the input so far. I'll check on the numbers tomorrow and post 'em. This whole deal is for a 75 year old pal of mine who is not internet savvy, so I volunteered to help out.
The devil is in the details.
Waldo
There can be MANY reasons to have a vibration that has nada to do w/the crank balance.

You need to describe in as much detail as you can- like will it vibrate if you are sitting still, in Park, by raising the RPM to the point it does it, or what road AND engine speed(s) the vibe occurs, whether it does it when in gear, coasting in neutral, under acceleration, during steady cruise, etc.

Things that could be involved are the various engine accessories like P/S pump, A/C compressor, W/P, a broken clutch fan assembly, the wrong or damaged flexplate, flexplate balance factor, flexplate fasteners, the torque converter and its fasteners, the motor mounts, the transmission mount, the drive shaft balance, the U-joints, the differential, even tire imbalance can cause vibrations.
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Old 08-20-2010, 02:08 AM
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Thanks for the input Cobalt. Only change was the crankshaft after a spun bearing (the rod was fine). The vibe is present most all the time. The intensity varies thru the rpm range, but is always there.
Other professionals (crank grinders and suppliers) acknowledge there is an issue with these two cranks not interchanging, which is why we're seeking the bob-weight specs.
Waldo
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Old 08-20-2010, 03:46 AM
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Waldo, if you're convinced the vibration is due to the crank/rods and piston mismatch, the best route for you is to just rebalance the crank in the normal way, IMHO.

To do anything to the crank involving balancing will require (in your case) removing weight, and this requires the crank to be removed to do the necessary machining ops. So if the crank needs to come out anyway, the weighing of the piston and rod, rings, bearings, etc.- to get the actual and not a theoretical bobweight- is a small additional effort to get it right and not just close.

That is, unless you were somehow planning on balancing the counterweights w/the crank in the engine...

In any event, possibly the following will be of some use to you:

IDENTIFYING ONE PIECE REAR MAIN SEAL CRANKSHAFTS
GM 5.0 & 5.7L (305 & 350 CID) Crankshafts

Identifying the Chevrolet 5.0 & 5.7L (305 & 350 CID) crankshaft used in various GM cars and trucks has not been made any easier since the introduction of the one piece rear main seal crankshaft.

Currently AERA is aware that casting numbers 14088526 and 14088535 are cast iron crankshafts used in either 5.0L (305 CID) or 5.7L (350 CID) engines. Again, GM is using the same crankshaft castings in two differently balanced engines. A steel crankshaft with the forging number 14088532 is used in the 5.7L (350 CID) Corvette engines.

Both engines are externally balanced and use the same counterweighted flexplate or flywheel and a non-counterweighted damper. Equally shared are the connecting rods which weigh 603-604 grams. The rotating (big) end of the connecting rod weighs 424 +/- 2 grams while the reciprocating (pin) end weighs
179 +/- 2 grams.

The weight for a 5.0L (305 CID) piston and pin is 646 +/- 3 grams, whereas a 5.7L (350 CID) piston and pin come in at 743 +/- 3 grams. The total bob weight for the 5.0L (305 CID) engine is 1815 grams in comparison to the 5.7L (350 CID) engine at 1916 grams.

AERA mrmbers have visually differentiated the crankshafts by the balance holes drilled into the front and rear counterweights. Machinists using these specifications should be aware that all of the measurements are approximate and may vary slightly from crankshaft to crankshaft.

The holes drilled in the 5.0L (305 CID) crankshaft front counterweight vary somewhat. There are either two 1 holes drilled to a combined depth of 2.563, or two similarly drilled 1 holes along with a third .750 hole drilled to a depth of
.125-.313. The rear counterweight is much the same with either two 1 holes drilled to a combined depth of 2.250, or two similarly drilled 1 holes along with a third .750 hole drilled to a depth of .188-.313

The 5.7L (350 CID) crankshaft's front counterweight has two 1 holes drilled to a combined depth of 1.188. The rear counterweight also has two 1 holes drilled to a combined depth of 1.563.

To-date visually separating the crankshafts using the specifications outlined above has been validated by later balancing.
The AERA Technical Committee
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