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Old 10-07-2007, 03:53 PM
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Need help with engine balancing

I decided a couple months ago that I would like to try and tackle the task of building an engine, even though I have no previous experience in the field (but I learn quickly). So I went with an SBC 350 (1970 2 bolt casting already machined 60 over), GM forged crank, and original heads 76cc. I bought the engine from a friend who ran it in his mud truck, and since I am quite familiar with his driving habits, I completely disassembled it to ensure I wouldn't run into any surprises in the future (even though the engine only had about 10K miles on a fresh rebuild). The cylinder walls and crankshaft were machined on the last rebuild and are still in perfect condition.

So I have all the parts I need to put the engine back together, and besides being completely clueless as to all the nuances experienced engine builders know how to handle, I feel fairly confident I can put it back together successfully.

But...do I need to have the crankshaft looked at by a machine shop before reassembling the engine? It was just machined 10,000 miles ago (plus it's already machined to its max tolerances). Also, I noticed the harmonic balancer has five consecutive holes drilled in the back side of it which were not stock. The dampener is in pretty rough shape so I was thinking about replacing it, but will I have any problems since the new one won't have the custom holes? Any other tips on the build would be helpful as well.

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Old 10-07-2007, 04:34 PM
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What pistons are in the motor now. If the GM forged crank is from a old GM Z28 or LT1 motor that used a forged dome piston then the balance could be far enough off with a typical cast, hyper dished or flat top stock or replacement piston.
If the bearings looked ok when ya tore it down then there is not likely a big,big unbalanced condition in your combo of parts. A motor that has had the bottom end rebalanced when parts are changed from stock will always run smoother, make a little more power and last longer.
Because you have a forged crank that could have been from a hi perf motor with pistons that are a lot different than stock I would tend to want to look into a balance job. Definatly replace the old balancer and maybe the flexplate too.
Ditch the old 76cc smog heads. Get something, anything better and bolt them on. There are lots of replacement heads available for less than $1000. that will really wake 'er up.
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Old 10-07-2007, 04:44 PM
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At the very least you should measure the cylinders and crankshaft journals while the engine is apart to verify that it doesn't need any machine work. Remember that at .060 over that block most likely won't be able to bored any further. It'd be in your best interest to know now that you've got good parts, rather than put it back together and find that it's deficient in one way or another and require another teardown.

As far as the balancer goes. He didn't by chance stroke it did he? If it's drilled because the engine is externally balanced and you put an internal balance balancer back on it'll be out of balance. It may not hurt to mock up one cylinder and measure the stroke. It'll be obvious because the 383 has a ~.250" longer stroke than a 350.
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Old 10-07-2007, 04:49 PM
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To me, proper balancing is a no-brainer with most anything that I'd bother to rebuild in the first place. For even stock rebuilds, I have a machinist that I utilize who's geared towards performance builds and whom I trust completely. First, weight-matching of all parts is important, as is ensuring the rotating balance of any applicable components and finally the rotating assembly balancing as a whole. Proper and precise balancing will pay dividends in all ways, performance, reliability, overall efficiency and longevity. Don't cut corners here.... ever.
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Old 10-07-2007, 07:08 PM
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Thanks for the help

The pistons that I pulled out were flat top, and that's what I'm putting back in (with valve depressions). The crank casting number is 3941182, which is from a 68-76 350 at 3.48 stroke.

I'm having my pistons pressed in this week so I'll take the crank in to get looked at at the same time. Thanks for all the help!
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Old 10-08-2007, 10:28 AM
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Quote:
Originally Posted by carlk4574
I decided a couple months ago that I would like to try and tackle the task of building an engine, even though I have no previous experience in the field (but I learn quickly). So I went with an SBC 350 (1970 2 bolt casting already machined 60 over), GM forged crank, and original heads 76cc. I bought the engine from a friend who ran it in his mud truck, and since I am quite familiar with his driving habits, I completely disassembled it to ensure I wouldn't run into any surprises in the future (even though the engine only had about 10K miles on a fresh rebuild). The cylinder walls and crankshaft were machined on the last rebuild and are still in perfect condition.

So I have all the parts I need to put the engine back together, and besides being completely clueless as to all the nuances experienced engine builders know how to handle, I feel fairly confident I can put it back together successfully.

But...do I need to have the crankshaft looked at by a machine shop before reassembling the engine? It was just machined 10,000 miles ago (plus it's already machined to its max tolerances). Also, I noticed the harmonic balancer has five consecutive holes drilled in the back side of it which were not stock. The dampener is in pretty rough shape so I was thinking about replacing it, but will I have any problems since the new one won't have the custom holes? Any other tips on the build would be helpful as well.
If the engine was balanced by a shop as opposed to using the OEM and hoping for success on a rebuild, if any part of the balanced assembly is replaced then the assembly needs to be sent to the balancer and sent thru the process to insure the new part works with the balance. This is because there are different ways to balance an engine the two most common you'll see as a hot rodder is total assembly balance and throw by throw balance. The total assembly technique is used by the OEM, they are interested in total crank balance as a unit, where individual cylinder's can have a fairly wide tolerance but the total is in balance. This is common to internal and externally balanced engines. The hot rod shop tends to balance each cylinder's hardware to all the others, they start with the lightest and machine every part to match, When doing this often it requires the factory balancer and or flywheel/flexplate be rebalanced compared to the factory's averaging of zero balance. This is why you end up scratching where ever your brain is located when you get your supposedly factory balanced parts back from the hot rod shop with holes and/or Mallory metal all over the place such that you think either the shop made a mistake or the factory never balanced anything.
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