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Old 02-04-2010, 01:37 AM   #1
slowstang70
 
Join Date: Feb 2010
Vehicle: 1970 Mustang
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Default Harmonic Balancer Weight

I have a quick question about the weight of the stock harmonic balancer. What exactly is the weight, I searched on the forums and did a quick google search but found nothing.

Also as a replacement i was thinking if this:
http://www.summitracing.com/parts/FMS-M-6316-K351/
but what effect does an imbalanced balancer have versus one that is neutrally balanced? What would you recommend and why? The motor is bored .030 over and that is it.

Thanks for the help!
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Old 02-04-2010, 05:13 AM   #2
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The bore doesn't effect balance. The balancer in your link is the 4-bolt type, requires pulleys and brackets for 70-73. If you have a pre-70, you'll want a 3-bolt type. Both are available new.
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Old 02-04-2010, 08:29 AM   #3
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Welcome!

The type of harmonic balancer (i.e. 50oz, 28oz, or 0oz) depends on the balance of the crank in the motor. Same goes for the flywheel/flexplate. They all have to match or you will get bad vibration. I run an internally balanced stroker crank in my 383W, so also run a 0oz balancer and flywheel. 351W stock run 28oz balance stuff. The 302s pre 1980 used a 28oz setup but post 1980 they ran a 50oz setup. In a motor that is running an aftermarket crank, it could be any of the three, so you would need to know what you have. Hope this helps.
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Old 02-04-2010, 10:59 AM   #4
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Thanks for the quick responses guys but I am still a little confused. Urban cowboy, since I have the stock crank and stock flexplate, I would need an externally, neutrally balanced harmonic balancer. The engine is going to be built back to oem specs except being bored .030 over.
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Old 02-04-2010, 02:32 PM   #5
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What engine do you have? What year was it made?

Given that it is completely stock, no you will not need a 0oz "internally balanced" harmoninc balancer. You will need either a 28 or 50oz balancer depending on what engine you have.
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Old 02-04-2010, 03:42 PM   #6
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It's a 1970 302.
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Old 02-04-2010, 05:58 PM   #7
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70 302 is a 28oz balancer and a 28oz flywheel. The weight is actually a small weight offset in the flywheel or balancer. In other words the parts are in and of themselves out of balance...and the weight isn't the actual amount of weight per se, but the 28 or 50 or whatever I believe is a measured quantity of offset force generated when rotating at a certain speed. A standard crankshaft is naturally out of balance, along with the flywheel and the balancer, but when they're all put together and rotating the entire assembly is balanced such that it prevents unwanted vibration.

Incorrect balancing leads to harmonic vibrations in the crankshaft that are created by torsional excitation(the winding and unwinding of the crankshaft like a rubber band as each cylinder applies power and then stops applying power). The balancing is designed to suppress those harmonics. If not properly controlled those vibrations will cause the crankshaft to fail quite spectacularly....the crank literally vibrates like a tuning fork until it fractures and breaks into multiple pieces. There's a few members around here that have pictures of cranks that failed from that.

0 balance is pretty much an aftermarket/race only deal that's not used in factory setups(expect maybe cars like Ferraris etc). It leads to a more well balanced assembly, but they can be more costly and difficult to balance, as heavy exotic material often has to be added, which is neither cheap nor easy. Hence it's not used in factory setups due to cost considerations.

You'll know immediately if you have the wrong balance factor anything, as the engine will vibrate like mad as soon as you start it.
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Old 02-04-2010, 08:51 PM   #8
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Thanks for the help urban cowboy and 67 mustang. All the imbalance, balanced and weight stuff really threw me for a loop but I am certain I ordered the right one.

Thanks again guys!
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Old 02-04-2010, 08:51 PM
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