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  #16 (permalink)  
Old 09-25-2008, 02:38 PM
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OH...while I was typing, another post came through!!!

I have some books by Dave Vizzard, but not that one....I must look it up. Thanks!

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  #17 (permalink)  
Old 09-28-2008, 02:33 PM
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What's the big deal?
Generally balancing only runs about $ 120-150.
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Old 09-28-2008, 02:51 PM
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The BIG DEAL is this:

If I rebalance by hard-to-get 428 crank for new-technology light-weight pistons, then it will be no good for my +40 pistons (which weigh the same as the broken +30s) in the eventual need for a re-bore. It will also be no good for any 428 pistons (which also run about the same as my broken +_30s).

So, why not balance it again (to these new light pistons)? Just ticks me off as I will eventually need to do a really major (expensive, mallory metal) rebalance to a well used 40-40 crank when I need to go to +40 or larger.

Anyway, the fact is 428 cranks are hard to get. A little easier now as SCAT makes them, although I've no experience with any of their products. So, one doesn't jsut do the first thing htat comes to mind.

Similarly, one just doesn't bore an FE block with wild abandon either. They just arn't that common anymore.

There are days I wish I was a SBC guy...so easy...so cheap........sigh....LOL.
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Old 09-28-2008, 04:08 PM
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I am not sure where the Mallory idea came from.
If a crank has been lightened for certain pistons, and then you want to install heavier pistons later, all you do is weld up the drilled holes until the crank comes into balance.

Also if a lot of additional weight is necessary to a crank that has balance holes in it, then lead is added to the existing holes, capped by a steel cap which is then welded into the crank.

If the +.040 pistons were already too heavy, then the intermediate balance would be irrelevant, wouldn't it.
FE 428 balance can be effected by altering the eccentric weight if necessary.
I personally have run the balance machine when balancing lots of engines.
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Old 09-28-2008, 07:52 PM
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If you don't rebalance you are just asking for trouble. You may wipe out the bearings in short order, which, at a minimum is going to cost you new bearings. You may also find or experience further damage due to the unbalance/imbalance. JMO
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Old 09-29-2008, 05:08 AM
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ScotF: Yes, you are right. I never thought about just filling in the drilled hoels with "weld", so I suppose it's not a big deal to get the crank re-balanced. The +40 pistons should work (without rebalancing) as they weigh within 5 grams of the existing broken +30 pistons.

Stroke: Your comments are well taken. In fact, the purpose of my post was to try to establish some limits to acceptable balancing. I've already had this engine balanced, so if all I am doing is replacing pistons, rings, snap rings and wrist pins, I can weigh them and compare to my existing parts. Then the question comes, how far out can I be from the original numbers and still have a reasonably well balanced engine? Well, from what I can see, Ford called for +/_ 0.5%, which is roughly 5 grams total difference. At least one response to this thread has stated 20-30 grams difference can be felt. So the "acceptable" amount seems to be coming out.

Thank-you everyone for your comments, information and opinions. I appreciate them all.
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Old 09-29-2008, 12:30 PM
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Quote:
Originally Posted by Argess
I have to replace pistons in my enigne that I had originally balanced. The same pistons arn't available anymore, but others are. Problem is, the replacements weigh in at 520 grams, and the originals were 597 grams.

Engine balance = 3 things

1/ Rotating balance (crank, flywheel, big rod-end, etc.)

2/ Reciprocating weight (piston, small rod-end, etc.)

3/ Equal balance (all pistons are the same weight, etc.)

Although I kind of understand the above, I don't have a feel for what is an acceptable deviation, and what's not.

So.....how much will changing piston weight throw off my engine balance and how much can I get away with before it becomes significant?
Reading thru this I have a couple comments:

First, over balance, this is acceptable in a high RPM engine around 6000 RPM and up it smooths the engine a bit and some people report seeing a little extra power. Below 6000 the engine will shake like crazy.

Second, is amount. Typically a couple grams is about the widest tolerance. This is less about how much out of balance will it take for you to feel and more about how much it's trying to make the crank find an instant center that's not on the main bearing centerline. These loads are reacted by the bearing webs into the engine proper. They are an alternating loads which is seen as using up the structure's fatigue life, rather like bending a paper clip back and forth till it breaks. The engine has a lot of mass, it can take quite a bit of out of balance before you feel it, but the bearing bulkheads and crank see it all the time.

You're in a position where the new pistons are lighter, you can go a couple different directions:

A) Find a pin heavier by the amount needed, thus avoiding re-balancing.

B) Re-balance, the lighter piston will result in metal removal from the crank counter balances, this is a lot cheaper process than if weight had to be added. The lighter crank assembly takes load off the bearings and will accelerate quicker.

Bogie
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