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Discussion Starter #1
I was searching around old posts and don't quite have a handle on the engine balancing. It seems to me that the literature says the dampener and flywheel have weights to balance them. However, some posts here in the forum allude to the idea that the crank is somehow part of the engine balance equation. In addition, some posts mention some changes to the crank must be made to use them on a newer block because of the rear seal.

I've got the crank, dampener, and flywheel from a 1968 302. I've also got the crank and dampener from a 1990 5.0.

I've got a good 5.0 block and a not as good 302 block.

My question concerns engine balancing. In a nutshell, is the engine balance determined by the dampener and flywheel OR is the engine balance determined by the dampener, flywheel, and crank?

Could I use the older dampener and a 28oz. flywheel on the newer crank?

Could I use the older dampener, older crank, and a 28oz. flywheel in the newer block? If so, what is the change to the crank to accomodate the rear seal?

My goal is to use the newer 5.0 block for a rebuild, but I'd like to try and reuse my original flywheel (28oz.) and dampener (also 28oz.).

Jerry
 
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Actually it has something to do with all of the above & the rods, pistons, rings & bearings. The difference is internal balance vs. external balance. Internal is all the balance weight is on the crank throws & external is on the flywheel/flexplate & balancer. You don't want to mix them up. Also on the later engines the cam uses the 351W firing order. If the 1990 5.0 is the better of the 2, use it & a hydraulic roller. Your machinist will know what to do.
 

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The balancing process is pretty involved. All of the rotating and reciprocating mass has to be balanced to keep things smooth.

Each of the connecting rods have to be balanced. (made to weigh the same)
Each of the pistons have to be balanced. (made to weigh the same)
Then the rods and pistons are assembled and have to be balanced as well.

There are two ways that a crank can be built; Internal balance and external balance.

An internally balanced engine has all of the balancing take place internally, which means that the flywheel and dampner have no offsetting weights on them.

An externally balanced engine has the dampner and flywheel play an active role in the balancing process, which means that they have offsetting weights associated with them.

In either case, the entire rotating assembly, including the dampner and flywheel is put on the balancing machine and spun to determine where heavy/light spots are and metal is either removed or added as needed until the whole thing is within tolerances (usually 1/10 of a gram).

All stock small-block Ford engines use the externally balanced setup.

On small-block Ford engines there were two weights used on the dampner/flywheel; the early engines used a 28oz offsetting weight and the later engines used a 50oz offsetting weight. You can't mix parts from the two types.
 

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Balancing is very important on high horsepower engines and not so important on stock horsepower engines. The reason for this is the horsepower math formula. At one point in the formula you multiply by horsepower so if you have a stock engine you probably will not rev it over 5,500-6,000 rpm. On a built engine you might redline over 7,000 rpm and a nascar engine over 8-9k. Think of a tire out of balance. At 35 mph you don't even feel it but the faster you go the more it shakes till the whole car feels like its going to shake apart and the amount the wheel is out of balance is only an ounce! Also thinking of a car tire they only have a limited selection of weights and they don't fine tune it, they just slap the closest one in size on and go for it. Its good enough for the application. So when balancing an engine you need to do what is good enough for your application.
 

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Discussion Starter #5
> On small-block Ford engines there were two weights used
> on the dampner/flywheel; the early engines used a 28oz
> offsetting weight and the later engines used a 50oz
> offsetting weight. You can't mix parts from the two types.

I read about all that. I guess maybe a more succinct question is, is a 1990 5.0L crank the same as a 1968 302 crank?

Obviously it would be bad to mix a early dampener with a later flywheel or vice versa, but the core question is whether the crank itself is different in a way that one could not use a late crank with early dampener/flywheel?

Jerry
 

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I don't have a straight answer to that question. I am sure someone else can tell you for sure.

If the rear main seal setup is the same then you should be able to use the '90 crank in the early block, but why?

Heck, if the '90 engine is in good shape why not just use the whole thing and get a conversion pulley set? You will be happier with a roller engine anyway.
 

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Discussion Starter #7
The rear main seal setup is not the same. The early block uses a two piece while the later block is one piece. Based on some posts I read, the crank is slighly different to accomodate this change. I've not sure if the late one is backward compatible or the early one is forward compatible. That's my first issue.

I do want to use the later block. As people have mentioned, it's a roller block. However, I don't want to pay for a new 50oz. flywheel and special front pulley since I've already got perfectly good 28oz. versions of each.

Jerry
 
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during the 1982 model year, the balance of 302s changed from 28 to 50 oz. It also is when the single piece rear seal was started. So no, the 68 crank is not the same as a 90 crank. You can have parts rebalanced to fit other specs. I have had 50 pieces changed to 28 and visa versa.
 

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<blockquote><font class="small">In reply to:</font><hr>

I do want to use the later block. As people have mentioned, it's a roller block. However, I don't want to pay for a new 50oz. flywheel and special front pulley since I've already got perfectly good 28oz. versions of each.

<hr></blockquote>
This I can address:

You can not use 28oz parts with a 50oz crank. Period.

If you want to use the 50oz crank you have to have the 50oz flywheel and dampener.
 

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Discussion Starter #10
> You can not use 28oz parts with a 50oz crank. Period.
> If you want to use the 50oz crank you have to have the
> 50oz flywheel and dampener.

Those are the answers I was fishing for. Thank you very much.

Jerry
 
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