could someone explain to me the difference between internally balanced and externally balanced flex plates, what Ive heard is that plates for internally balanced engines are zero balanced which would mean they are balanced, and plates for externally balanced engines have a balance weight on them. would'nt that mean they are both balanced? If so why would it make a difference which one you used?
thanks
steve

 

What year and series truck is it? What engine and transmission does it have?

 

Billy
Its a 1977 f-350 with a 460 and a c-6 Dive block. which shows to be internally balanced so the flex plate is supposed to have no balance weight on it but it does.

 

D1SZ-6375-A (replaced C8SZ-6375-A) .. Flexplate - 14 1/4" diameter = Use with internal balance / Obsolete

1968/73 429 // 1968/78 460 // Some 1979 460.
------------------------------------------------------
D9TZ-6375-A .. Flexplate - Use with external balance (has a weighted crank spacer) & block casting number: D9TE-6015-AB

Some 1979 460 // 1980/97 460.

 

Thanks Numbers
I still would like to know the difference between the two, or does the externally balanced plate just replace the obsolete internally balanced plate. I've read from someone on this forum that they had the wrong one on their vehicle and it created an out of balance vibration.

 

You cannot use an externally balanced flywheel or flexplate in place of one that is internally balanced, because a vibration will result that will tear the center from the flywheel or flexplate.

 

The difference is that with INTERNAL balancing only the stuff inside the engine is part of the main balance, and the associated harmonic damper/balancer and the flywheel/flexplate are not needed (for balancing) and must be zero balanced.
The good part of that is that it's easy to take an already fully custom balanced engine and add some new stuff on the outside without worrying that they're matched. Just zero them and you're good.

Alternately, an EXTERNALLY balanced engine cannot be balanced without those two external items added to the reciprocating mass and balanced as an assembly. They "can" be balanced separately, at least to production standards. But not to perfection. For that they have to all be attached.
This is why you can get away with attaching a new damper or flywheel to a previously balanced engine and retain a reasonably balanced engine. But racers are going to do them all at once every time.
As ND said, if you mix and match external balance and internal balance parts, you'll never get to even a remotely balanced or usable engine.

Not sure why the 460 engine had both types, but a typical reason for designing an externally balanced engine is space. Perhaps there is not enough room for larger counterweights on the crankshaft, maybe when used with a longer stroke especially, and the need arises for more material to be added then there is a convenient spot for inside the crankcase. So they remove a little from the inside and add extra on both ends at the outside.

This is my non-engineering take on it anyway.
An example of the different ways an externally balanced engine can be designed, the "V8 Family" (commonly known to most of us as the Windsor engine family, after the 351 of the group) originally utilized a 28 ounce imbalance factor. Not sure the engineering/technical details, but oversimplifying maybe, essentially without the external bits the crank assembly would be 28 ounces out of balance. Necessitating the extra counterweights on the damper and flywheel.
When the 302 member of the family went through one of it's changes, in about '81 this time, they changed the imbalance factor to 50oz instead of the old 28. I believe this was to accommodate some changes that took up some room inside the crankcase area. I have no idea what the changes were, but perhaps it was to allow for higher rpm running by stabilizing the crankshaft with more weight at the ends and less in the middle? No idea...
The 351W with it's larger crankcase, larger main bearings and lower rpm expectations perhaps, stayed the same 28, but the 302/5.0 went to the 50oz.

This is just blabbering on my part. Based wholly on assumptions made over the years that may, or may not have any basis in fact. So wait for the engineers of the group to weigh in on the subject. Especially the definition and especially the reasons for and changes made, to make an engine internal vs external.

I'm hoping to learn the correctness-of-things myself!

Paul

 

1 Ton
Thanks for the help, so, just to confirm, the flex plate for an internally balanced engine is zero balanced and the flex plate for an externally balanced engine is out of balance to compensate for the engine not being balanced thus creating a complete balanced assembly. So can both flex plate have weights on them, because the one off of my 460 internally balanced engine had a small balance weight, the new one which i believe to be the wrong one "externally balanced" had a larger weight on it. My new rebuilt engine which was balanced seems to have a slight vibration and I am thinking that the problem is the wrong flex plate.
thanks again for the help
Steve

 

Sorry, can't say with 100% certainty. We've been trying to figure out how to tell the difference between the 28oz and 50oz dampers and flywheels for years, but the size of the counterweights seems inconsistent between manufacturers.
Possibly depends as much on how many other holes and things are on a particular model flywheel or flexplate as to how big the weight must be. In theory you could tell for sure just by that size, but I don't know.

When the incorrect type is installed on a 5.0 it will idle fairly smooth, but the moment the rpm start's to climb the vibration gets steadily worse. Starting at about 1200 rpm you know something is really wrong.
Not sure if a slight vibration on a 460 indicates an incorrect part, or something more normal going on. Have you tried all the usual suspects yet? Disconnecting fan belts, pulling individual spark plug wires to see if a single cylinder is causing trouble, and that kind of stuff?

Paul

 

Paul
Yes, we did all the diagnosis for misfire, plug wires, plugs, did compression test, removed belts, all is new and all was good, but researched the flex plate and found out it is for the externally balanced engine, so thats where we are, going to r and r the flex plate next week and go from there.
My fault, when I bought the plate I wasn't fully knowledgeable of the two choices and the parts expert at XL Parts wasn't either I guess. live and learn
thanks for your help
steve