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Discussion Starter · #1 ·
I have a 1971 351C 4v engine and have taken the disassembled engine to the local machine shop for balancing etc. He has the new pistons,
bearings, harmonic balancer, 164 tooth flexplate (28oz imbalance weight), rods etc. The rods will have the new ARP bolts. The machinist says
I have the wrong flexplate, and he needs one with a 50oz weight. I've never heard of a cleveland engine using a 50oz weight. I want to believe
him, but something seems wrong. Is this normal? The doubling of flexplate weight? It's almost as if the harmonic balancer was 180 degrees
off, but theres only one way it goes together. I'm confused. Anyone with any experience, thoughts, ideas, etc would be appreciated :)
 

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Hey Red - welcome to the Forums. :tup: You are correct, all 351W, C or M of any year is 28 oz/in. Assuming your balancing man knows this, then you need to ask why. If any substantial changes in reciprocating parts weight is happening, that might help explain it, such as going stroker, H-beam rods, etc. In that case it may be less expensive to balance with more external weight, but is not the normal course taken. However, you didn't mention changes like that, so he may just have had a brain fart.

I am suggesting you humbly ask him, as you may learn something about the guy that you need to know. Perhaps he's the front man and doesn't know, or maybe he's really new, or he's trying to short-cut some weight and you need to know your options. If he's up-front about the answer, that's good. If he dances around about it, you may want to go somewhere else. Let us know what you find.

David
 

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Discussion Starter · #3 ·
Hey Red - welcome to the Forums. :tup: You are correct, all 351W, C or M of any year is 28 oz/in. Assuming your balancing man knows this, then you need to ask why. If any substantial changes in reciprocating parts weight is happening, that might help explain it, such as going stroker, H-beam rods, etc. In that case it may be less expensive to balance with more external weight, but is not the normal course taken. However, you didn't mention changes like that, so he may just have had a brain fart.

I am suggesting you humbly ask him, as you may learn something about the guy that you need to know. Perhaps he's the front man and doesn't know, or maybe he's really new, or he's trying to short-cut some weight and you need to know your options. If he's up-front about the answer, that's good. If he dances around about it, you may want to go somewhere else. Let us know what you find.

David
Thanks for the input. Very sage advice.
Bob
 

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Discussion Starter · #4 ·
Ok, here's what happened. The TCI 50oz flexplate (529620) will not work (no surprise). The 28oz TCI flexplate (529628) would not work. I ended up with a flexplate I ordered from NAPA (600-5009) for a 1973 351c 2v 28oz imbalance weight (C6 transmission). That did work, and now the balancing is successful. Hurray!
I did not use the stock D1ZZ-6375-C flexpIate I have because of the ditzy 9.2 converter bolt pattern.
I'm thinking that there IS a difference in the balancing of a 302/351w and a 351c. Perhaps the exact weight location on the flexplate? Anyways, I still need to make sure the new flexplate has the correct offset, but I won't know for sure until I pick up the stuff at the machine shop. I eyeballed it yesterday and it looked good, but the real check will happen when I get it back, but thats what converter shims are for, right?
Thanks for your response David. I had thought I was losing my mind.
 

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I'm thinking that there IS a difference in the balancing of a 302/351w and a 351c.
No, but the parts that matter beyond balance are what's necessary to then fit the transmission and torque converter, including the tooth count, converter bolt pattern, and offset (different for C6 vs C4, AOD, etc.)

David
 

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Discussion Starter · #6 ·
Could be the balance problem was with the aftermarket heavy duty flexplates quality control and or balance checking. They were way off. Its ironic the cheap plate is more accurate. The napa flexplate I will be using (BK6005009) has the converter about 1/16th of an inch closer to the transmission compared to the D1ZZ-6375-C new factory part. Should this be a concern?
By the way, the machinist was very up front and approachable. He showed me the balance machine and I was in the room with him as he tried the different flexplates. A very pleasant surprise.
 

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So long as the rest of the spec's are good for your setup, within 1/16" offset should be fine. It's interesting ot hear about the aftermarket plates being far off-balance, and one reason everything from the pulley to the plate is included in the balancing process. I'm curious how far off they were? Also good you are making a working relationship with the shop.
:tup:
David
 

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Discussion Starter · #8 ·
I did not jot down the values as the flexplates were tried, however it appeared that the balance weight locations were also off because when he rotated the crankshaft to where weight should be added or removed, there was no counterweight on that part of the crank to add/remove the weight. Kinda weird.
Bob
 

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.
plate might have been chinese.

after your trans is installed, pry the converter rearward gently with the nuts removed . . it should go back around 1/4" . . if barely moves at all, it might be too close . . this can kill main bearings in a high perf app.
 

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Discussion Starter · #10 ·
.
plate might have been chinese.

after your trans is installed, pry the converter rearward gently with the nuts removed . . it should go back around 1/4" . . if barely moves at all, it might be too close . . this can kill main bearings in a high perf app.
I will do. Thanks.
 
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