1st timer --with a ?
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1st timer --with a ?
Hello,
I Have been reading some posts, you guys are great!
My dad has a 76' with a 390 that broke the cam, so he found a used 391 industrial and had it put in. It runs great but seems to have a vibration someone told him that the flywheel was balanced for a stick tranny and he has an auto. and that Ford did not make a flywheel to fit a auto trans for a 391. I find it hard to swallow that the flywheel is that much different. Does any one have any ideas?
Thanks
Jason
I Have been reading some posts, you guys are great!
My dad has a 76' with a 390 that broke the cam, so he found a used 391 industrial and had it put in. It runs great but seems to have a vibration someone told him that the flywheel was balanced for a stick tranny and he has an auto. and that Ford did not make a flywheel to fit a auto trans for a 391. I find it hard to swallow that the flywheel is that much different. Does any one have any ideas?
Thanks
Jason
Super Moderator
Joined: Apr 2004
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Sticks use a flywheel, autos use a flexplate. Different animal, made to mate to the torque converter for the auto trans, instead of having a pressure plate bolted to them.
Anyway, your dad no doubt used the flex plate from the 390 on the 391.
The 391 has heavier internal parts, especially pistons and the crank itself. Thus, the balance for a 391 would be different than a 390.
However, the 390 is supposed to have a neutral balance flex plate or flywheel. So, look at the old flywheel and see if there is a weight on it--I do not know if 391's are internal or external balance.
The other question would be the torque converter pilot. Check this out:
Torque Converter Installation Instructions
Anyway, your dad no doubt used the flex plate from the 390 on the 391.
The 391 has heavier internal parts, especially pistons and the crank itself. Thus, the balance for a 391 would be different than a 390.
However, the 390 is supposed to have a neutral balance flex plate or flywheel. So, look at the old flywheel and see if there is a weight on it--I do not know if 391's are internal or external balance.
The other question would be the torque converter pilot. Check this out:
Torque Converter Installation Instructions
Elder User
Joined: May 2004
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From: saxton pa.
FE to FT
The 391 uses a flywheel/flexplate that is balenced differently than one from a FE . If you have the 391 flywheel you can have a neutral balenced FE flexplate balenced to match. This should get rid of your vibration.
Have A Good Day ------- Hotwrench
Have A Good Day ------- Hotwrench
FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,561
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From: Long Island USA
If you had a spare 391 crank, you take the crank, balance it to match the flywheel that came off YOUR 391.
Then, take a new flexplate (or the old one if the teeth are good) and balance IT to the spare crankshaft.
I had this done with a 289 crank and flexplate and the flexplate for an externally balanced 302, when the 289 flexplate had already been balanced to my new/reground crank.
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Thanks,
I am not familar with Ford, I know what a flexplate looks like and i Know what a flywheel that you bolt a cluch to looks like. The guy who sold my dad the motor said he had to use the flexplate off the 391. I am confused hear because if that motor had a flexplate than it had to have a auto trans on it before right? or do Ford have some two piece flexplate/flywheel that can be used with a stick or an auto?
I am not familar with Ford, I know what a flexplate looks like and i Know what a flywheel that you bolt a cluch to looks like. The guy who sold my dad the motor said he had to use the flexplate off the 391. I am confused hear because if that motor had a flexplate than it had to have a auto trans on it before right? or do Ford have some two piece flexplate/flywheel that can be used with a stick or an auto?
Laughing Gas
Joined: Aug 2008
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It sounds like the 391 came from a manual transmission, so the externally balanced flywheel was not used as it was going to be mated up to your automatic transmission.
The neutral balance flexplate from the original 390 was used as it was going in an automatic, so without the extra balancing, the vibration started.
PS: For those who know the answer (I don't), should he check the balancer? I'm wondering if he might have also used the 390 balancer. Does a 391 balancer have a counterweight in it.
The neutral balance flexplate from the original 390 was used as it was going in an automatic, so without the extra balancing, the vibration started.
PS: For those who know the answer (I don't), should he check the balancer? I'm wondering if he might have also used the 390 balancer. Does a 391 balancer have a counterweight in it.
Elder User
Joined: May 2004
Posts: 565
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From: saxton pa.
AT Flexplate
The part number for the correct flexplate for a 391HD is C4TZ-6375-F.
The Hollander interchange number is 286 -- Fits 330HD / 361HD/ 391HD
except allison AT540. FT and FE harmonic balencers will not swap between
HD/FT's and FE's The HD/FT's use a large diameter crank snout. Medium duty FT's and FE's have the small snout cranks.
Have A Great Day. ------ Hotwrench
The Hollander interchange number is 286 -- Fits 330HD / 361HD/ 391HD
except allison AT540. FT and FE harmonic balencers will not swap between
HD/FT's and FE's The HD/FT's use a large diameter crank snout. Medium duty FT's and FE's have the small snout cranks.
Have A Great Day. ------ Hotwrench
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Laughing Gas
Joined: Aug 2008
Posts: 840
Likes: 8
Ahh.....good point.....I forgot about the crank snout diameter. I forget a lot of things lately.....er....just which forum am I in ????
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