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Old 10-11-2018, 09:28 AM   #1
Jimjific
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Swapping a 307 for a 350

Hello all,

I plan on pulling a crate 350 from an automatic truck I have and putting it into a 1969 truck with a 3 on the tree 307. Are there any balancing issues to worry about or can I use the 307's flywheel?

Thanks,
Jim
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Old 10-11-2018, 10:08 AM   #2
Warrens69GMC
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Re: Swapping a 307 for a 350

As long as the crate 350 is 85-under, you are good to go.
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Old 10-11-2018, 12:56 PM   #3
Mike C
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Re: Swapping a 307 for a 350

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Originally Posted by Warrens69GMC View Post
As long as the crate 350 is 85-under, you are good to go.
Warren is absolutely right, but to clarify in detail: In '86 GM switched to the one piece rear main seal that removed the rear counterweight. Any V8 after that point has an externally balanced flywheel and a different bolt circle so they require the new style flywheel. If you have an aluminum seal adapter on the crate motor behind the flexplate, it is a one piece rear seal block so you will have to find a later style flywheel.
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Old 10-11-2018, 01:17 PM   #4
Jimjific
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Re: Swapping a 307 for a 350

I'm not sure what year the motor is. It is a GM Goodwrench one though. The guy I bought the truck from (it is also a 69 but automatic I bought it in 2015) said he was going to get the original one rebuilt but the builder told him it was cheaper to get a crate motor and put it in.

If he was just doing a replacement and being that it is an automatic, I would assume he would have had to replace the torque converter plate if the balancing was different. So if I see no weights on the torque converter plate would that mean it is internally balanced and I could just use my flywheel I have?

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Jim
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Old 10-11-2018, 02:20 PM   #5
Boog
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Re: Swapping a 307 for a 350

You will likely need to put a pilot bearing in that 350 crankshaft.
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Old 10-11-2018, 04:24 PM   #6
68Gold/white
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Re: Swapping a 307 for a 350

You can look at the ends of the crank shafts to tell, what's going on.

On the later engines (with one piece rear main seals), the end of the crank shaft (where the flywheel or flex plate bolt to), the crankshaft is perfectly round, a smooth machine surface.

The earlier crank shafts have an oblong shape where the flex plate or flywheel bolt to. Part of this odd shape helps balance the crank shaft.

IF, the end of the crate engine crank is round, this swap will need a standard fly wheel made for the later engines!!!
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Old 10-11-2018, 05:21 PM   #7
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Re: Swapping a 307 for a 350

Am I right, if the 307 flywheel fits the 350 then it's a 2 piece rear main seal and he should be good to go.
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Old 10-11-2018, 05:34 PM   #8
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Re: Swapping a 307 for a 350

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Am I right, if the 307 flywheel fits the 350 then it's a 2 piece rear main seal and he should be good to go.
Yes!!!
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Old 10-12-2018, 08:09 AM   #9
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Re: Swapping a 307 for a 350

Thanks for the info everyone. I will be pulling the 360 first and see what I have.

Jim
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