Swapping a 307 for a 350 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Jimjific · 10-11-2018, 09:28 AM

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

**Warrens69GMC** · 10-11-2018, 10:08 AM

As long as the crate 350 is 85-under, you are good to go.

Mike C · 10-11-2018, 12:56 PM

Quote:

Originally Posted by Warrens69GMC

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.

Jimjific · 10-11-2018, 01:17 PM

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?

Thanks,
Jim

**Boog** · 10-11-2018, 02:20 PM

You will likely need to put a pilot bearing in that 350 crankshaft.

68Gold/white · 10-11-2018, 04:24 PM

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!!!

garyd1961 · 10-11-2018, 05:21 PM

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.

68Gold/white · 10-11-2018, 05:34 PM

Quote:

Originally Posted by garyd1961

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!!!

Jimjific · 10-12-2018, 08:09 AM

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

Jim

10-11-2018, 09:28 AM	#1
Jimjific Registered User Join Date: Aug 2013 Location: Albany, NY Posts: 145	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 __________________ Thanks, Jim - 69 Short Step CST 350 AC/PB/PS - 69 Short Step 307 Man Column/PS

10-11-2018, 10:08 AM	#2
Warrens69GMC Registered User Join Date: Jan 2003 Location: Safford,AZ Posts: 3,625	Re: Swapping a 307 for a 350 As long as the crate 350 is 85-under, you are good to go. __________________ 1969 307, t350 1950 Chevy Wagon 1978 Big 10 1967 C10, 250,3-OTT

10-11-2018, 01:17 PM	#4
Jimjific Registered User Join Date: Aug 2013 Location: Albany, NY Posts: 145	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? Thanks, Jim __________________ Thanks, Jim - 69 Short Step CST 350 AC/PB/PS - 69 Short Step 307 Man Column/PS

10-11-2018, 02:20 PM	#5
Boog laying low Join Date: Mar 2001 Location: Searcy, Ark. USA Posts: 13,534	Re: Swapping a 307 for a 350 You will likely need to put a pilot bearing in that 350 crankshaft. __________________ Boog 69 Chevy stepside, 358/T350, 4.11 posi, 4.5/4 drop, rallys, poboy driver primer is finer 91 Chevy sportside, Tahoe, Yukon & GMC Crewcab All GM..'nuff said. I stand for the flag and kneel at the cross

10-11-2018, 04:24 PM	#6
68Gold/white Registered User Join Date: Sep 2011 Location: Ark City, Kansas Posts: 3,411	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!!!

10-11-2018, 05:21 PM	#7
garyd1961 Registered User Join Date: May 2016 Location: Henderson NC Posts: 975	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.

10-12-2018, 08:09 AM	#9
Jimjific Registered User Join Date: Aug 2013 Location: Albany, NY Posts: 145	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 __________________ Thanks, Jim - 69 Short Step CST 350 AC/PB/PS - 69 Short Step 307 Man Column/PS



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