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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old May 6th, 2022, 02:23 AM
MoscowMule MoscowMule is offline
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Default Piston swap

I have a question.Has anyone punched out a 305 block to accept a 351 standard piston? I don't see why this couldn't be done.The stroke is the same 3.58.The pins are the same.Only the bore is bigger.I am trying to save my V-6.I can not find oversized pistons anywhere.
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  #2  
Old May 6th, 2022, 06:02 AM
Prowbar Prowbar is offline
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Default Re: Piston swap

Think you hit the water jackets by enlarging the bore that much, maybe one could sleeve the block?
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Old May 6th, 2022, 09:15 AM
MoscowMule MoscowMule is offline
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Default Re: Piston swap

I thought I read somewhere they are the same block just bored different sizes.
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Old May 6th, 2022, 02:18 PM
James James is offline
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Default Re: Piston swap

The following is base on my own observation, measurements, research, and opinion. Your results will be different.

I have measured the wall thickness on a GMC 305 V6. This is what I have found (see attachment):

Base on my research a minimum wall thickness for a naturally aspirated engine is .100", thicker if turbo/super charged/NOS injected, running near stock compression.

You will need to have the cylinder walls ultrasonically tested and base on the minimum thickness you find, of the cylinders, and subtract .100" this will be the most material that can be remove.

Example:
Thinnest measured wall thickness: .232"
▼▼▼▼Minimum wall thickness: -.100"
▼▼Material that can be removed: .132"

▼Minimum wall thickness times 2: .264"
▼▼305 V6 stock bore diameter: +4.250"
Maximum cylinder bore diameter: 4.514"
▼GMC 351 V6 cylinder diameter: 4.560"

I do know some natural aspirated racing engines will go down to a lower minimum thickness depending where it is located in the cylinder. Also some machine shop will offset bore the cylinder in favor of a thicker thrust side.

Bottom line it depend on the engine and if you want to risk boring it out that far. No two engine are alike.
Attached Files
File Type: pdf Cylinder Wall Thickness.pdf (34.5 KB, Multiple views, 9 clicks)
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Old May 6th, 2022, 06:18 PM
BobBray BobBray is offline
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Default Re: Piston swap

How bad is your block? Generally speaking 351 pistons are harder to come by than 305 pistons. Anyway, you are talking about going from 4.25" bore to 4.56", that's an awful lot of material to remove and because of the high nickel content of the V-6 block they don't cut easy. And that's assuming there is enough cylinder wall to go that far as others have pointed out. A quick check on Ebay shows 2 .060" 305 pistons available now. Your best bet is probably just keep looking for 305 pistone, they do show up.
https://www.ebay.com/itm/31265662593...hoCn-IQAvD_BwE

I have no connection to the seller.
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Old May 6th, 2022, 11:02 PM
MoscowMule MoscowMule is offline
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Default Re: Piston swap

Quote:
Originally Posted by Prowbar View Post
Think you hit the water jackets by enlarging the bore that much, maybe one could sleeve the block?
I asked about that.The machine shop guy said that would be a lot of money in labor.
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  #7  
Old May 6th, 2022, 11:12 PM
MoscowMule MoscowMule is offline
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Default Re: Piston swap

Quote:
Originally Posted by BobBray View Post
How bad is your block? Generally speaking 351 pistons are harder to come by than 305 pistons. Anyway, you are talking about going from 4.25" bore to 4.56", that's an awful lot of material to remove and because of the high nickel content of the V-6 block they don't cut easy. And that's assuming there is enough cylinder wall to go that far as others have pointed out. A quick check on Ebay shows 2 .060" 305 pistons available now. Your best bet is probably just keep looking for 305 pistone, they do show up.
https://www.ebay.com/itm/31265662593...hoCn-IQAvD_BwE

I have no connection to the seller.
I thought I read somewhere before that the 305 and 351 shared the same block just a bigger bore on the 351.I know where I can pick up a 351 for a few hundred.If nothing else I guess I could rebuild the 351 if the block is decent.
My block has one cylinder that had a broken ring and a double ridge in it from it.I asked him about pulling a sleeve in that cylinder .He said the rest of the cylinders are to bad to make it worth it.Also I can't find .060 rings so pistons won't do any good without them.
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Old May 7th, 2022, 06:00 AM
BobBray BobBray is offline
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Default Re: Piston swap

You could use a 351 short block with your 305 heads and manifolds and create a 351C, it would run fine. Did you try Hastings for rings? I have always had good luck with them:

https://www.hastingspistonrings.com/
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Old May 7th, 2022, 08:26 AM
Prowbar Prowbar is offline
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Default Re: Piston swap

If you get the chance I'd get the 351 if it is a good runner. Swap that for the 305.

Machine shop work costs much more...
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  #10  
Old May 7th, 2022, 01:04 PM
MoscowMule MoscowMule is offline
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Default Re: Piston swap

Quote:
Originally Posted by Prowbar View Post
If you get the chance I'd get the 351 if it is a good runner. Swap that for the 305.

Machine shop work costs much more...
I think I will go check it out next weekend.It is a complete motor with everything on it.The good thing about this place is he let's me tear thing down to check them out before I would have to buy it.He has several 305 but there is nothing in them that would help me.
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