307 to 403 swap

fleming442

Captain Tenneal
Supporting Member
Dec 26, 2013
13,061
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That's what I'm trying to tell ya. Olds aren't entirely different from anything, but there are a few things that require extra attention. Speed Pro/TRW pistons come with an incorrect spec. Increase the piston to bore clearance. Olds cranks are traditionally wet noodles. A forged 330 crank helps you immensely. Lighten up the pistons and rods, get a good balance, and it'll be ruthless.
 

lkovacs

Not-quite-so-new-guy
Thread starter
Dec 4, 2020
25
3
ok. so tell the guy to take 0.060 inches off the top and bottom of the heads, make sure it has 004-005 piston to bore clearance, lighten up the pistons and rods, get a good balance, and that's all for the actual machining of the engine?
 

lkovacs

Not-quite-so-new-guy
Thread starter
Dec 4, 2020
25
3
I'm just combining what other people have replied with; I think 307 was the one who told me .060" off the heads.
got it. don't lighten the pistons. just the rods?
 
Oct 14, 2008
7,102
113
Melville,Saskatchewan
The Speedpro forged pistons have .001" clearance on the box and .002" in the instructions, both wrong. Tell them .004" minimum. For milling, 0 deck the block and 72cc should put you at 9 to 1, about the minimum you want with big port heads. As said, make sure the intake surface on the heads is milled enough for the thickness of gaskets you plan on using. Mine weren't milled enough and only shim gaskets would work. I don't know of any shim intake gaskets in the BBO port size other than the turkey which would need to be cut off.
 
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Oct 14, 2008
7,102
113
Melville,Saskatchewan
Sort of.... where did you come up with 060 off the heads? The combustion chamber size will play into overall compression.
Get lighter pistons if you want; don't lighten yours.
It is an educated guess BUT the heads should be CCed to make sure they are still 80+ CC stock combustion chamber size. 1cc is a .006 cut, says the experts on Olds V8's. They may have already been cut.
 

lkovacs

Not-quite-so-new-guy
Thread starter
Dec 4, 2020
25
3
ok thanks guys. Also quick question. Would it be stupid to use a manual transmission instead of an automatic? is it a crazy amount of work or is it viable? what would need to be done.
 

fauxfour2

Greasemonkey
Apr 21, 2020
101
43
ok thanks guys. Also quick question. Would it be stupid to use a manual transmission instead of an automatic? is it a crazy amount of work or is it viable? what would need to be done.
I dont think its stupid its just more work, fabricating and sourcing. Work on what you know and what you need to know. The 403 is a direct drop in and a 200-4r or thm350 will also fit right in with no real work and allow you to use your driveshaft (iirc). Im sure many someones here have put a manual in a gbody but im pretty sure there was more work involved. I love a manual in certain cars but couldnt see myself putting one in my gbody. Thats my personal preference but whatever you do I hope it goes as smoothly as possible.
 

fleming442

Captain Tenneal
Supporting Member
Dec 26, 2013
13,061
113
The G body bellhousing and release fork are odd ducks, and I don't know if you can get one in a BOP pattern. BUT, I do think I've seen people post about having T5s behind 260s. If you could track the stuff down, it might not be too bad. It would definitely be easier and cheaper to just slam an auto in, tho.
 
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lkovacs

Not-quite-so-new-guy
Thread starter
Dec 4, 2020
25
3
yeah an auto would be easier and more original... this is gonna sound dumb but I feel like I would enjoy a manual more. My time driving my car is like an escape from the world. I don't put music on unless i have passengers because listening to my motor is like meditation. I feel like if I had a manual it would be a more enjoyable experience for me... I don't know. I'm trying to get my dad to teach me stick as a graduation present so we'll see. I know I'm probably gonna suck at it lol
 

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