AnthonyH'79Delta88

Hey, I have a 350 rocket in my car and I was wondering what the max over bore is. Can I bore it all the way out to a 403? What is the distance between cylinders on a stock 350? Thankyou.

jims2000lt

I think the most you can go is sixty over. If you wnet to 403 specs there wouldnt be anything left of the block. Someone else will chime in on the maximum. Welcome.

citcapp

30 to 40 best 60 at most for long life. If you want more then go with a stroker kit or move up to a 455

Oldsguy

Welcome to CO Anthony. Are you considering an overbore for performance reasons or is the engine just worn out?

cutlassefi

I did a '71 350 a little while back. Bored it to 4.125 then sonic tested it, still had .200+ wall thickness. More than enough.

AnthonyH'79Delta88

For performance but I would still like it to be reliable. It is a daily driver.

Oldsguy

Well I would suggest you find yourself a trustworthy machinist to help you out. You will eventually need one anyway to do the actual work so why not? Over boring is usually done to true the cylinders not to increase performance, it is true that total engine volume will be increased with an over bore but you will also decrease the static compression ratio of the cylinders if you don't make other modifications. If you can formulate a total plan for your engine with the help of your machinist (engine builder) you may be pleasantly surprised. Also, I think there are quite a few members here that would be happy to help you out with specific suggestions and general engine build combos that have proven to work well with Oldsmobile engines but just boring the cylinders out the maximum allowed won't increase your performance.

cutlassefi

I beg to differ. increasing cylinder volume by increasing bore, then packing that increased volume into the same head/gasket/piston volume will increase static compression not decrease it. If you bore it .030 over, you're right, minimal performance gain. Mostly done to true up cylinders as stated. However if you bore it .100 over now you've increased the displacement maybe as much 15 cubic inches. If you assemble an engine that will make a modest 1 hp per cubic inch, that increased displacement coupled with slightly higher compression could easily amount to another 20hp. Would you be inclined to feel that? I think so. Plus by increasing displacement as well as compression, that will typically show up as a positive across the entire rpm range, another benefit. And increasing bore size also typically enhances breathing capabilities as well. Sounds like a win-win to me.

Oldsguy

My understanding of the physics that create the compression in a cylinder is flawed then, sorry for the mis-information Anthony.

380 Racer

My 380 was bored out .098 to 4.155. It was a 70 block.

Oldsguy

Ok, thinking about this some more.....larger volume heads can lower compression because they are on the end of the equeation but larger total cylinder volume (bored out) increase compression because they are on the front of the equation. i.e. you start out with an amount and cram it into a certain space you have a certain compression ratio, you start with the same and cram it into a smaller space you have a higher compression ratio. You start with more and cram into the original space you still have higher compression ratio, which is what we are talking about here. Is this right?

jensenracing77

that would be right.

70 cutlass s

A 403 has a larger piston than a 455. If you want a small block with the largest piston you can put in it than buy a 403. Don't use a 350. You'll have problems with it if you put that big of piston in it, and won't make the power your hoping for. With a 350 you make more power with a bigger crank and head work and the right cam and adding roller tip rockers, headers and the main thing is the right gears in the rear end.

joe_padavano

Yeah, considering that the 403 bore will break into the water jackets on a 350 block.