boring out a block

[1990LTD]

I've often seen the term "bored over .somethingorother" thrown around when talking about engine builds. I understand the concept (essentially making the cylinders wider) but what I'm curious about is - what else needs to be changed when you bore out a motor like that? Seems to me the pistons wouldn't seal properly anymore but besides those, what do you need?

[86VickyLX]

I've often seen the term "bored over .somethingorother" thrown around when talking about engine builds. I understand the concept (essentially making the cylinders wider) but what I'm curious about is - what else needs to be changed when you bore out a motor like that? Seems to me the pistons wouldn't seal properly anymore but besides those, what do you need?

Pistons, rings, head gaskets need to match the size that it's being bored to.

[gadget73]

the head gaskets usually aren't a concern unless you're going way oversize. Modern blocks don't really support much over .030 or .040 safely, so thats not much of a concern. As long as the firing ring in the gasket is bigger than the hole in the block, its fine. Generally speaking, boring an engine out does not give you enough extra displacement to be significant. A .030 over 302 motor is a 306. 4 cubic inches in the grand scheme of things is nothing. Most of the time, the block is bored out because the cylinders are no longer round and straight, or they are worn beyond spec so the rings won't seal properly.

[1990LTD]

ahh, makes sense. So it would only really be done as a repair on one of these motors?

[sly]

or to build up a performance engine using a "seasoned" block.

[86VickyLX]

As long as the firing ring in the gasket is bigger than the hole in the block, its fine.

That's what I was getting at, the firing ring.

[DuceAnAHalf]

also on some motors, boring the cylinders gets the walls further away from the valves, which helps them flow a little better.

[P72Ford]

Alot of times going too far with the bore leads to issues with cooling, in engines with thin wall castings.

Some people have bored the 283 Chevy an 1/8" (makes a 4" bore) with success. My Dad did one in the late 60s, and it had issues with overheating at low engine speed.

[86VickyLX]

Alot of times going too far with the bore leads to issues with cooling, in engines with thin wall castings.

Some people have bored the 283 Chevy an 1/8" (makes a 4" bore) with success. My Dad did one in the late 60s, and it had issues with overheating at low engine speed.

It all depends on the castings too, how much meat are in them. I heard the best 302 blocks were found in Mexico.

[gadget73]

old 302 blocks, like pre-72 and the Mexican blocks are supposed to have the thickest walls. You can sonic check a block to find out how much material is there so a safe over-bore can be worked out but generally speaking, the less oversize you can make the hole the better your chances of not having stupid problems.

[1990LTD]

so not all blocks are identical in size? or does thickest in this context mean higher grade/content of metal?

[gadget73]

thicker in this context just means there is more material between the water jacket and the inside of the cylinder, so basically there is more you can cut out of the block before you run into problems. The more material there, the stronger the block is as well. Thinner blocks are lighter, but they are not as strong, and will not support as much of an oversize bore. they're cheaper to make too.

[1990LTD]

thanks.


how about parts that need changing when stroking the motor out? i would imagine heads, pistons, rod, crank

[gadget73]

stroker kit usually involves rods, a crank, and maybe different pistons. There is also sometimes internal clearancing depending on the stroke and the engine. You don't have to change the heads, but if you're spending the bucks on a stroker kit, it would be moronic to bolt crappy stock heads on top.

[1990LTD]

so the stroke doesn't dictate I put certain heads on?


I remember being given a very vague explanation of a 383 stroker...the guy said it was a 350 with 400 heads?