Boring out a 402 [Archive] - Chevelle Tech

: Boring out a 402


Fireman512
Aug 8th, 08, 9:29 AM
I need a little help sorting out some conflicting information. I want to bore out my 402 a bit, but I am uncertain about how far I can go. I just read in a couple of references that the 402 can be taken to a bore of 4.25 in, which would give me more choices as far as pistons go, as well as make it a 427. But, I just got an email from a shop I contacted about it and they said that you can't go more than .060 over on this block which would put me at 4.185 on the bore and limit me to oversized 402 pistons.

Help me sort out who's right, please?:confused:

Dorian

Tex66
Aug 8th, 08, 9:54 AM
Isn't a 402 a 396 bored .030 over? I'm not sure you'd want to go much more than .030 more on the bore.

Fireman512
Aug 8th, 08, 10:16 AM
Isn't a 402 a 396 bored .030 over? I'm not sure you'd want to go much more than .030 more on the bore.


One of the published references that I bought stated that the 396/402 series of engine blocks could be bored as far as .155 over putting them at 427. I found it hard to believe myself which is why I'm putting this out there in case someone has actually done it.

Schurkey
Aug 8th, 08, 10:53 AM
Wouldn't surprise me if a 396/402 block would go to 4.25".

BUT: You better sonic test the cylinder walls first. Not all those blocks had enough cylinder wall to go that far over.

ricks_67
Aug 8th, 08, 10:55 AM
I was told by my machinists that early 396 blocks 66-67 had thicker cylinder walls and could go to 427 cuin. He said years ago when they use to do circle track motors they would fill the water jacket to the bottom of the freeze out plug to strengthen the bottom of the cylinder walls when boring block to 427. he said you should run an oil cooler then cause the oil will get hotter. the rocking motion of the pistons and rods angles are what would crack the bottom of the cyl walls. Deffinately have the cyl walls sonic checked for thickness. I opted for another 402 block and had it bored .030 and just transfered my rotating assembly to the new block.

Bubba's 69
Aug 8th, 08, 11:14 AM
Is there deep gouging in the cylinder walls?

You won't be able to notice a huge difference in power between a 413 and a 427 or even a 408.

I would only bore it enough to clean up the walls, this way somebody down the road can rebuild it again. Good blocks are getting hard to come by. We need to save some now so our kids and grandkids can experience Big Block power. :thumbsup:

Fireman512
Aug 8th, 08, 11:36 AM
Is there deep gouging in the cylinder walls?

You won't be able to notice a huge difference in power between a 413 and a 427 or even a 408.

I would only bore it enough to clean up the walls, this way somebody down the road can rebuild it again. Good blocks are getting hard to come by. We need to save some now so our kids and grandkids can experience Big Block power. :thumbsup:

The walls look very good IMHO. I could quite possibly build it as is. My issue with it is that it is already .020 over and I was having quite a time finding pistons that size. I just found a place that offers a master rebuild kit for it that has pistons that size, so that might be the best option for me.

pdq67
Aug 8th, 08, 2:35 PM
You can use the old rodder's trick of trying to stick a wooden pencil between the cylinder walls through the side freeze plug holes and if it doesn't go between them, it's probably a good candidate to get sonic checked for a 4.25" boring.

And I wouldn't stop there b/c I figure a stock 454 rotating assembly will fit as well as a 454 w/ a 4.25" long stroker crank that will make a 482.6" engine will also fit!

pdq67

ss3964spd
Aug 8th, 08, 4:50 PM
I was told by my machinists that early 396 blocks 66-67 had thicker cylinder walls and could go to 427 cuin.
Definately not a completely factual statement by the machinest.

My '66 -961 block is currently at a trusted machine shop and he sonic checked the block. The cylinders are too thin to go to 4.250. I also did the "pencil test" on this early block; it went between the walls.

If contemplating boring a 396/402 block to 4.250 be sure you insist on having it sonic checked.

Dan

Busted Knuckles
Aug 8th, 08, 6:49 PM
Check the distance between cylinders by looking thru the center core hole. Paul got it partly right - a wooden pencil will barely fit between the cylinders of the thick wall blocks, but as Dan said, that's not a guarantee it'll go 4.250 - but some will. The problem with that is it'll call for custom pistons. If it's more like 1/2" between the cylinders, you'll be limited to about .060 over.
If you don't have much wear, ask the machinist if it'll hone another .010 with straight, round cylinders.

Dan Orgill
Aug 8th, 08, 8:06 PM
My '69 402 was .060 over when we went to rebuild it, and my engine guy advised me to get a new block, which I did. I would have been looking at custom pistons as well. It was cheaper buying another block.

Kevin R
Aug 8th, 08, 8:14 PM
Just go pick up a 454 block and use that then your done......:yes: Drop in your 402 crank,rods,get some 427 pistons,killer solid cam and it will be plenty of fun:thumbsup:

jimlogan
Aug 9th, 08, 8:56 AM
My local machine shop has tried boring 2 -402's to make a 427 and neither one of the blocks would go that far. There were some 396 blocks that would go that far, but you have to have the right block.

pdq67
Aug 9th, 08, 1:08 PM
Per Ed Staffel, some # 3855961 396 blocks will bore an 1/8" over and swallow a 454 rotating assembly w/ no grinding.

pdq67

mwiggett
Aug 9th, 08, 4:07 PM
My 402 had never been bored when I got it from the salvage yard. Unfortunatly water had gotten into it and seized it. Ended up going .40 over and still had some marks on 2 cylinder walls up high. The guy at the machine shop said it would be fine but was willing to go .60 over if I wanted. I looked at it and agreed that it posed no problem. .60 over would have been the end of that block and I really wanted to save it if possible. As it is it may run a bit warm but I'll live with it.

pdq67
Aug 9th, 08, 5:54 PM
I doubt .060" over will end it's life.

pdq67

Bubba's 69
Aug 10th, 08, 1:39 AM
I doubt .060" over will end it's life.

pdq67

I agree

.060" over is 99.9% not an issue at all on a big block. Small block .060" over, yes definitely, it's life is over after that unless sleeved.

I would take it to .030" over. Then you won't have any problem finding pistons.

ddeennis
Aug 10th, 08, 2:00 AM
my bbc 396 is now 4.185" bore. .090" over or a 402 .060" over ( ya a real 396 is a 4.094")

does anyone know if they make pistons to go over alittle more without being custom?

4.195"
or
4.205"
or
4.215" which would be a .090" over 402 bbc

pdq67
Aug 10th, 08, 8:45 AM
You will just have to look???

pdq67

rustbucket79
Aug 11th, 08, 2:36 PM
my bbc 396 is now 4.185" bore. .090" over or a 402 .060" over ( ya a real 396 is a 4.094")

does anyone know if they make pistons to go over alittle more without being custom?

4.195"
or
4.205"
or
4.215" which would be a .090" over 402 bbc

I've seen a few .070" overs for the 454's, but I doubt you will find something that odd for a 402. Long before you order pistons you need to have that block sonic tested. I bored a 402 .060" over recently for a guy who already had pistons and was aware of the risk of splitting a cyl wall. The block showed all the signs of "flexy" cylinder walls while honing it, even though it had a partial fill. I expect that a sonic test would show the wall thickness to be very thin, but he wants to try it.

If your block passes sonic test, I would still do a minimal overbore, and be sure to find a ring set that fits your desired bore before ordering pistons. (for obvious reasons :D)