I was just thinking about a mystery big block I used to have. The part number on the block pad said it was a 402 when I looked it up, but I remember taking the heads off and putting closed chamber ovals on it and the bore measured to make it a 427 dead on. Being a kid back then I never gave it much more thought, but now I am wondering if it could have actually been bored out that much.

I know there are some 396/402 castings that can be bored .125". I dont recall the number off the top of my head, but I've read it more than once. They were used to make both 396/402's and 427's.

A factory 396 came with a 4.094 bore and 3.76 stroke. 402 was just a .031 overbored 396 from the factory to increase the displacement to 402 sharing the same stroke as the 396 @ 3.76 stroke. Even if you were to over bore a 396 +.125 the resulting bore would only be 4.219. Short of the 4.250 bore required with a 3.76 stoke to make the 427 displacement. I would not bore a 396 .125 over. Even sonic checked most of these casting would not do well with that kind of bore increase. .090 over is about your maximum. .060 over is a safe limit. Perhaps some others can give you some more in depth info as to wall thickness issues related to specific bores.

A factory 396 came with a 4.094 bore and 3.76 stroke. 402 was just a .031 overbored 396 from the factory to increase the displacement to 402 sharing the same stroke as the 396 @ 3.76 stroke. Even if you were to over bore a 396 +.125 the resulting bore would only be 4.219. Short of the 4.250 bore required with a 3.76 stoke to make the 427 displacement. I would not bore a 396 .125 over. Even sonic checked most of these casting would not do well with that kind of bore increase. .090 over is about your maximum. .060 over is a safe limit. Perhaps some others can give you some more in depth info as to wall thickness issues related to specific bores.

OK, this made me dig out the numbers.
Block number 3855961 1965-1966 was used for both 396 and 427. In the 369 it was used in both 2 and 4 bolt applications. In 427s it was 2 bolt only. This info is from "Maximun Performance Chevy Rat Motors", by Ed Staffel.

That might be what it was. What made me probably think that it was a 402 was it came in a 70 El Camino donor car.

Now thinking about it more I remember looking in a book about big blocks at Pep Boys and seeing it was an Impala motor from 66 or 67. I didnt do drugs back then or now, but I sure feel like I did. So probably it was a factory 427 transplanted into that 70 El Camino.

Thanks for the replies! Mystery probably solved!!

Watch out for the infamous rear cam bearing issue on that block. If there's no groove around the rear cam bearing bore in the block the rear cam bearing install is tricky and the cam must have a groove in the rear journal. The shallow groove in some rear cam bearings is not near large enough. If the block has the groove it it's not big issue and the cam doesn't need grooved.

Years ago when I was in the automotive machinist program at JCJC junior college, we took a junk 396 block and bored it until we hit water (after reading in one of the books that some early 396 blocks could safely tolerate that much overbore). The block was junk, and it was a good learning experience (learned how to sharpen a bit before it was over, that's for sure!)

I forgot what it took before we actually hit water..but it was a lot more than .125" It probably would have run on that particular block and been OK...but it's way to expensive and risky to try it in the real world.

Auto machine shop school was fun until it was time to get a real job :(

There are some guys that run a 454 crank in a 396 and bore it to a 4.250 bore.When I was looking for a shortblock ealier this year I had a guy in Michigan that builds these engines all the time email me.Im not sure if they do it as kind of a stock look and fool people but according to him they do it alot.They obviously sonic check the blocks to make sure they can bore them that much.Im not sure how much work goes into one of these but the guy also told me that Ohio I believe makes the kits special for them.I didnt end up going with it because it did make me alittle nervous since I was going to use the car mostly for street.I had a 125 over 454 that had a offset ground crank that I ran for 3 years and it blew up big time.It was a 289 casting block so they will take the bore but I think all the street driving finally got to it.

I have seen the later model 854 402 blocks bored to 4.250....some of those castings have the wall thickness to get it done. Obviously sonic testing is the only way to now for sure....as for the 396 stroker concept...it sounds cool...have read several articles about it....easy way to produce a ton of torque!

wilma

I have a '66 #961 block that has "standard bore" 427 pistons in it...

Tony,

Do you know what the casting date of the block is?

Dan

Tony,

Do you know what the casting date of the block is?

Dan

Sorry Dan,

Its in the car, which is parked out in the rain right now (wet ground, so I can't get under it to check)... I did have it written down, but I recently moved, and can't find alot of my notes... I might have some digital photos of the date, I'll check...

If I get the chance, I will try to check the assembly stamp (pretty sure its still there)...
Tony

Thanks Tony.

Getting ready to pull the 961 block 396 from my ride for a little build up.

Dan

Let me just throw something in here as food for thought.
Rather than boring the 402 all the way out to make a 454 (.125 over bore), take it out .060 over, stick a 454 crank in it and you will end up with about 440. This will only be about 14 cubic inches short of being a 454, you'll have thicker cylinder walls and if built up just as you would build a 454, it is very unlikely that you would not see a noticeable difference between the 440 and 454. Furthermore, this would leave room to bore it another .030 for a future rebuild.

Good points Tom.

Legend has it, however, that the early 961 blocks, having been used for both 396's and 427's, have thick cylinders, expecially with the 396 size bores. Therefore, one can bore a 396 961 block to a 4.250 bore and still have plenty of wall thickness left.

Supposedly.

It is often talked about but I don't think I've ever read conclusive proof that someone has done it, and provided wall thickness measurement before and after boring a 396 961 block to 4.250.

I plan to find out though. I also plan to find out if a 4.25 crank will fit in the 961 block.

Dan

Legend has it, however, that the early 961 blocks, having been used for both 396's and 427's, have thick cylinders, expecially with the 396 size bores. Therefore, one can bore a 396 961 block to a 4.250 bore and still have plenty of wall thickness left.


I think this motor was supposedly built by a well-know builder ( I am thinking, Kenny Duttweiler?). The entire lifter valley (and rods?), etc have been ground smooth, all head bolt holes Heli-coiled...

But I am curious Tony, did it start out as a 4.094 bore, or did it start out as a 4.250 bore?

Dan

But I am curious Tony, did it start out as a 4.094 bore, or did it start out as a 4.250 bore?

Dan

The deck stamp indicated it was a 325HP 396 (I think, been a while)... So 4.094"... Pistons are TRW forged (I forget the number now), 427 standard bore...

Very interesting Tony, thanks.

Can't wait to find out what the block casting date of yours is, or even the final car assembly date.

Dan

When I bought my camaro years ago it had a 402 block that was bored to 427. If I remember correctly it was dated as a 70 and the casting # was 3999290. It too had standard 427 pistons in it. I know the walls were thin because #6 cracked and that was the end of it.

Fwiw, Ed Staffel say's at least some -961 block's will bore to 4.25" and will swallow a 454 rotating assembly..

And I figure where a 454 crank will fit, a 4.25" stroked crank will fit too!!

And for that matter, a 4.375" stroked crank!!

pdq67