[pjdutton]

I have a serious question to ask.Can a p350 block be overbored to the 400 bore size?I have seen so many people ask this question but there has been no clear answer.

[tom s]

would only know with a sonic test.Tom

[screamingchief]

I'm generally not a betting man,but I'd bet $20.00 that most any 350's will not be able to go that big.

That's nearly a .250" overbore! (.245" to be exact)

Think about that a second...

That's nearly an 1/8" off each side of the bores.

I dont care what a person does to the block (filling,caps,etc.),I just dont think it'll go that big...

Going to a 4" stroke is the best option for gaining some cubes in a 350 IMO,talk to Ace @ PPR,he said something about offering a combo for that.

I do wish someone would prove this out though,seems like this question just keeps coming up all the time lately,would stop this line of debate dead in it's tracks,one way or the other.

[Reed179]

I'd like to see someone try to bore a 350 block .245 over. as well, and make it live on the street. I don't think so....

[bobzdar]

Well, according to some analysis done in this thread:

http://classicalpontiac.com/qa7/disp...-+why+no+350?&

All of the pontiac blocks appear to be the same casting, so it should be possible provided there isn't any shift moving the water jackets closer to one side of the bore or the other. I think it needs some more in depth analysis, but per above the 350 casting should provide a wall thickness of .207" at 4.181" bore (455 + .060" overbore) which is what the recommended minimum wall thickness is, and is probably very comparable to what an .060" 455 or .090" 400 has...

Somebody should really take one of the thousands of 'scrap' 350's (or even 326's) around and try it or at least cut it up so we know for sure, it seems like everyone just assumed it couldn't be done. In the 7 years I've been around Pontiacs I've never heard of somebody trying it, only that it couldn't be done.

[engineer]

so now I should save all the 326 and 350 pontiac blocks I can find to make them 400 or 455s, when those cores are gone. Hmmm!!! what am I missing. I have a 326 right now if someone wants to bore it over to confirm this. Block is in Houston.

[77TA406]

Now I'm glad that I saved all those 350 blocks that I bought just to get the cranks...

[screamingchief]

I've seen that thread over there on CP before,don't really agree with it entirely,and would'nt bet much on it being the final word on this either.

As I remember they said they cut a 400 block,then based their assumptions on the fact that the spacing between the cylinders in the water jacket were the same between the 400 and 350,therefore the castings were all the same.

And like I said before,I would'nt bet the farm on it,thats all.

But I could be wrong.

Like I said before as well,wish someone would prove this one way or the other,we need a pontiac "mythbuster" on this one!

And IMO it'll take more than cutting any one single block to prove otherwise,more like a couple of blocks from various years of each casting #.

Too many chances for variations to be 100% sure as a "fact" that they all would be able to go that big.

Sonic testing is the only real way to know for sure before any machining is done.

Me myself,I dont like to run "minimum wall thickness" just to run a 400,if I want a 400,I'll use a 400 to begin with.

If I want more cubes from my 350,I'll use the 4" stroke crank!

The rest of ya'll can do as you see fit,good luck to all.

[bobzdar]

A standard 400 bore would leave much more than minimum wall thickness, the .207" wall thickness was for a 4.121"+.060" (455 +.060") or a .091" over 400...

[Thumpin455_GTO]

theoretically... I have a few 350 blocks if anyone wants to bore one out to 4.181, just sittin there in Nebraska without cranks. I doubt they would just use the extra iron, they have different casting numbers, and cast in designations in more than one place on the block. Why would they have an extra 1/8" of iron around the cylinders on every 350 block, the bean counters wouldnt have let that one slide.

[Jeff Walker]

I thought I remember reading somewhere that this was tried and they wound up boring into the water jacket.

[Tom Vaught]

I dis agree with the statement that all late 350/400/etc pontiac blocks
are the same.

Who is going to cut up a couple of blocks?

Tom V.

[WARPed]

Do a sonic test on a standard bore 400 and 350 you will find they will have similar cylinder wall thickness and not having a “thicker” wall on a 350 block.
As mentioned above the 350,400 & 455 all had there own separate castings.

[chiphead]

Yeah, I'm gonna call "BS" on this one... LOL. Think of how many 350s could be made into good running 389s if a 389 slug would fit.

[bobzdar]

Well, this is verbatim from the above link if you don't want to wade through it, I definitely think it's worth investigating. However I don't have the equipment or space to do it currently as my pontiac and all related stuff is in NY while I'm in VA working, maybe this summer if nobody else gets around to it...

" broke out my trusty $6 calculator and crunched some numbers real fast:

350 standard bore 3.875"
400 Standard bore 4.120"

4.120-3.875 is a total bore difference of .245" (or approx 1/4 inch).

Now, the 350 std bore block I cut had a cylinder wall that measured about 3/8 of an inch at it's thinnest section or .375"

The .245" difference between the 350 & 400 bore size gets divided by 2 resulting in .1225" needed to be removed from the 350 cylnder wall.

.3750" minus .1225" leaves a wall thickness of .2525" or a little more than 1/4 of an inch.

Punching in the numbers for a 455 is also interesting.

350 standard bore 3.875"
455 standard bore 4.151"

Overall bore difference of .276"

.276" divided by 2 is .138" leaving a wall thickness of .237" or just a tad less than 1/4 of an inch.

Going a bit farther, let's do a .060 over 455:

350 standard bore 3.875
455 .060 over 4.211

Overall bore difference .336 divided by 2 is .168 which would leave a wall thickness of .207

Please note that these minimum 350 wall thickness would not be for the entire lenght of the bore but only for a very small section, about 1/2" or so in the middle of the bore as the walls get progressively thicker toward the top & bottom reaching a thickness of about 1/2" or so at the very top & bottom.

So, punching a 350 block out to a std bore 400 leaves a wall thickness of .2525", to a std bore 455 leaves .2370" and a .060 over 455 would leave .2070"

Now, the 50 million dollar question is either

A: Just how thick are the walls on a 400 block (or a 455)?

or

B: What is the safest minimum cylinder wall thickness you should allow?

We can't answer "A" until somebody actually cuts a 400 block, measures it and posts the results.

On "B", doing a quick check on the web shows that a minimum cylnder wall thickness of .18 to .20 seems to be the general concenus of being "safe", although some have gone as thin as .10 (with admitted risk).

Based on just the "considered safe" minimum wall thickness of .18 to .20, it would appear that a 350 Pontiac block bored to either a std bore 400 or even a std bore 455 or even a .060 over 455 would be well within the "safe" zone.

Keep in mind that it's somewhat "iffy" when boring a true 455 block .060 over when it comes to hitting water if the block has bad core shift.

The link above references a simular delima faced by a Mercedes owner as far as max safe overbore to upgrade a "smaller" Mercedes engine into it's later model "larger" engine of almost identical design."

[tom s]

in my experiance of having block soniced,1 cyc does NOT show a good example of the rest of the bores.Tom

[screamingchief]

in my experiance of having block soniced,1 cyc does NOT show a good example of the rest of the bores.Tom

Took those words right out of my mouth.

I'll go even further,one cylinder,in one block is hardly representitive of every cylinder in every "other" block.

Block to block there are going to be pretty significant differences very likely.

Just when you think you know the answer to all this,you'll be getting a call from your machinist telling you your "big bore" 350 block is a paperweight now!

I would like to believe it could be done,dont get me wrong,just that I'm a "glass half empty" kind a guy (ie. a pessimist).

With my luck it would work for everyone but me!

[Jack Gifford]

Without looking through receipts, I'd say I only paid about $50 for sonic wall measuring of a block. I'll chip in a buck if 49 more of you guys will.

[WARPed]

I truly hope there aren’t any green horns out there that would try this. Sure would be a major letdown when they find out they just wasted all that time and money. Lets face it, if this was a true viable option it would have been done years ago and not “just discovered” now.

["Puddles the Pontiac"]

here is my results from my last sonic check. it is on a '72 .060 455 over 30,000 VERY hard and abusive miles. pistons/wall clearance at about .019. who would consider this thin? i would be more then happy to have a 350 bored out to 400 bore if anyone near by has one to donate. h&)), i'll even by the block if there is one near by. by the way, wouldn't a thin walled engine run cooler do to less iron to keep the heat trapped?