OVERBORE A 350 TO 4.12"?
 

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.

would only know with a sonic test.Tom

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.

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....

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.

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.

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

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.

:Peace:

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...

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.

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

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.

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.

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.

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."

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! :doh:

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! :rolleyes:

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.

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.

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?

If somebody wants to ship a 350 to me in Las Vegas,Nevada, I can sonic check it and then throw it on my boring bar and cut it until I hit water. Then everyone will know.

Why bore it to 4.120" ? Bore it to SBC size of 4.00" for 383ish cu in and open up a world of cheap ,lightweight pistons!

Seems to me that's what the knock on the stock 400 chebby engines were. They had heat problems because a 350 block was cut to 400 bore and the walls weren't thick enough. They had a tendancy to heat up. That was the buzz back in the day anyway.

Robert

That SBC info is'nt exactly true,the 400 SBC is a siamese bore block,ie. the cylinder bores are all connected together internally to gain the needed cylinder wall thickness,thus they lack cooling between the cylinders,hence the heating issues.

Now a 4" bore would be a much more realistic "maximum goal" bore to consider for a deal like this IMO,and with a 4" stroke you would have a "square" combination B/S wise.

That combo would yeild 402 cu.in.

The pin heights will still likely be different though,so dont count on being able to use "off the shelf" pieces for other brand engines.

And it would still need to be verified by a sonic test of all the bores on the block in question,and it may take checking a couple blocks to find the "right one".

As well I'd want to fill the block to the freeze plugs for some added support for the bores (just to be safe),and I'd also plan on using an oil cooler for any extended "street" miles.

what i heard was the sbc 400 had siamese cylinder's
and that was the cause of the cooling/overheating prob's

A little off subject but I believe that whole chevy 400 thing was a myth-or they just didnt have an adequate cooling system.Ive built a few 400 chev engines back several years ago with zero issues-not even a hint of overheating with highflowing three cores or stock four core radiators.The big thing was to build 383 strokers but I always preferred the 400 for its ability to make the same h.p. at lower rpm and provide superior torque-great for a street/strip car.

the front of that #5 cylinder, is that really 0.028" thick???

no, that was an ink splotch. the reason i showed that was because this was the engine i used everyday. driving and racing. for three years, it was used in my daily driver.most would think this is too thin. i want to offset bore it even more. thought it might relate to the topic.
p.s. anyone ever go more that 4.211?

I have a couple of 350's at the shop. let me know if I'm close enough. I'm in Johnson City.

I would like to know if a 350 can be bored to 4.00" this is about a .120 overbore 4" bore/ 4" stroke sounds like a cool 350. looking at a 400 compared to 350 I don't think there is anyway to bore it to 4.12

I have a 1975 standard bore 350P in the hot tank right now and will do more measurements after it is cleaned up, but for now, I don't think it will be possible to bore a 350 out to a 400. Here's why...correct me if my math is wrong. Through a frost plug hole, I measured the distance between the cylinder walls and got .381...I also measured at the top of the deck and got a measurement of .748 between two cylinders. This leaves a distance of .367, but since that figure includes the thickness of two walls, it must be divided in half, which only gives us a wall thickness of .1835. As stated in the post by bobzdar,a 350 will need to be bored out .245 total, or .1225 per wall. This would only leave .061. Maybe my math is wrong...let me know opinions.
Chris

If the minimum 'safe' wall thickness is .200...How could a standard bore block only have .1835 thick walls? I'd have to check it out to see, but perhaps that is correct, or perhaps there's a lot of core shift in that particular block?

Hi all. I have now measured four different blocks...three 350's ( 1974 & 75 ) and one 400 ( 1977 ). and here is what I have found. The measurements are not accurate to the thousandth, but will still give everyone the answer they are looking for. When measuring the distance between the cylindrer walls through the frost plug holes, all the 350's were approx. .370 inches. When measuring the distance between two cylinders on the deck (where the head bolts up), all the 350's were approx .745 inches. If my math is correct, that gives a cylinder wall thickness of approx .187.
On the 400 block, the measurement through the frost plug hole was approx .210 inches and the top measurement was approx .495 inches, which, if my math is correct, gives a cylinder wall thickness of approx .142. I know that seems pretty thin, but thats how I calculate it. Either way, it's now obvious that 350 and 400 blocks do not use the same casting, based on the distance between the cylinders when measured through a frost plug hole. Earlier blocks might be different...who knows. If anyone else wants to recalculate my figures, I would sure like to know what they find...
Chris

Thank you Chris, for the research.
To he who said they seem awfully thin- remember that Chris is measuring fore/aft numbers- the wall thicknesses in the "thrust" plane may well be more.

PJ...you are correct about the thickness in other areas of the cylinder. My measurements taken through the frost plug hole were at the widest...or thinnest point of the wall. The cylinder wall tapers at the top and bottom, so those areas are considerably thicker.
Chris

I just bought a 1970 350 for $3.50. I did some research and .060 or 3.935 final bore size is max. Checkout this article link.

http://popularhotrodding.com/enginem...9PHREMPontiac/
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Here's an article out of my 2003 Hot Rod magazine.

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