.60 overbore on a 1968 340

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efar

efar

Coastal Cuda
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My engine builder is recommending a .60 overbore on my 1968 340, it's at .40 now. I know the books say you can do this but does anyone have any first hand experience with this? Pros and cons? The car is a 1968 Barracuda, building a nice hotter than stock weekend and show driver, original X heads, 3.55 gears, automatic, LD340 intake, Holley 600 carb.
 
A good builder would sonic check the block and see if there's enough meat left to go .060 If he says it's okay, then I would go for it.
 
If the block did not have excessive core shift when it was cast it wont be a problem but a sonic check sounds good to me.
 
The 340 will have a tendancy to run hot at .060 over. As others here have said, Sonic test first.
 
Demon416 said:
The 340 will have a tendancy to run hot at .060 over. As others here have said, Sonic test first.
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I have heard that also. I will have a worn out .030 over 340 for sale soon. :banghead:
 
I've done it at least 20 plus times...it will not run hot! You will have plenty of wall"thrust" for a carb set up.If your going to spray 250 or more I'd get it sonic tested..I've sprayed with .080 thin spot and it's still clucking along to this day.
 
There are really no pro's except having straight walls. Con's are weaker cylinder walls that flex more under a load. Flexing cylinders mean less ring seal which in turn means a power loss. Definetly have it sonic checked because you never know what you can get away with until you know how thick the walls are. I've seen a few blocks that were ok to bore .100 over and others that were only safe at .030 over.

The old controversy of an engine bored out excessively will run hot will be a controversy forever. Some guys claim it can't happen while others (me included) say it can because the thinner the cylinder walls are the easier the heat disapates into the cooling system. I might be all wet on my thinking (no pun intendeed) but that's my thoughts on it. Take it for what you will, I'm not a professional engine builder but have built a # of engines and saw on at least 1 occasion a buddy that had his 327 Chevy block bored .060 over and he had a hard time keeping it cool afterward when before he didn't. And before anyone says it's possibly because the clearances were tighter after the rebuild I can say for sure they were just barely tighter. It was already .030 over and didn't need bored again but he was greedy for the extra 5 cubic inches.
 
Sell it to someone else or store it and stroke a 360.
You can buy a virgin 360 block for less than the sonic test.
Or..... you could sleeve the 340 back to stock and still have a
weak motor when you could of had a monster for the same money.

Then you can add NOS.
 
i tend to think engines that have been bored a few times run hot only because the water passages are not cleaned out properly. just my opinion ;)
 
my 318 is bored .060 over...i dont have any cooling problems...must be a chevy thing.
 
Sonic check should be done at that overbore. I've checked blocks that at std bore were thinner than blocks that were .060 over. There is no rule when it comes to these things. Some guys get lucky and just bore 'em 60 and have no problems, while another guy does the same thing and comes back 6 months later with a broken out cylinder. I own 3 340 blocks and not one of them would be suitable for a .060 over bore.
 
Thanks guys, I've since taken the block to 2 other machine shops for second opinions. One said the cylinders are fine, they just need a bit of honing, and the motor would be fine for the driving I will do with it. The other was on the fence about whether it need boring or not. I also found a lead on an original 1970 340, rusty but never bored for 350.00. This may be the ticket.
 
fishy68 said:
Not really. 318's seem to have really thick walls. I've seen them sonic test thick enough to bore .090 over
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WOW. Thats impressive! Maybe I have some meat left in my 318 after all? Of course I'd sonic check it. Where the heck would you find a .090 over 318 piston though??
 
skrews said:
Sonic check should be done at that overbore. I've checked blocks that at std bore were thinner than blocks that were .060 over. There is no rule when it comes to these things. Some guys get lucky and just bore 'em 60 and have no problems, while another guy does the same thing and comes back 6 months later with a broken out cylinder. I own 3 340 blocks and not one of them would be suitable for a .060 over bore.
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x2 - Except I only have two of them...lol.
 
What is the number after the 340? If its a 340-1 it would be more concentric and even on the cylinders wall thickness. the higher the number the more likely that it will not sonic test as good. Such as 340-8 being the worst.
 
Quick! Everybody run for your life, its a .060 over 340, ahhhhhh...LoL!!!!

I love reading these hogwash threads, I've almost become bored with car sites....till now ;)
 
old thread but worth comment- I have a sonic tester and have checked many blocks, various makes, for myself and a few local shops.

the 340 Mopar will go .060" but the resulting cylinder wall thickness may be as low as .095" on the nonthrust walls, between the middle cylinders.

you should really strive to leave .130" remaining all around, for good ring seal.

having said that, the particular 340 I checked, only had .124" on one wall, and .130" on another wall, 2 separate cylinders, even at standard bore. after being bored .030" it had .109" and .115"

I would not bore a 340 .060" unless absolutely necessary, like a rare oem block numbers matched to a car, etc. sure it will run, but my experience with blocks bored to under .100" wall remaining, is they don't make as much power, due to cylinder wall flex. If you can find one at standard bore that would clean up with a hone at standard bore, or only .010" over with .010" pistons, use that.

by the late 1960's all the American car companies went to thinner wall castings compared to the 1950's
V8's, to save on material and lighten up the cars. there are a few exceptions like Oldsmobile, there are 455's that will take .125" over. by contrast, the old 1950's blocks had very thick bores in general, and many of those can go .150" and even .180" I had a 1949 Rocket Olds 303 V8 that I had poked .150" and it could have gone another .030" before hitting .130" wall remaining. the thinnest cylinder walls in that block were over .220" and many were over .250"

the only way to know, is check them with a sonic tester. the 340 I checked, had core shift towards front of block on both banks, making the forward walls thinner than the rear facing walls in every cylinder. there would be .220" on one wall, and .125" on the opposite wall on the same cylinder (non-thrust walls) and all 8 holes were shifted like that. in a perfect world with no core shift, that bore would be centered and have about .170" on both sides, then it would take an .080" overbore and still have .130" wall remaining.

core shift is the final arbiter here, I'd suggest pay the extra $75 or $100 and have it sonic tested first. make sure the operator knows how to calibrate the sonic tester. a part of the block has to be measured with dial calipers or a micrometer, then checked with the sonic tester, then the tester calibrated to that value. otherwise the readings will be invalid.
 
.................I have had 340s that went .060.............I've also had a few 318s bored 1.30 to the 4.04 340 size.....only 1 318 ran hotter than b4.....kim...........
 
Have it sonic checked. If it checks out fine, bore build and enjoy. I know guys running the snot out of 340s .040, .060, and .070 over. As long as cylinder walls have enough meat you will be fine. Every block is different.
 
5-360 casting date 6th January 1971

Already +0.040"

Sonic checked as follows (1 reading every 1/2", full height and all round each cylinder):

Min thickness found:
Major Thrust = 0.232"
Minor Thrust = 0.201"
Non Thrust = 0.177"

Thickest reading:
Major Thrust = 0.354"
Minor Thrust = 0.342"
Non Thrust = 0.303"

Fair bit of core shift. Now bored +0.070". Based on the above theory of maintaining at least 0.130" min all round (in this case on non thrust) this block would be good for 4.130" (I have no intention of going further than 4.070", the thicker the better)


Cheers
Jamie
 
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