340 wall thickness & deck height

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Jadaharabi

Jadaharabi

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What is the thinnest the cylinder wall should be if you bore out a 340?

I recently had a block ultrasound and magnafluxed and they told me I could safely bore this block out .080. It is a stock bore 340. Meaning a maximum bore size of 4.120.
 
The second part of my question is the deck height on the driver side I have a measurement of 9.606. At the rear on the driver side I have a measurement of 9.601.
On the passenger side front I have a measurement of 9.600 and then on the rear passenger side I have a measurement of 9.610.

Would this block need to be decked or squared up?
 
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Jadaharabi said:
The second part of my question is the deck height on the driver side I have a measurement of 9.606. At the rear on the driver side I have a measurement of 9.601.
On the passenger side front I have a measurement of 9.600 and then on the rear passenger side I have a measurement of 9.610.

Would this block need to be decked or squared up?
Click to expand...

Yes, very much so..........though it is typical of what production machine tolerances were back in the day.........before square decking the block it would be good to know desired piston, rod, and crank, to know min/max cut required........cut once only
 
you want to bore to the least amount as possible to get straight cylinders....as far as decking...that is about normal for a small block mopar...left to right and front to back different...If you want a perfectly square block then deck it.....you need to figure out where YOU want for a deck height....your pistons..rods and crank...

and unless the rods are blueprinted to 6.123 center to center and the crank throws are perfect at 3.306. and indexed....perfectly 180 degree opposite each other on the journal....your deck heights will vary even with the block squared... cause there is tolerates in the rods and crank...
 
The first question is more one of opinion. He may feel it can go that big. Others may not. In terms of my minimums - for a std stroke application, a minimum of .150" on the major and minor thrusts would be about it. For a stroker up to 550hp I'd be looking for a minimum of .180, with no minimum less than .090 for the pin axis. One or two cylinders that are badly shifted or thin can be fixed with sleeves or moving the bore centers to address the issue(s). If it's more than that, test another block.

Decks are always off, and always tall on factory original blocks. I will always square deck anything that matters. That's not "decking". That's square decking, meaning using the proper fixture to ensure the cylinder bores are exactly perpendicular to the crank axis.
 
Personally I would only bore the minimum required to clean up the cylinders. A rigid cylinder will provide better ring seal, providing more power than the extra cubes. This also gives a safety factor if this block ever needs to be refreshed again for any number of reason. Taking it to the max now means it is the last rebuild.
 
My machine shop guy told me that I had six cylinders that were at .005 wear. And I have one cylinder at .007 and 1 cylinder at .008 wear.
 
Then it ought to clean up at .030" bore and maybe even .020" bore. My son's 340 block's bores were all exactly .005" over and we went .020" over. I like the philosophy in post #7 about extending the block's life with minimum boring.
 
If they pistons you want are available at .020...that is what I would buy
 
70aarcuda said:
If they pistons you want are available at .020...that is what I would buy
Click to expand...

Agree. I would definately pay the little xtra to have 20 over slugs made by by diamond or somebody and have the xtra meat available for down the road than just go 30 because pistons are a little cheaper.
If your building something that is going to make power its extra important. Good ring seal is a big key to a motor that runs like it should, and good stiff round bores are a good bit of that
 
The last oval track engine I built for a guy, we used a magnum block, went .010 over to keep walls as thick as possible, got custom pistons made, and used metric rings
 
Agreed - to a point. I'm not for spending on custom pistons to save .010 of bore size. But's me. I wouldn't blow anything out to max thinking it's going to give some result - because it really doesn't.
 
moper said:
Agreed - to a point. I'm not for spending on custom pistons to save .010 of bore size. But's me. I wouldn't blow anything out to max thinking it's going to give some result - because it really doesn't.
Click to expand...
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Was not planning to bore for max. I was trying to get some opinions on what was a safe minimum for wall thickness.
The shop that tested my block said it could go 080.
After some of the replys I don't know if it would go .030.

top to bottom
#5 is fr .142, .152, .141 valley side .226, 227, 251
rear .128,143,159 outside .163.168.184
#6 is close to the same measurements
 
Let me recap
just because you can does not mean that you should
let's clear something out
did you mean to bore to 4.080 or .040 over
or as you said bore .080 which is to 4.120 BVVC
your ring seal is better with a thicker wall- as has been said above
you can loose power with larger overbores- so bore to 4.060 which is .020 0ver-
a common size for pistons and rings
find a shop with the tooling to square deck the block off the crank and cam centerlines
.010 is too much if you are going to dial in your quench
do get your rod lengths and piston CH in hand before decking
what heads are you going to run?
glad you checked your block
 
Jadaharabi said:
.
Was not planning to bore for max. I was trying to get some opinions on what was a safe minimum for wall thickness.
The shop that tested my block said it could go 080.
After some of the replys I don't know if it would go .030.

top to bottom
#5 is fr .142, .152, .141 valley side .226, 227, 251
rear .128,143,159 outside .163.168.184
#6 is close to the same measurements
Click to expand...
From Moper's preferred numbers in post #6, you can almost meet his numbers everywhere for .020" . If you can have the bore shifted off towards the valley side, then the numbers say you probably could go past .030" and still meet those minimums. (IMHO, I get the impression that Moper does not take un-necessary chances with machine work....)
 
Generally I try to balance value (return on investment) and expectation. There are times when something is needed to accomplish the goal and the money has to be spent. The only alternatives to spending are gambling the final results, or lowering expectations. There is no "middle ground" there.
Also times when spending money is not going to return any value. IMO - bore size is one of those areas.
It's my opinion that the .005" of local thickness one saves by going +.020 rather than +.030, when the sonic testing shows thick bores (that one cylinder is really good for a factory block BTW) provides no return on the investment of custom .020 over pistons. Go +.030, spend less, and get the result you want. That's my calculus anyway. Ultimately it's up to the builder and customer. They have the relationship here.
 
Just to point out.... many pistons are easy to get in .020". Had no problems getting flat top KB's for a 340 in .020".. took about 2 weeks to ship. ICON FHR forged .020" flat tops currently show a ship date of 11/2 from Summit.... so 2-3 weeks.

Can't say on the other brands/models.
 
Jadaharabi said:
What is the thinnest the cylinder wall should be if you bore out a 340?

I recently had a block ultrasound and magnafluxed and they told me I could safely bore this block out .080. It is a stock bore 340. Meaning a maximum bore size of 4.120.
Click to expand...
I have never heard of going more than .060 over on a 340 block, and these are known for overheating issues with more than .040 overbore

you need to speak with a reputable engine builder preferable with experience and success in small block mopar. Square decking, align hone mains, boring with torqueplates are all absolute necessities if you want to build a powerful motor that will stand up and not break.

See @steeldustmachine on Instagram
Moonshine speedshop in NC

There are plenty of other good ones also
 
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