Max bore size for a 1969 340 Block

-
Texas Red

Texas Red

Old Flunky
FABO Gold Member
Joined
Mar 25, 2020
Messages
438
Reaction score
605
Location
Lake Corpus Christi
Hello Everyone----- Because I am not sure and want to know .... Ok from the factory a 1969 340 block would have come with a 4.040 bore size. So the million-dollar question is How far will the block bore out to? Safely?
I'm not looking to re-invent the wheel here just bring it back to its once former glory. I need this info to look for pistons for the build plus other parts........
A very special thanks to everyone who helps keep the ball rolling around this place....... plus helps guide old farts like myself in the right direction.
This is the very best forum on the net ... PERIOD. Texas Red..... Gary
 
Agree with RRR on the sonic check. Most "competent" machine shops "should" have one.
 

I should have one, the machine shop I use has one so..... It's not like they are super expensive.
 
Mine was .030 over for years, then sat for over a decade. When I looked at a rebuild, i had it sonic checked and was told "a lot of core shift, probably ok to go another .030 if I was just going restoration and cruises etc, but for any performance minded use, best to find a different block or spend the $ and sleeve it.
Someone suggested that the cost of custom sized pistons is a lot less than it used to be, possible to bore just enough to clean up, then spend the extra for custom pistons over the "off the shelf" over sized price.
I ended up going with a stroked 5.9 dressed with a LA front end.
 
Last edited:
The real issue is those blocks are over 50 years old now, and corrosion inside the water jackets eat away material over time. A sonic check is a great idea, but one small pinhole is easily overlooked even in a sonic test.
 
I have several 0.060 over engines running around.
Several built before Sonic checking was readily avail. ( I'm that old)
The 340 in my avatar, still has original engine, raced with 12.5 pistons @0.060, that have since been trimmed to 10ish compression.
Engine is # matching original.
Lotsa pistons were avail yrs ago, eBay had some a while ago.

Good luck,
People were throwing away 0.040 engines cuz someone started bad info.
Gotta remember, going 0.020 over is only removing 0.010 material, aluminum foil is 0.0065 thick. (perspective).

Gotta 0.060 on a stand now .
 
Last edited:
Sonic test then you will know for sure, every 340 block i had sonic tested always had that one thin wall cylinder.
 
Inertia said:
Gotta remember, going 0.020 over is only removing 0.010 material, aluminum foil is 0.0065 thick. (perspective).
Click to expand...

I have 2 340s that I cleaned up at .020, one in my truck, the other waiting its turn. I have a .040 block that was free because of a dropped valve, it busted the cyl. wall and was really thin. I am sure a lot of blocks are able to go .060, but there are some that won't.
 
Mopar Sam said:
I have 2 340s that I cleaned up at .020, one in my truck, the other waiting its turn. I have a .040 block that was free because of a dropped valve, it busted the cyl. wall and was really thin. I am sure a lot of blocks are able to go .060, but there are some that won't.
Click to expand...

I'm not suggesting Sonic check isn't wise, just sad about blocks I know we're tossed for no reason.
"Oh 40 over, toss it"
"Back in the day" - pretty much everything/breed would go 60 over,
You could get 60 over, diff compression ratios for 340s in most popular brands.
Sonic check wasn't always available.
Cheers
 
NC Engine Builder said:
The real issue is those blocks are over 50 years old now, and corrosion inside the water jackets eat away material over time. A sonic check is a great idea, but one small pinhole is easily overlooked even in a sonic test.
Click to expand...
That's a great point lots of people forget. Probably a good idea to sonic test and pressure test.
 
Texas Red said:
Hello Everyone----- Because I am not sure and want to know .... Ok from the factory a 1969 340 block would have come with a 4.040 bore size. So the million-dollar question is How far will the block bore out to? Safely?
I'm not looking to re-invent the wheel here just bring it back to its once former glory. I need this info to look for pistons for the build plus other parts........
A very special thanks to everyone who helps keep the ball rolling around this place....... plus helps guide old farts like myself in the right direction.
This is the very best forum on the net ... PERIOD. Texas Red..... Gary
Click to expand...

I always bore the minimum. I have some that will go back to standard bore + a smidge. I measure first to see where it will "clean up", then decide what bore should work. Then rough bore to make sure it will clean up. Then order pistons.
 
Inertia said:
I'm not suggesting Sonic check isn't wise, just sad about blocks I know we're tossed for no reason.
"Oh 40 over, toss it"
"Back in the day" - pretty much everything/breed would go 60 over,
You could get 60 over, diff compression ratios for 340s in most popular brands.
Sonic check wasn't always available.
Cheers
Click to expand...
Same with Clevelands. Sometime, somewhere in the annals of automotive literature, somebody printed that Cleveland blocks were no good after a .030 overbore. Couldn't be further from the truth. The one I have is .040 and I've sonic tested it. You have to SEE and not GUESS.
 
I have one 69 block at .080 over she been blown up a couple times and a 70 block at .080 with sleeves in every cylinder and that one has .060 over Pistons in it they both run warm on the street. They cruise at 200 and heaven forbid you hit a stop light and they idle at 220-230
 
Lots of thin-wall talk with Mopar blocks.

Think there is a lot of bad info out there, no such thing.
Core shift and bad indexing on the original factory milling machine mainly.

I rebuilt a +30 427 Ford engine and now that's THIN-WALL territory!
Bored out 406 and they is paper thin.
 
polyjohn said:
Lots of thin-wall talk with Mopar blocks.

Think there is a lot of bad info out there, no such thing.
Core shift and bad indexing on the original factory milling machine mainly.

I rebuilt a +30 427 Ford engine and now that's THIN-WALL territory!
Bored out 406 and they is paper thin.
Click to expand...
My Machinist made mention that he measures in the center for that bad indexing stating the hole is not bored perfectly yet still functions just fine but noting that this is something he learned from another old Machinist. Sonic testing is on the list as well. All good info. Thanks very much
 
polyjohn said:
Lots of thin-wall talk with Mopar blocks.
Click to expand...

Larry Shepherd should be kicked in the crotch for at least perpetuating this BS rumor.
Many of us shook our heads as the claim of thin wall 440s but it took some actual testing to clearly disprove it. In the case of the 440s, the later blocks that were supposed to be thin wall were as thick or thicker than earlier blocks.
The same could be said for the other engines.
Yes, sonic testing does make sense. There have been rare cases of extreme core shift or thin spots where the casting just wasn't as thick as everywhere else. Imagine how frustrated you'd be to spend the money to build an engine and have a cylinder split while racing or on a long road trip. Not good.
 
You must log in or register to reply here.
-

Similar threads

Wally-T
Max bore out size for a 225?
Replies
13
Views
717
Charrlie_S
C
2
Max bore size on 340 Resto Block
2
Replies
36
Views
10K
2009redrt
2
N
Max bore size
Replies
11
Views
13K
swinger340
swinger340
A
5/64 & 1/16th ring and max bore size
2
Replies
29
Views
4K
AJ71DUSTER
A
A
5/64 & 1/16th ring and max bore size
Replies
0
Views
627
AJ71DUSTER
A
Back
Top Bottom