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Old 12-19-2001, 04:13 PM
k13 charger k13 charger is offline
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Question 340 pistons

Out of interest. Has any body out there ever investigated if standard bore 340 pistons could be used in a plus 40 thou bore 360. Could this be a cheaper way of getting extra compression in a 360? Or would it be too much compression to run on pump gas. Can somebody please help!
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Old 12-19-2001, 04:47 PM
Jims451 Jims451 is offline
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There is a pretty big difference in the compression height od the 340 and 360 pistons, about 0.200" give or take a few thou depending on application. At the least you would have to mill the 340 pistons dow some, if the tops have the extra thickness to even do so?
By the time you spend the money for that, it would be cheaper to buy some new 360 pistons from Keith Black, or other performance piston manufacturer.
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Old 12-19-2001, 05:13 PM
HankL HankL is offline
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This actually is a pretty famous subject.

Legendary Chrysler Engineer Tom Hoover built a 380 ci engine like this and it got a lot of attention in the press at the time.

You can do it if you have an early 70's 360 block that sonically checks out to have enough wall thickness in the cylinder.

You could also do it by purchasing one of Mopar Performance's sleeved aluminum/iron blocks - but if you did that you could go up to something like 4.125 inch dia pistons, maybe a little more.

Check out this website which has quite a few details:

http://www3.sympatico.ca/big-d/MYWEBPAGE.htm
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Old 12-20-2001, 12:43 AM
k13 charger k13 charger is offline
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Thank you Jims451 and HankL. I was hoping for a bit of luck, but I suppose that if it was a common mod it would have been pretty well know. Even in Australia. Chrysler powered cars aren't all that common down under. And, as such parts are just that little bit harder to get. Plenty of fords and chevs. I just like being different. I was hoping that the deck height in the block between the 340 and 360's might have been enough to facilitate the piston swap . As i think the rods are the same lenght and the pins are the same daimeter. Any way, Thanks again guys
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Old 12-20-2001, 12:52 AM
sanborn sanborn is offline
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Yes, you can do it. And being down in Australia I understand why you might want to.

The stroke of a 360 is 3.58" and stroke of a 340 is 3.31" giving a compression height difference of .270" divided by 2 or .135".

If you use late 340 pistons(4 eyebrow), they were actually down in the bore about .080"(those were the first days of low compression). You can bring the pistons out of the block about .040"(like the early HP 340s) to give about 11.5 to 1 compression. In order to do this you would need to mill about .015" off the tops of the pistons.

Please double check the dimensions I gave because we did this a very long time ago(and my memory may be bad). I do remember doing this but don't expect this to be a competition engine-the pistons are OK for the street but not for serious racing.
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Old 12-20-2001, 12:53 AM
duster360 duster360 is offline
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i read in a mopar mag that it can be done if you use a pre 73 360 and standard size pistons from a 72-73 340 because the piston is set in the bore further to lower the compression. according to the artical it wil give you a zero deck. i do agree with jims451 though, as cheap as KB's pistons are why bore a block so thin.
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Old 12-20-2001, 04:43 AM
k13 charger k13 charger is offline
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Smile

The plot thickens. The pistons that I have in mind are std size bore 340. Sanborn, the eyebrows you refer to would be the valve reliefs i presume? As for the blocks i have a 72 and a 75 block. Being in australia they could be as much as a year earlier than the car that they came in. I will investigate a bit further and find out what brand and part number the pistons are. Does any body know what the deck height is on a early 360? And as for performance I just want a good street engine, with a bit better than 8.8:1 compression . Thats what made me think of std size 340 pistons
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Old 12-20-2001, 09:09 AM
sanborn sanborn is offline
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The four eyebrow pistons I referred to are the later pistons with four valve reliefs. Just to be sure , measure the distance from the pin hole to the top of a standard 340 piston(4 eyebrow) and a standard 360 piston to check my dimensions.

The deck height of a 360 is the same as 340, about 9.60". As for blocks, use the 72 model. Legend has it the earlier blocks had the same water jackets as the 340. Don't know if its true but I do know they could stand more overbore than later 360s.
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Old 12-20-2001, 12:40 PM
mr_340 mr_340 is offline
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Default Sanborn is Right!

As I recall on the Tom Hoover 360, he used the '72-'73 340 pistons with the 4 valve notches. These have a compression height of 1.74" from the center of the pin to the deck. A 360 would need a compression height of about 1.68" to get the piston to zero deck height. I think Tom took less off the pistons than .060", but he was running close to 12:1. If I remember right, he machined the combustion chambers for piston clearance.

The early 340 pistons had two notches and the compression height was 1.84". The tops of these pistons are about a 1/2" thick, so there's plenty of meat there for notches, milling, etc. They are some heavy slugs though.
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Old 12-22-2001, 04:53 AM
Brian Mills Brian Mills is offline
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Default Pistons

Had a friend in the '70's looking at this. He got a 340 block and scrapped the idea. As I recall he was about to trace down a lead that Ford pistons in a 390 were the same bore as a 340, he was about to look at pin location and the availability of a low compression truck motor piston. Might be worth a look.

All the Best!
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  #11  
Old 12-22-2001, 12:25 PM
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MOPARVANN MOPARVANN is offline
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This is the combination we use due to our rules for circle track racing. A 73-360 block bored .040 over with stock 72-73 low compression 340 pistons. Had .040 milled off the top of the pistons and conservative valve reliefs cut. The result is a 12.5:1 engine with flat top factory pistons. Great combination that makes many horses and still abides by the rules. The original plans that Chrysler had for this set-up was circle track racing, and in that area it is great. For a street engine on pump gas I would not use this set-up today, it is more expensive than going with the new pistons available. When the rules say"a factory flat top piston or a 9:1 replacement" what other choice did we have? We have run this combination for years and make plenty of power using racing gas. Good luck and HAPPY HOLIDAYS.
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Old 12-22-2001, 07:25 PM
k13 charger k13 charger is offline
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Thanks Brian and Moparvann for the comments. This piston swap is not quite as easy as I thought it might be. Then again I suppose the extra "270 " thou of stroke is what would push the compression through the practical limit of running on pump gas. We have gas of about 97 octane here in the land down under. I would think that "10:1" would be about the limit on that sort of fuel. And may be that is a bit optimistic !! cheers from George to "everybody" on the mopar chat site.
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Old 12-23-2001, 12:13 PM
mr_340 mr_340 is offline
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Default Thanks Mate!

Drink a Toohey's or a Castlemeine XXXX for me during the holidays. Stay away from those pork pies (yuck!)
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  #14  
Old 12-23-2001, 12:35 PM
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Default PISTONS-

Years ago hot rod mag did a build up on a 372" small block using this combo.After discussing it with my machinist it came down to this,"yes,the money u save on not buying custom pistons is offset by the labor of the machine work to cut yours dwon and recut the valve reliefs" In short not economically feasible.But MCG feb issue said that mopar performance was releasing a 435 horse 402 ci crate motor....thats pretty cool huh?
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