Pistons Needed

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wigsplitter74

wigsplitter74

The Mopar Kid
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Janesville, WI
I've got a 340 that needs to go to .060 over and I am having trouble finding a set of pistons for it that will give me a 10.5-11:1 CR with about 70cc heads. It has a stock stroke and stock rods. I would greatly appreciate it if someone could post some part numbers to some pistons that would work for me, I know that asking you guys to come up with part numbers is a lot to ask but I'm a mopar guy in need of speed parts. Thanks Much.


Or, can a 318 be bored to 4.07" ??????
 
Thanks guys I appreciate the help!

One more question, I've heard of people using "fat rings" like my current motor has a stock piston in a .030 over bore with "fat rings" and there hasn't been any problems. I bought this 340 that's +.030 but the machine shop says they think it needs to go to +.060, could I use the high dollar .030 pistons with some other rings in a .060 bore??? The 340 shortblock was built 3yrs ago and he changed plans so he oiled it down and it sat in storage so everything was freash I'm just having it gone through and hoping to finish the build and have some fun with it.
 
No. Pistons must match the oversize. The rings only seal. They are held and stabilized by the piston. The more they rock due to loose fitting, the worse the ring seal gets. They are supposed to run a tolerance around .004". You are talking of running them at .030" plus any wear and taper. BAD news...
 
If its already at .030 why does it need to go to .060??..and if it is bored that far i'd suggest having in sonic checked first...
 
stroked 340 said:
If its already at .030 why does it need to go to .060??..and if it is bored that far i'd suggest having in sonic checked first...
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There's some pitting on the walls from it sitting in storage for 3 years, tho everything was oiled, moisture got in where the rings were and caused some pitting, the machine shop recomends going .060 over, it's a 70 340 block now I wish we wouldn't of had it assembled before letting it sit then I'd probably be good to go. Now I'm kind of considering selling the block and aftermarket Billet MP crank but IDK it'd be hard to get what it's worth out of it
 
Rust pits like that are rarely that deep. They look worse than they are. I would have it rough bored and then order the pistons. Also, in order to get 10.5:1 with the TRWs, they need to either pop out of the block or the chamber size has to be reduced. I would recommend having the chambers set to 66ccs.
 
70aarcuda said:
how about .040 over?
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That's what I was thinking but the machinists don't think it will do it, as he told me "well that's only .005 more off the walls" . . . okay bummer

moper said:
Rust pits like that are rarely that deep. They look worse than they are. I would have it rough bored and then order the pistons. Also, in order to get 10.5:1 with the TRWs, they need to either pop out of the block or the chamber size has to be reduced. I would recommend having the chambers set to 66ccs.
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I was kind of wondering that, they say they're about 9.5-9.9ish with my X-heads that have been worked on in the past so I don't know what's already been taken off them but the 13:1's that came out were way beyond the deck.

What would need to be done to achieve 10.5:1? How do I get the pistons to pop out of the bore? or how do I get the chambers to 66cc's?, just by milling? I would think that would screw up my intake seal
 
There are a few ways to skin this cat. You need static compression. The only way to do that is make the chamber space smaller. If your heads were already cut (at 70ccs I'd think they have been) then you may or may not be able to trim a lot more without intake issues. You will want to be careful using that option because of this. If the block is getting bored anyway, have the deck height set to 9.590 so the pistons will pop out of the bores by .025" or so. Then, you have the chamebr size and gasket to play with. The gasket you need is the MP thinner ones, or similar MLS types that are .028 compressed. Lastly, the heads can either be milled down and the intake fitment carefully checked and adjusted later... or you can replcaee the valves with nailhead style stainless valves that will lose a cc or three. All together you'll be around 10.5 or just over it. I am curious tho. You said you have domes ones... Why not just cut the domes down if the bores are not that bad? I have a hard time thinking the bores rusted that deep in a few years... Can you post any pictures of these pits? Good clear ones?
 
moper said:
There are a few ways to skin this cat. You need static compression. The only way to do that is make the chamber space smaller. If your heads were already cut (at 70ccs I'd think they have been) then you may or may not be able to trim a lot more without intake issues. You will want to be careful using that option because of this. If the block is getting bored anyway, have the deck height set to 9.590 so the pistons will pop out of the bores by .025" or so. Then, you have the chamebr size and gasket to play with. The gasket you need is the MP thinner ones, or similar MLS types that are .028 compressed. Lastly, the heads can either be milled down and the intake fitment carefully checked and adjusted later... or you can replcaee the valves with nailhead style stainless valves that will lose a cc or three. All together you'll be around 10.5 or just over it. I am curious tho. You said you have domes ones... Why not just cut the domes down if the bores are not that bad? I have a hard time thinking the bores rusted that deep in a few years... Can you post any pictures of these pits? Good clear ones?
Click to expand...


The problem with using the domes that I have is that a few of the walls are ruff at the current .030 bore. I don't have the ability to put up pics but I have had a few experienced engine builders look at it and they said they would expect it to smoke right of the bat and probably tear rings up on a few cylinders, the pits can barely be felt with a finger tip in my opinion but I'm new with a lot of this stuff. I'm at the point where I'd really consider selling off this stuff IDK yet
 
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