Just Purchased Virgin 340 Short - What to do with Pistons

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cudaracer

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The 71 340 block is in great condition, and the rotating assembly spins easy. My plan is to obviously strip it down and check everything carefully, but I am not a fan of those stock 10.5:1 heavy pistons. Assuming they are in good shape, should I not stuff it with a modern set of KB's and rings?
 
Then why not use them?
CC your heads and figure what compression it will really have.
Then go with a cam that will like that compression.
That's low buck (cam and lifters aren't expensive compared to psitons and balancing etc) and effective use of the set up.
 
The 71 340 block is in great condition, and the rotating assembly spins easy. My plan is to obviously strip it down and check everything carefully, but I am not a fan of those stock 10.5:1 heavy pistons. Assuming they are in good shape, should I not stuff it with a modern set of KB's and rings?

The KB piston doesn’t have any better ring pack than what you have.
 
You've been reading too many magazines and listenin to too many forum gurus. Those "heavy" pistons have been around for decades. They aren't the plague. They will run fine.
 
You've been reading too many magazines and listenin to too many forum gurus. Those "heavy" pistons have been around for decades. They aren't the plague. They will run fine.
I couldn't figure out if that comment was for the OP or YR.. LOL...
 
These 340's ran wicked from the factory, even with the mild cam. I agree with the others... assuming everything checks out, toss a cam in it and run it.
 
Great, I was hoping there would be a general consensus that stock pistons are fine, I was just checking to see if I was missing out a bunch on modern new pistons, which I know nothing about.

So, next question, I have 65cc heads. It will take me some research to see where I would be with CR......does anybody know have the calculations handy, to report back where my CR would land? And assuming it is close to 10.5:1, how is that OK, when everything I have read, suggests I want something close to 9.5:1 with iron heads? Would I not need premium fuel or higher?
 
416.....pffft, If I am to stroke it, I would make it a 426ci from Hughes.

But I purchased the short for $1K, and I am trying to keep my price low.
The KB Pistons are lighter and that's the upgrade part of it ...but then you'll have to rebalance the assembly, though...you'll be removing weight and not adding, so it's on the cheaper side.
I think if I were you... I wouldn't change the Pistons unless I was going to over bore it.. like if I decided to put new bearings and everything in it... and the bores are out round ,for instance...cause then its a waste of parts.
 
Did you cc the heads?
if not, spend the time to do it.
There's some cheap ways will get you close.
 
I was under the impression that 340s head above deck Pistons and you would either need an open chamber had or thick head gaskets?.. I'm just asking I've never had a 340...
 
if those pistons are fine consider yourself lucky.If the heads are stock 71 or 72 cc's you'll be at a perfect 9.4 or 9.6 and with careful tuning, cam, gear and tire selection you'll have a little Rocket!
 
Did you cc the heads?
if not, spend the time to do it.
There's some cheap ways will get you close.


^^^^^^This...and you need to know how far out of the deck the piston is. OE was .017-.018 depending on whose literature you read.

If you have 65 cc heads and if the piston is out of the hole you’ll need to figure out what the CR is. If you are willing to do the work you can be 10:1 on pump gas fairly easily.

If they are OE heads, it’s likely they aren’t that small, which is ok too because if you end up at 9.5:1 is there is nothing wrong with that either.


But you MUST measure to know. Guessing at CR has made engines that should run hard run like a wounded hog with no legs, so don’t guess on your CR.
 
The factory pistons are probably the lightest ones you’ll find. If you can check the deck height, most of stock motors are way under. THe piston should be .040- .045 out of the block. Like ir3333 said the head cc is most likely 70-74 cc stock. This knocks the compression down quite a bit. I had a ‘71 dart with ‘69 340 all stock, pistons, heads cast iron intake and a .480 lift cam. With 4.10, headers, car ran mid to low 13’s. Raced it for years.
 
416.....pffft, If I am to stroke it, I would make it a 426ci from Hughes.

But I purchased the short for $1K, and I am trying to keep my price low.

I wouldnt. For the few extra cubes you get worse rod ratio, more piston side loading and wear.
4 inch is plenty
Unless you need a tractor motor to plant corn and soy beans. If so, all bets are off.... lol
 
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No offense but if you are assembling it yourself, then you should be able to figure out compression ratio. It's a math equation based on measurements of your motor and gaskets. I'm in same situation as you. You'll get satisfaction doing it yourself.
Google Wallace compression calculator.
Give it a go and post your results.
Someone here will double chk results.
Hope I didnt sound like a dick.
CC's are 65, and the piston are in the bore by 0.07"

Can anyone calculate my CR, assuming deck height is typical
 
how long a rod can you use using chevy rods?
now figure your dynamic compresson using several sceaniros
 
The 71 340 block is in great condition, and the rotating assembly spins easy. My plan is to obviously strip it down and check everything carefully, but I am not a fan of those stock 10.5:1 heavy pistons. Assuming they are in good shape, should I not stuff it with a modern set of KB's and rings?
nothing wrong with the stock pistons they work well , i have 2 1970 340's one totaly stock one with iron heads with a little work on them and mahle forged pistons , not much of a difference between the 2 340's except price . I'd just leave it stock until something breaks , if ever
 
Just Fyi, check the ringlands on those stock pistons real good. The high mile or hard run ones tend to bevel the square edge on the outside of the ring groove, opening it up, making the ring looser in the groove. It once took me two sets of stock pistons to make one good set. Best of luck.
 
One of the funniest builds I ever did was with stock 340 pistons. I sold my buddy my 1972 Duster and he needed an engine. I told him I would build him one for 5.00 labor. He laughed and said 5.00 and I said ya go get 5.00 worth of jawbreakers and as a group we ate them while assembling his engine. Stock crank, rods, pistons, 484 purple shaft cam, reworked and gently polished set of 2.02 valve heads I sold him, portosonic intake, 850 Holley, and headers. He was a dirt track guy so this was his first drag car. He ran 12.40’s the first weekend out. We got it in the 12.20’s the next time out with some tuning and he took 2nd place. The story turned bad after the car sat for a few years and he got in with a bad crowd. We nicknamed that engine the jawbreaker engine.
 
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416.....pffft, If I am to stroke it, I would make it a 426ci from Hughes.

But I purchased the short for $1K, and I am trying to keep my price low.
340 would be the 434 ci stroker , I built the hughes 426 stroker with a 360 block tons of torque but for every day driving I prefer the stockish 340 cause I'm cheap . 426 eats close to twice the amount of gas.180 km drive with the 340 used less than 1/4 tank 426 uses 1/2 a tank
 
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