340 pistons

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713404spd

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Yesterday my younger brother bought what he thought was a 71 model 340 but turned out that it is a 72 model. I think it is a cast crank because the balancer on the front has the offset weight in the front of it. Am I correct? And also I was wondering if I could put a set of 71 style pistons in this motor. Thank you, seth
 
look at the casting date on the block..if it was cast in 72, the engine would actually be a 73 model...

yes..you can change out pistons...but the engine will need to be rebalanced.

as the earlier pistons are heavier then the later low compression pistons,,,,,

or use KB243 pistons which seem to be lighter ...
 
It could be a late 71 built engine witch would be for a 72 model car. Big balancer w the counterweight = cast crank and 8.5:1 comp. I have swapped pistons in the cast crank 340s. I usually use a complete early assembly w a forged crank. The assembly will need to be balanced.
 
If it were mine I would swap in the forged crank but he is a senior in high school and working on a grocery store budget. Thank you all for all the help!
 
713404spd said:
If it were mine I would swap in the forged crank but he is a senior in high school and working on a grocery store budget. Thank you all for all the help!
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I have a cast crank 10:1 340 now w some flat tops, lunati cam, LD340 intake, Holley 650 DP, and 2:02 J heads. Goes plenty good enough, mabe even better then I would want my high school kid driving.
 
713404spd said:
And it is a 72 casting so it would have came in a late 72 or a 73
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Look for VIN on block...that will tell ya if it is a 72 or 73,,,,which probably does not matter...

as you know it has a cast crank...nothing wrong with a cast crank..
 
Cast crank will be plenty strong. Go with the lighter KB pistons and get it rebalanced.
 
70aarcuda said:
look at the casting date on the block..if it was cast in 72, the engine would actually be a 73 model...

yes..you can change out pistons...but the engine will need to be rebalanced.

as the earlier pistons are heavier then the later low compression pistons,,,,,

or use KB243 pistons which seem to be lighter

Will these kb243 pistons put out the 10.1 compression as the 71 340?
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I got 10:1 w KB107s, 68cc X heads, and a Fel Pro .039 head gasket. KB243s would be a little higher. With the cast crank and the light pistons it winds up real quick (light rotating assembly). I was told not to turn a cast crank over 6500 RPM. I will sell the complete engine for $1800. Only has 2500mi. on it. I will be swapping engines in my car within the next couple weeks.
 
back in the 90s..i was spinning a cast crank to 7500 rpm for two years...with heavy stock rods and heavy trw pistons...changed gearing from 5.38 to 5.13....spun engine to 7000 rpm for another 5 yrs before stock rod broke...crank never got damaged...

with today's lighter rods and pistons.....running a cast crank should not be a problem
 
70aarcuda said:
back in the 90s..i was spinning a cast crank to 7500 rpm for two years...with heavy stock rods and heavy trw pistons...changed gearing from 5.38 to 5.13....spun engine to 7000 rpm for another 5 yrs before stock rod broke...crank never got damaged...

with today's lighter rods and pistons.....running a cast crank should not be a problem
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Good to know. I guess at 6200 Im not hurting mine @ all.
 
I don't know what engine he is replacing but if it's a 318, I'd use it's crank, whatever pistons you decide to use, and the 318 balancer and converter/flywheel. Those externally balanced 340's are getting harder to find certain parts. Making it an internally balanced engine will make parts swapping a whole lot simpler and cheaper. You could sell the 340 cast-crank-only parts to someone doing a restoration and help fund a decent rebuild.
 
Hmm that's a great idea. We are replacing a 318 as a matter of fact. There ain't a dang thing wrong with it except for the fact that it's not a 340. Haha jk ( I am actually a big fan of the 318. I'm building one for my daily driver) Would a 73 model 318 be internal or external balanced
 
Great news. We got the 340 tore down tonight and it already has a set of sealed power 70-71 style replacement pistons (alittle heavier than the kbs but that's ok) so basically I'm working with what I originally wanted which was an early 340. I plan on 355 gears (plan to cruise about 25-2700 rpm) and since It has the cast crank I probably won't spin it past 6500 rpm. What kind of stall would y'all run? I was thinking 2300-2500 and also what would be a good cam. Right now all I have is a single plane intake but I am looking for an edelbrock performer and I'm thinking a Holley 650 dp. Feel free to critique my combo so far and any help with the cam and converter would be greatly appreciated. Thank you
 
It will depend on what you're going to do with the car. Street/Cruise? Race? Combo? Is mileage a concern? Pump gas? Which single plane intake do you have now?
 
It's just gonna be a street car and he wants to run 93 octane and he is more worried about power than mileage
 
713404spd said:
It's just gonna be a street car and he wants to run 93 octane and he is more worried about power than mileage
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With cast iron heads, 9.5:1 is about the most you'll want in compression ratio if you want to run any kind of advanced timing without it pinging. 10.5:1 if you have aluminum heads. Something in the range of a .480 lift and 280 duration should be fine with those gears, carb, and intake. Maybe a little more if he has headers and likes a lumpier idle but I wouldn't go over .500 without other engine mods.
 
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