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Discussion Starter #1
well we are switching gears here again, I mentioned in an earlier thread that my son and I purchased a 360 for his s/b step truck, the motor was supposed to be good but it turned out it it two low cyls and needs to go to the machine shop, yesterday at a swap meet we purchased a "340" block and crank, supposedly un-bored and the seller is known to be reliable, so our thought here now is to send the block and crank to the machine shop and have it done up as a short block, probably .020 or .030 over, new rods and pistons etc, we have two good sets of 360 heads, nothing special, not "j" heads but machined ready to go, what do you guys think they will perform like?, this truck is used for cruzin and local shows, no quarter mile stuff, thanks
 

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If those heads have pushrod housing castings in the exhaust ports, (Large obstructions), you will be much better off with new heads. Mopar Magnum heads have no obstructions and flow like crazy. The part number is 5007950, but finding any could be difficult. Try Indy for the same head as Mopar. The 7950's have 2.02/1.65" valvews, 60 cc heart shaped chambers, and in my opinion, the best small block iron head made. They can be vertical or angled intake bolt pattern. You won't believe the torque!
 

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The 340 and 360 used the same heads so for a stock setup, mild performance, out of the box and clean will work fine. they should have the 1.88 intakes, and even if they aren't ported, less hassle than swapping to the Magnum heads.The big key with the 340 in a low performance stock form is to get the compression up to at least 9:1. Anything higher, a little tuning will get it to run on regular 87 octane without pinging problems easy enough and still outperform anything else 340 in a truck. We can work on the tuning setup to do this, just get the compression up with the tallest flat topped pistons you can find for a good running engine. Congrats, 340s are hard to find these days and are a very good engine.
 

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Discussion Starter #4
ok thankyou, next question, forged or cast crank?, the one we have is a cast according to the casting number, the block is a 69 according to the casting number which should have come with a forged crank this will be mated to a 4 spd std, see any problems here, apparrently the cast crank is externally ballanced and may need a difficult to find harmonic ballancer?
 

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Although a 340 did offer a beefed up crank for some engines, it is the same for a 318 (interally balanced, same size main and rod journals, etc.), much better than the cast crank. Did you do the ring test or look at the seams? Wide, ground seams are steel, along with a nice ring to it when you tap it with a piece of metal, cast has a tiny seam and rarely ground any, kind of clunks when tapped with metal object. Being your level or performance needed, drop a 318 crank in and have fun, have the oil holes chamfored really good for better oiling, use a high volume oil pump for longevity, things like that.
 

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Discussion Starter #6
i did not do the"ring" test or check the seems but I did check the casting number which I do not have here at work now but will post when I get home but according to a list I found here on allpar it says it is cast for 72-73 and forged for earlier blocks, the casting number stamped into the block is 69, I guess my fear here is there were three blocks and cranks available and I think I have the later crank which if it fits isnt a big deal, I also have a 318 long block core which I could get the crank out of as well, should I be concerned, I have the guys phone number I got the 340 from he MAY still have the forged crank???
 

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I will always go with forged if possible, especially if you can simply trade it for the right one. It really saves the hassle of externally balancing the crank and getting the proper flywheel on top of it, and besides, it will look nicer and be more durable. Hate to have a proper dated block with the wrong crank cobbled together for it.
 

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If you can't afford a forged crank, go for a cast steel crank instead of a cast iron crank.
 

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Discussion Starter #9
ok so I got a hold of the fellow that sold us the 340, he sold the other forged crank along with the other blocks BUT he dose have the correct harmonic ballancer for the cast crank 340 which apparently is unique to the late 72-73 340 so this should solve that problem, should this be sent to the machine shop when the engine is rebuilt?, also on the 360 that we were origanally going to build it required a ballance plate for the flywheel which we ordered ( alternatly we could have used a special drilled ballanced flywheel but hard to find) this was because the crank was externally ballanced, will we need this with the 340? my understanding is no
 

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For safety sake, I would give both the balancer and the flywheel to the machine shop so they can balance it as a unit. Better safe than sorry.
 

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Discussion Starter #11
hmm, good idea on the flywheel but it is currently in use on the 318 on the truck and wont get swapped until engine swap time, I will send the ballancer though, thanks
 

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That could be a problem... They really do need the entire rotating assembly to do it right.
 

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This is the point when I say use a 318 crank, and none of this is an issue, but that's fine, send that offset balancing plate, much easier since that is the direction you have decided to go.
 

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Discussion Starter #14
well the ballance plate is supposed to do the trick? in theory it is supposed to have the same ballance as a ballanced flywheel, yeah I could use and may use the 318 crank but I just thought it would be good to keep the bottom end all 340, guess I gotta give this some more thought, we arnt in a rush to get to the machine shop, car show season is upon us and we wont take this motor back out untill the fall
 

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Like I said, the 340 and the 318 cranks are the same. Same main and rod bearing sizes, same weight, everything. There were some heavier duty 340 cranks, but they were in things like the 340 6 pack engines, otherwise, no difference, both are internally balanced, and that is the important part, along with being forged cranks instead of nodular steel (Mopar never made a cast iron crank in that era I am aware of).
 

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You will have to take a physical look at the end of it to see casting markings, but according to rockauto, it is a fit for a 1967 and up block, so it may be.
 

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Discussion Starter #18
ok I will check casting numbers, thanks, all in all still exited to finnaly get our paws on a 340, we measured the bore crudely with calipers the other day and it seems to match factory spec, therefore un-bored, very happy with that!
 

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That's always a good sign. A good fine hone and moly rings and you will be good to go for a lot of power.
 

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Discussion Starter #20
well I pulled the set of 360 heads off a 318 I had sitting in my shop and much to my supprise they are "j" heads, it was mentioned earlier in this thread to look for large obstructions in the exhaust ports which may obstruct flow but I dont see any?, the machine shop said they should be able to get this motor to 10:1 and run on pump gas with these heads?, unfortunatly it isnt in the budget for Indy heads or anything special, however there is a huge swap meet this weekend a couple hundred kms south of us that my sons and I are going to this weekend so we will see if something unique shows up there, any special casting #'s we should be looking for?
 
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