I have a 1969 340 in the machine shop. I bought it used... the previous owner had it in a Ramcharger 4WD and took it out when it spun 2 rod bearings. My machinist said the thrust flange is worn so bad that it would need to be welded and machined, as well as the rod journal that had the bad bearings. $ 350 !!! I know the 273 318 and 340 engines had the same stroke, but can the forged cranks really be interchanged as I have heard? What about the difference in weight of the pistons?

yes ...318/273 is same bearing sizes and stroke...

will be needed to be balanced for the heavier 340 pistons...

Pretty interesting. Thanks!

Also check the converter/tranny input shaft hole size as they were smaller on the earlier l.a.s

If you have an old 318 Poly crank they are steel.
273's are steel but as mentioned may have the wrong convertor register depending on year (I think 65 and back) Later 273's (66-68) seem to be a direct bolt in.
A 318 has a steel crank in a factory truck motor. You would need to verify its an original.
Only thing I have seen that makes them different is none of these were cross drilled for oiling like an original 340 was.
I used what I thought was a 273 crack and rods in the Dart. I had several different steel cranks laying around. In the end I found out the one I had all the machine work done to was from a 65 truck 318 motor. It also had the incorrect crank register even though it was a stick motor. (Had a pilot bushing)When we did the balance work the new 340 pistons even being .060 over were lighter than the original 273 slugs. For my application using the lighter 273 rods it made for almost an even swap.
In case your wondering I learned most of this the hard way. Nothing wrong with using the 273 cranks. Just be sure to verify everything is correct for your application BEFORE you assemble it.

John..What is the year-split on small to large register for the 273-318? At 67 as you stated, doesn't make any sense. In different stuff I have taken apart I've never seen the small register crankshaft or convertor after 65.

The 273-318 crank is heavier, the crank pins do not have lightening holes drilled into them.

The small register is for a TQ converter and will not matter on a manual trans unless you try to run the latemodel roller pilot bearing. My 68 273 had the big register, my 66 273 had the small one.

Sure, use any 273/318 crank, right up through the Magnum motors.

Stock 340 pistons weigh 720 grams without pins. Stock 318 pistons weigh 591 grams without pins. There's a 32 gram difference in rod weight between the smaller and larger rods, 726 and 758.

So try to buy pistons that weigh the same or less than 318 pistons and balancing will be easy. Or, buy stock 720-gram clubs and step right up to extra dollars for balancing a 273 or 318 crank.

R.

Using the 340 crank you have in mind & with the right piston/rod etc choice it might be possible to get the bobweight(s) to what the crank is now with the 340 piston/rod (if you had one assy to weigh) but even so with Ma's poor balancing I would instead get a ten dollar HF gram scale & rig up a fixture to hold the rod horizontle such as your dial indicator stand & trim/deburr the rods till the rotating and reciprocating weight totals are the same on all (8) assys then give that single gram number (you add the rot/recip together (one is times 2 cuz there is (2) rod assys on a journal and the other is 1/2 of the total for (2) rod assys times 2. Google the formula it is simple) & give that total number to your balancer & it'll greatly reduce your bill as he ain't doing the deburring/trimming he just drills (hopefully) or adds Mallory to the crank plus you can smooth the rod shanks and trim the top/bottom balancing pads to get the assy as light as possible as your time is free (as opposed to his). you can trim the rod as much as you desire as long as you dont compromise its' strength and keep the (total assy) weights equal (individual components weights can vary no big deal) as the eng only sees the total assy wt that is on each journal. The pad on the small end manipulates the reciprocating (piston/rings/pin/locks (if used)/rod small end) weight and the big end pad manipulates the rotating (rod big end/bearings/rod bolts) weight. You grind on the rod to get em both where you want em (all (8) assys equal). EDIT Oops I saw that you have a 340 block & might use a 273/318/340 crank, same deal & if those cranks used a lighter rod/piston then you for sure want to lighten the rods you intend to use (Mallory ain't cheap) and get everything equal & have that gram number in hand for him

John I wanted to ask, when did the 904's go to the larger nub TC/diff input spline input shaft? & while you're here, the 60 deg approach angle for the .040" hole in the orifice in the leakdown tester, is that 60 the included angle or 60 on one side of the drill bit I will use? Thank you!

904's went to the large pilot hub in '68.

Orifice angle is included angle. I'd use a countersink rather than a drill bit.

John you are the man! Thank you