You're correct that all the parts are internally balanced, BUT, the assembly was balanced to the weight of a 273 piston. I'm willing to bet those .060 over 340 pistons are a lot heavier than those standard 273 pistons that it came balanced with from the factory. Using those 340 pistons with the other 273 stuff will create an out of balance issue.

OOPS- gotta get back to the honey-do list.

Yup, the 340 pistons are heavier by quite a bit. Usually takes mallory metal to balance a 273 crank to a 340 (I hear it's expensive). 340 rods were also stronger than 273 units.

man these early 273' give everyone a headache sooner or later,

A little story to make you feel better and your entertainment as well.

7 years ago i purchase my 71 dodge challenger.

The transmission would always slip into 3rd gear.

One hot 100* i took the car for a drive and got stuck in bummber to bummer traffic,

I tooasted the Trans and it wouldn't gointo any gear.

Get the car towed home order myself a fresh 727 and a 2500 stall convertor that same day.

Next week end I get my transmission install the lines bolt up the transmission go to slide the convertor foward

Learned real fast about the early 273's that day

I missed that this is using 340 pistons on a crank balanced for 273 pistons (& the OE balance was not dead on at that). Plan on taking all 8 piston rod assy's in along with the crank

Quint, get it balanced. Mallory metal is expensive. We got a new crank for my son's car and an extra $300 was added just for the mallory metal plus drilling the crank for the mallory metal when we had it balanced. We had around $600 in balancing the crank itself (the rods, pistons, flywheel and dampener were already balanced).


Donnie Carns

to all who commented.

Pulled the balance of the motor apart today and loaded it up to get balanced. Converter, flex plate w/ bolts, crank, rods, pistons, balancer and bolt, everything less the front pulley. I sent an extra crank and the original 340 rods along that if the bob weights are too far off, he has other options rather than spending a small fortune on mallory. I also decided to have new rod bolts installed.
This like most other projects has gotten a little out of hand. What started as a ring and bearing freshen of a good running motor ended up being entirely different. It got a little bigger cam than expected, new pistons, Eddy aluminum heads, M-1 intake, MSD ignition, and a nice new set of Doug's 453 powder-coated headers. Now there are a lot of new parts involved that will undoubtedly produce more power making the initial idea of using the 273 crank and rods somewhat less desirable.
This combination less the 273 rods has been together for about 35k miles. I guess I used the 273 crank in 1995 when the motor was rebuilt, not realizing that it wasn't a 318 truck crank. The motor had no issues the whole time I had it in my p/u. Lord knows it got the snot kicked out of it all the time towing and messin around in the mud.
When removed 2 of the old pistons had a crack at the skirts so we replaced the whole set. The heads needed a lot of work involving new guides and valves and were going to cost about 700.00 so we ordered a set of iron replacement heads. After 3 weeks of waiting on the iron heads (backordered) the aluminum heads were purchased. Camshaft is a Racer Brown solid stick that has been in another 340 for over 35 years. I wanted to use it because I know it has no issues at all. Lifters and camshaft show no wear whatsoever and this thing just has an old school thump to it that is perfect for the car. After a phone conversation to Racer Brown valve spring rate required is just below that used on the Eddy heads. Seems to me the #'s on the cam are around 304-525 or something in that area. Racer Brown recommended going to a 1.6 ratio rocker to increase lift numbers.
In the end all I want is a dependable driver with some attitude that does not require racing fuel and is durable enough for an occasional flogging. Maybe I'm wrong but with a 3500 converter, 456 gears and 30" slicks it should run mid 12's all day, maybe a little more. Now after being balanced it should be a pretty tough little piece.
Donnie here is a pic of kinda where the car is now. Since the pic the windshield, back glass, head-liner,and black vinyl top have been installed. Rear 1/4 windows are in and done along with the package tray. We moved the battery to the trunk. K-frame and all front suspension is out now and everything will be installed in one shot from underneath. No hood removal required.... Thx all Quint

I got everything back from the machine shop. The crank was a 1968 318 truck steel crank, notice the small harmonic balancer. Turns out that the new pistons with the 273 rods we put in were a little lighter than the crank was originally balanced for. Now that everything is balanced it is back together again. Crank was drilled at the rear to accept the converter prior to being balanced. Now that everything is bolted back together here is a shot of long block on the dolly with the K frame, trans, headers, steering box, dist and plug wires almost ready to go in. I have a few more odds and ends to finish before I bolt this in as I don't want to have to lean over the fenders any more than I have to. I posted some other pics on the restoration forum of the dolly and the engine. Thx to everyone for the advice.