Can a 273 crank be balanced to 340 specs ?

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Kern Dog

Kern Dog

Build your car to handle.
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I was told that since these cranks share the same rod and main journal size and stroke, a 273 forged crank can be balanced to fit a 340.
I have never tried this.
I have a friend that had a 340 built at a shop and the engine developed a spun rod bearing at less than 100 miles. I helped with the teardown, it has a severely grooved crank. I told him that I have a 273 crank he can use BUT I need to know if it can even work.
The forged 340 cranks seem to have that drilled section on the 1/2 and 7/8 rod journals. Can a 273 crank me modified like that to work? If not, what else needs to be done?
 
I am no expert, but I dont see why it wouldent work. All depends on what it spins up like on the balancer with the Bob weights clamped on to simulate the 340 piston, rod, and ring weights. That's gonna determine where it will need to be drilled and lightened, or have Mallory added to it to balance it to your application. But assuming like you said , stroke, and rod, and main journal sizes are the same as a 340, and providing it can be balanced to your other rotating assembly parts, it should work. I know your mentioning forged, but even a 318 cast crank would fit and work for that matter, it has the same stroke as a 273 and 340, though you are looking at the forged 273 because of durability
 
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Of course it can. I've always wanted to build a 340 with the smaller 273 connecting rods. Would probably rip it up.
 
I can’t remember who posted it, but he said back in the day when he was poor and used what he had, he installed a 273 crank in a 340. No balance job - just installed and go. Maybe he (and possibly others who’ve actually done this) will see this thread and comment.
 
You could it would just take a lot of weight added to the crank throws. Mallory is quite expensive and in the long run the lighter 273 crank would be as heavy as a 340 crank when it's all done.
 
Kern Dog, what year of a 273 did the crank come out of? Crank hub on rear is smaller for the early 273.
 
Yes... it will work. I did it along time ago but I used the 273 rods and silvolite pistons to keep the weight down. Still cost a bit to balance it but it did wing up the RPM's damn quick! As said though you need the later year crank to make sure the index is correct.
 
I've done it. But I was using aluminum rods. Still had to take weight out.

Or, you can just put the 340 crank in a lathe (better be DAMN ridged) and turn the OD of the counter weights down. I've went all the way down to the rod throws. Even with aluminum rods I had to add Mallory.

In the end...I don't think any of that was worth a damn, and I was shifting at 8500 or more.
 
Today's pistons can be quite a bit lighter than the stock 273 or 318 pistons, you can also get lighter pins and rings
so all is not lost
with careful parts selection you might even duplicate your OEM balance and not have to balance :)
Weigh your old parts and do the math
but also what YR just said after I wrote the above
I do know when we were racing 340s they did lots to get the rotating weight down
 
The "complete" core 340 I purchased had a crank installed without the drilled 1/2 and 7/8 rod journals. Don't know if it was from a 273 or 318 or what. Now I am hesitant to put it back together with new rings and bearings because I don't know the history of the rotating assembly balance.

Eventually, for my own peace of mind (and longevity of the motor) I will insist having the crank checked and/or rebalanced. I just wish I knew how the 340 was running with the crank I found in it, but have no history on the performance or the build. Did not even realize the 273/318 had different characteristics than the 340 with the obvious rod journals drilled until I ran into a comment somewhere here on FABO.
 
Certainly could be done . Drill out some of hole weight like in the 340 crank in 1-2 & 7-8. Probably still needs some metal added. Use SCAT rods on some KB or similar piston s and you'll end up taking weight off. Or you could use the 360 external weights front & back with stock 340 pistons & rods and I'd expect to be close or take weight off.
 
It can be easily done by a good performance machine shop or the 340 crank could be welded up and fixed too. Last time I was at Schmidt automovtive I saw there crank welding machine
 
Yeah, I have a 340 of my own that needed repair to a journal via welding and grinding.
This question is for a friend with bad luck!
 
Are there any crank shops around that can repair the crank....like weld up the journal and grind it back to the size of the rest....?

should be someone in the Sacto area...
 
Yeah, there is. I had my own 340 crank welded and reground.
I was just wondering if it was cost effective to just balance the virgin 273 crank.
 
I think the cost of the mallory metal would be the deal breaker.....
 
***UPDATE***
The friend of mine asked for my help today. He wanted to go to the machine shop to check progress on his engine.
This engine has a checkered past.
The machine work was done, the buddy brought it home in pieces with the plans to assemble it himself. Months passed by with no progress so the machine shop guy offered to assemble it and came over to pick it all up. The owner and the buddy of mine went to the shop and within a couple of hours, their shop guys screwed the engine together.
A couple of months pass by. I help install the engine, the buddy does all the rest....alternator, fuel pump, add oil, cam break in, all of it.
20-25 miles in, it has no power and is starting to rattle. Oil pressure is 45 at idle in Park. I hear with a stethiscope a rattle coming from the oil pan area. I pulled the pan and a rod cap to find severe gouging. Another cap revealed a spun rod bearing. The lifters look new, cam looks fine but the crank is obviously wasted.
The shop agreed to rebuild the engine but The buddy wanted to make sure they were not using a wrong crank. They had a 318 crank in there but said they were going to balance it. Fair enough...as long as they do it right.

Question though....
The machine shop owner has dug in his heels saying that the failure to "Pre-oil" the engine resulted in the bearing and crank damage. My buddy says he was never instructed to pre-oil, the factory never pre oiled them AND the engine was not properly cleaned before assembly...It sat for months. Also, he recalled the shop guy assembling the engine with an impact wrench!
What ???
So now, it looks like the engine will go back together but who knows if it will be paid for by the shop or the buddy. Small claims court is likely.
 
Pre oil b/s but a wrong comnbination of components. The question in the thread title tells it all.
 
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318 and 273 cranks are balanced to the same bobweight, so either will need to have weight added to the crank for use with 340 rods & pistons, unless either are lighter aftermarket parts. Not enough info to really know. This sounds like it will come out badly to me. Why is anyone in their right mind going back to that shop?
 
Kern Dog said:
***UPDATE***
The friend of mine asked for my help today. He wanted to go to the machine shop to check progress on his engine.
This engine has a checkered past.
The machine work was done, the buddy brought it home in pieces with the plans to assemble it himself. Months passed by with no progress so the machine shop guy offered to assemble it and came over to pick it all up. The owner and the buddy of mine went to the shop and within a couple of hours, their shop guys screwed the engine together.
A couple of months pass by. I help install the engine, the buddy does all the rest....alternator, fuel pump, add oil, cam break in, all of it.
20-25 miles in, it has no power and is starting to rattle. Oil pressure is 45 at idle in Park. I hear with a stethiscope a rattle coming from the oil pan area. I pulled the pan and a rod cap to find severe gouging. Another cap revealed a spun rod bearing. The lifters look new, cam looks fine but the crank is obviously wasted.
The shop agreed to rebuild the engine but The buddy wanted to make sure they were not using a wrong crank. They had a 318 crank in there but said they were going to balance it. Fair enough...as long as they do it right.

Question though....
The machine shop owner has dug in his heels saying that the failure to "Pre-oil" the engine resulted in the bearing and crank damage. My buddy says he was never instructed to pre-oil, the factory never pre oiled them AND the engine was not properly cleaned before assembly...It sat for months. Also, he recalled the shop guy assembling the engine with an impact wrench!
What ???
So now, it looks like the engine will go back together but who knows if it will be paid for by the shop or the buddy. Small claims court is likely.
Click to expand...



And your buddy didn't prelube because????????????

How does your buddy know the engine wasn't properly cleaned? Lots of questions that are going to be hard to answer.
 
Aside from assembly Lube and priming the pump before I bolt it on so that it draws immediately I just fill it with oil and a little gas down the bowl vents to fill them and fire it off...
Oil pressure is immediate. That's what the assembly lube is for...otherwise we could just use motor oil to assemble it...which some do as well.

They screwed up.
 
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MOPAROFFICIAL said:
Aside from assembly Lube and priming the pump before I bolt it on so that it draws immediately I just fill it with oil and a little gas down the bowl vents to fill them and fire it off...
Oil pressure is immediate. That's what the assembly lube is for...otherwise we could just use motor oil to assemble it...which some do as well.

They screwed up.
Click to expand...
That is how I see it too. Pre-Lubing is something that is common knowledge for engine builders but maybe not for people that have an engine built for them.
The engine does have aftermarket pistons but who knows what the balance factor is.
 
yellow rose said:
And your buddy didn't prelube because????????????

How does your buddy know the engine wasn't properly cleaned? Lots of questions that are going to be hard to answer.
Click to expand...
He said he went to the machine shop with the owner, they had the shop guys pull the parts from the truck and they immediately started assembling it.
No bore brushes through the oil galleries.
No hot tank.
No jet wash.
He asked why they didn't use Plastigage. They said they machine the journals to fit so there was no need to measure anything.
My friend owns a Carburetor rebuild and repair shop and is quite skilled at THAT....Engines themselves are another matter. He is experiencing a steep learning curve here though.
The engine sitting at his shop for months surely collected some airborne dust, right? How much is necessary to contaminate an engine enough to lead to rod and main bearing carnage?
He said the guy assembled the engine using an air powered impact. When he told me that I was shocked. He simply didn't know that the assembler absolutely should NOT have done that! After some conversation, my friend started piecing things together that didn't seem unusual to him at first.
A month ago, he took the engine to a different shop. The man there put a digital torque wrench on a main cap and it cracked loose at over 131 ft/lbs. What is the spec on mains...85 ft/lbs ???
Obviously, the shop was to blame but how do you make a case on it to get money back?
 
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