Are 340 Connecting Rod Large Ends Chamfered on both sides?

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dibbons

dibbons

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According to a Hot Rod Magazine internet article:

"10 Critical Mopar Engine Building Specs!"

"Beginning with the rods, close examination will reveal that one side of the big end has a larger chamfer than the other. The large chamfer always faces the cheek of the journal, therefore the rods are orientated in opposite clocking between the left and right engine banks. It pays to mark the large chamfer side for reference during assembly. Next, the rod caps have a specific orientation on the rods. This is with the bearing tangs on the same side of the rods as installed."

Well, my own "close examination" can find no difference in chamfer on either side of these reconditioned 340 rods (see photos-I believe the first and last photos are one side and second and third photos are the opposite side)

IMG_1162.jpg


IMG_1163.jpg


IMG_1159.jpg


IMG_1165.jpg


IMG_1160.jpg


IMG_1164.jpg
 
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I just orient the production rods like you are showing so
that the numbers are in order and facing outside so you can
read them.
 
I always go by the bearing lock tang slots. On these engines, they always face the outside of the block. Also, you need to keep in mind, most of those magazine articles are geared toward Chevy engines and there could be differences there.
 
There are chamfers on both sides of the connecting rod, a narrow one and a larger one. The larger goes towards the crank. The narrow ones go towards the other connecting rod. It is important to check all orientation of the piston to the rods and the rods to the crank when assembling.
 
It is obvious on the engine I am building now. I can tell you from personal experience, if the chamfers and the valve reliefs in the pistons are not oriented correctly, it doesn't make a happy engine. My buddy had a local machine shop do his Pontiac engine, and they numbered the new pistons and pressed them onto the rods. But not all the rods were correct, ruined a bunch of rods and the crank

PXL_20250407_221724642.jpg


PXL_20250407_221730226.jpg
 
The chamfer is important. Very important. Since no one has told you why, I will elaborate. The crankshaft has an area between the crankshaft throw and the rod journal called a "radius". This radius is ground into the crankshaft and is there to keep the connecting rods from binding against the crankshaft and breaking it. Here is a picture showing the radius. The large chamfer on the big end of the connecting rod installs toward that radius. If you do not have a large chamfer, I would recommend taking the rods into a good machine shop and letting them put a chamfer on them. It's not expensive or time consuming. Here is what the radius looks like. This is my Ford 400 crankshaft, but the radius is the same. See where I am pointing with my pick. That's the radius and where the large chamfer faces.

IMG_20250407_185357866_HDR (1).jpg
 
dibbons said:
Then I'm just having a devil of a time seeing the difference for some reason.
Click to expand...
Have they been re-sized? Then either the champher that's supposed to be there is gone.... or both sides were champhered equally. I'd be VERY careful during rod/piston hanging, to make sure the rod doesn't rub the crank!
 
As you well know, sometimes the connecting rods have a copper coating of some kind. I was able to locate the chamfer on the single rod with that characteristic. Turns out the larger chamfer is on the opposite side of the connecting rod from where the casting number is located. I can now find at least the semblance of a chamfer on the other seven rods on the same side. (large chamfer visible in photo #1, tiny chamfer on opposite side in photo #2)

IMG_1167.jpg


IMG_1169.jpg
 
You are correct about the chamfer and the radius being involved.
However,,,,,Mopar factory cranks are made with Undercut fillets ,,,,and don’t truly require the chamfer .
The chamfer also helps strengthen the rod caps and the rod journal area .
It helps to relieve a stress riser area by giving a blended radius to the journal so it doesn’t have a sharp edge to cause a fracture .
And I think at one time I read that the chamfer aids the oil buildup ( Dynamic Wedge ),,,,that helps them float the bearings ?

They still need the chamfer area,,,,but it will survive with a marginal chamfer as well .

Tommy
 
Desoto291hemi said:
You are correct about the chamfer and the radius being involved.
However,,,,,Mopar factory cranks are made with Undercut fillets ,,,,and don’t truly require the chamfer .
The chamfer also helps strengthen the rod caps and the rod journal area .
It helps to relieve a stress riser area by giving a blended radius to the journal so it doesn’t have a sharp edge to cause a fracture .
And I think at one time I read that the chamfer aids the oil buildup ( Dynamic Wedge ),,,,that helps them float the bearings ?

They still need the chamfer area,,,,but it will survive with a marginal chamfer as well .

Tommy
Click to expand...
I'm not up to speed on what Mopar used and why, BUT I can tell you this. I've torn down a LOT of original 340s and every single one had chamfered rods. You are correct though that there are several different types of radii. Undercut, half fillet, full fillet and so on. I used to know most, but it's been since about 1986 since I've done any crank grinding.
 
RustyRatRod said:
I'm not up to speed on what Mopar used and why, BUT I can tell you this. I've torn down a LOT of original 340s and every single one had chamfered rods. You are correct though that there are several different types of radii. Undercut, half fillet, full fillet and so on. I used to know most, but it's been since about 1986 since I've done any crank grinding.
Click to expand...

No Sir,,,,you are correct,,,,I wasn’t disparaging anything you were saying .
And you are correct ,,,,the 340 rods and any high performance rod was well relieved by Mopar .
In fact,,,most factory rods had an almost excessive chamfer sometimes .

Tommy
 
The Mopar factory cranks always seemed to get a lot of bad press long ago .
They always talked down Mopars use of the undercut .
But,,,,Mopar cranks we’re not known as being fragile or prone to break easily .
They were a very robust crankshaft and very durable.
Much stronger than the GM cranks that had been twisted after forging to form the throws .
And Mopar used a much softer steel,,,,1540 I believe,,,,,but they worked very well .

Tommy
 
Desoto291hemi said:
The Mopar factory cranks always seemed to get a lot of bad press long ago .
They always talked down Mopars use of the undercut .
But,,,,Mopar cranks we’re not known as being fragile or prone to break easily .
They were a very robust crankshaft and very durable.
Much stronger than the GM cranks that had been twisted after forging to form the throws .
And Mopar used a much softer steel,,,,1540 I believe,,,,,but they worked very well .

Tommy
Click to expand...
I've never seen one "just break", if that makes sense. There was always "something else" afoot, if you will. Broken rod bolt, cracked piston that finally fell apart.....there was always something else. A "contributing factor" you might say. I've never seen a Mopar crank......cast or forged just "give it up". I'm pretty sure it's happened, but it's rare, IMO.
 
RustyRatRod said:
I've never seen one "just break", if that makes sense. There was always "something else" afoot, if you will. Broken rod bolt, cracked piston that finally fell apart.....there was always something else. A "contributing factor" you might say. I've never seen a Mopar crank......cast or forged just "give it up". I'm pretty sure it's happened, but it's rare, IMO.
Click to expand...

I agree,,,,very rare I feel .
Yes,,,I have seen them taken beyond their limits,,,,,and fail,,,,but it was after repeated abuse .
I had a friend that had a factory 440 crank give up the ghost,,,,but it was after 2700 passes in his Cuda !
That car would carry the front wheels close to 4 feet off the ground !
Even the people that didn’t like Mopar would line up at the railings to watch the show he put on .
He left on the two step about 3800 if I remember correctly,,,,,man,,,,what a show .
When that engine finally let go,,,,the crank was in 3 pieces,,,,and the block had all the main webs broke .
It was all contained in the pan mostly,,,it let go on the line at launch,,,,so that was a blessing .

We’re still not sure if the block failed first or the crank ?

Tommy
 
Desoto291hemi said:
I agree,,,,very rare I feel .
Yes,,,I have seen them taken beyond their limits,,,,,and fail,,,,but it was after repeated abuse .
I had a friend that had a factory 440 crank give up the ghost,,,,but it was after 2700 passes in his Cuda !
That car would carry the front wheels close to 4 feet off the ground !
Even the people that didn’t like Mopar would line up at the railings to watch the show he put on .
He left on the two step about 3800 if I remember correctly,,,,,man,,,,what a show .
When that engine finally let go,,,,the crank was in 3 pieces,,,,and the block had all the main webs broke .
It was all contained in the pan mostly,,,it let go on line at launch,,,,so that was a blessing .

We’re still not sure if the block failed first or the crank ?

Tommy
Click to expand...
I KNOW Chrysler and Ford both used nodular iron. I'm not sure Chevy ever did. I think all their cast stuff was just grey iron. Nodular cast iron is STRONG. I always bring up the Ford 351 Cleveland engines when discussing cast crank strength. That's all they HAD back when the Clevelands were running routinely 9500 RPM down the back stretches at Talladega and Daytona. They didn't have mass crank breakages, either. Same with Chrysler.
 
Update: I just examined the rod journals of the .020 under 340 crankshaft and .010 under 318 crankshaft I have on hand (using a magnifying glass). I don't see much, if any, fillet radius. In fact, looks more like a absence of material in that area of the rod journals.

On the other hand, I remember when I ordered the billet crankshaft (4330M USA Timken Steel) from Bryant Racing for the 2.96" stroke small block crankshaft I am building for the 305 CID in our '72 Satellite Sebring Plus (see "Project Odyssey" on FBBO), I opted for a full radius design. In that case, the machine shop which assembled the short block for me needed to order "narrow" main and rod bearings so that there was clearance with that more pronounced radius.

The full radius 305 Direct Connection forged crankshaft may have used part nos. P3577538/P4452974. I used stock 12:5 to 1 TRW forged pistons (L2322F) and custom Cunningham rods @ 6.298" (6.123" + .175)

https://www.hotrod.com/features/basic-tech-series-crankshaft-982-1621-124-1/
 
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dibbons said:
Update: I just examined the rod journals of the .020 under 340 crankshaft and .010 under 318 crankshaft I have on hand (using a magnifying glass). I don't see much, if any, fillet radius. In fact, looks more like a absence of material in that area of the rod journals.

On the other hand, I remember when I ordered the billet crankshaft (4330M USA Timken Steel) from Bryant Racing for the 2.96" stroke small block crankshaft I am building for the 305 CID in our '72 Satellite Sebring Plus (see "Project Odyssey" on FBBO), I opted for a full radius design. In that case, the machine shop which assembled the short block for me needed to order "narrow" main and rod bearings so that there was clearance with that more pronounced radius.

The full radius 305 Direct Connection forged crankshaft may have used part nos. P3577538/P4452974. I used stock 12:5 to 1 TRW forged pistons with custom Cunningham rods @ 6.298" (6.123" + .175)

https://www.hotrod.com/features/basic-tech-series-crankshaft-982-1621-124-1/
Click to expand...
That is a crank radius. There are several types.
 
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