Anybody have a link to the picture....

[Vaughn MacKenzie]

fkovalski basically they manufacture the rod with both ends closed, then use a process where they fracture the big end at the two places to separate it to make the rod cap, instead of machining it. Makes for a perfect mating situation which minimizes distortion when torquing it down = longer bearing life. Comprende?

 

[NPloysa]

I just saw the fractured rod technique recently in person at the dealer. BMW does it on their bike rods also. Caught me offguard at first but makes sense.



Nick

 

[Hohn]

But, if we agree that the cap screws see a shear stress in an offset design, doesn't that make the screws themselves slightly more prone to failure since a steel beam cannot handle as much shear load as tensile (or compressive) load? What I'm saying is, I'd rather my bolts saw only tensile (or compressive) loading rather than any shear load.



-Ryan

I agree, and I'm not certain that the angled cap MUST make the rod stronger, just that it MIGHT.



Notice, I said that the angled cap makes the CAP stronger-- it does. But it does this by shifting load to the rest of the rod. So the overall strength of the rod is probably about the same.



Here's a good pic of the new HPCR rod:

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And here's a view of the cracked cap:

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A couple interesting things. Cummins has switched to a smaller capscrew on the newer rods. I suspect it's because the interference fit of the cracked-cap design means that the cap needs less clamping force to prevent walk.





In the context of cap angle and rod strength, I think that the cracked cap design can make the rod stronger when combined with the offset cap.



As I posted before, the angled cap just swaps some tension for shear, right? But with the cracked-cap design, the screws don't have to take much of that shear at all. The interference fit of the cap itself takes that shear, which takes the strain off the screws. All the screws have to do is provide enough clamping to prevent the cap from "jumping a tooth", so to speak.



The cracked cap makes the cap fasteners' job much easier. I suspect Cummins saw that, and that's why they went to the cheaper, weaker screws on the HPCR cracked caps.



Capscrews have gotten smaller as designs have improved. They went from an M14 on the 1st gen engines to an M12 on the 24Vs, then finally to the M11 screw on the HPCR engines.



I had a good discussion with Don M on this, and he's the source of this info. The pics I posted are from him, too. Thanks to Don.



Here's a link to a DTR thread with good pics and rod info.

http://www.dieseltruckresource.com/dev/showthread.php?t=47264&highlight=cracked+cap



Justin

 

[Hohn]

fkovalski basically they manufacture the rod with both ends closed, then use a process where they fracture the big end at the two places to separate it to make the rod cap, instead of machining it. Makes for a perfect mating situation which minimizes distortion when torquing it down = longer bearing life. Comprende?

Correct-- the rods start as one piece of forged or PM steel.



I've heard that they can dip the one piece in Liquid Nitrogen, then they can fracture split it with remarkable precision.



Anyone know if they fracture the rods at room temp, or do they superchill them to make them good and brittle?

 

[rbattelle]

Justin, are all the HPCR rods fracture split and look like the one you've posted? I don't think they are, as my service manual indicates that my rods look exactly like the 24v (VP-44) rods, which have a MUCH different cap than the fractured version...



-Ryan

 

[Hohn]

I'd imagine that all the HPCR rods look like the pics. However, DC, Cummins, and other corporations are infamous for production overlap as they use whatever parts they have on hand before they switch over. (i. e. , the 53 blocks went on even in 2001).



It's also possible that the decision to switch over the rods occurred after the manual was written.



Sorry-- I can't give you anything conclusive, just a bunch of hypotheticals.

 

[klenger]

The rod pic has been posted on rv.net for comment. Sure brings out the best from the Ford guys.

 

[TJany]

ok... another thought... when you look at a piston you've pulled, ever noticed there's more wear on the sides parallel to the cap? this is called side thrust (i believe) and is due to a sideways force on the piston. This tells me that there is a side ways force actin on at least the pin on the piston. there is also going to be a sideways force at the crank end. an earlier post eluded to the force being strictly along the centerline of the rod, however as direction changes in the movement of the crank cap, and at the piston pin, side forces are acting on the cap etc. In fact think about a simple change in direction, "an object in motion tends to stay in motion, while an object at rest tends to stay at rest, until acted on by an outside force," anotherwords i'd feel pretty confident saying that there are forces acting on the rod (and therefore cap) in all directions. given this scenario i can see an offset cap providing more strength, as the majority of the forces will be acting along the centerline of the rod, but not all. however without runnin some numbers on it i cant offer up any proof on the matter.



just some more thoughts on the issue



Tim

 

[NPloysa]

I think they forge the rod, then hit it on the side of a table like you do with a new roll of coins

 

[rbattelle]

TTT... someone was asking about these pics...



-Ryan

 

[klenger]

While the size of the rods is interesting, the smaller size of the Ford rods seems to be the least of their problems.