Quote:
Originally Posted by Diesel Rookie
Exactly what do you guys mean by PMR why are they weaker? I think it has something to do with the piston conecting rods but am not sure. From what I can tell on the forum my 2003 F350 7.3L may be a PMR truck, is that corret?
Joe
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Yes, the term PMR (powdered metal [connecting] rod) refers to the connecting rods. It is so named because of the process used to manufacture them.
Forged rods are constructed from molten metal poured into a forging press and pressed by many tons of force. They are then allowed to cool and machining processes take place to cut the "big end" bearing journal bore in half, install rod bolts (studs in this case) and then hone the journal bore to size.
Powdered metal (or sinter forged) connecting rods are created when a fine metal powder is heated to near melting and then pressed into the desired shape. The rod "big ends" are actually broken off instead of cut. This creates a unique mating surface that more perfectly aligns the two parts. Then the bolt holes are drilled/tapped and the final machining begins.
The problems associated with PMRs are plentiful.
1. No set grain structure.
2. High probablility of porosity (pinholes/air pockets in forging)
3. Very brittle. There is very little flexibility and resilience inherent in the material.
4. The 7.3L PMRs were designed like the forged rods. Comparing forged to powdered metal rods shows that they are very similar in their construction. This leads to weak spots and areas that should have been more carefully developed to take advantage of the different manufacturing processes/materials.
There is nothing wrong with powdered metal connecting rods so long as they were DESIGNED to be PMR. Take the 6.0L Powerstroke and 6.6L Duramax for example. They are able to handle much more than the 7.3L PMRs due to their design.