Rod Bolt Question

[Jim-W]

Which combination of standard FE rods with ARP rod bolts is stronger, 13/32” or 3/8”? I ask because I thought I once heard the smaller high tensile bolt is stronger than oem, and leaves more meat in the rods creating a better stronger combination.

Thanks all,
Jim

[FrozenMerc]

I hope a couple of the engine builders that have real world experience will chime in, but the engineer in me would answer the question like this. 

I would want the larger rod bolt as the larger bolt will produce a higher clamp load when torqued to a significant percentage of it's proof load (assuming the bolts are made from the same tensile strength material)  The higher clamp load of the larger bolt would do a better job of holding the cap to the rod, preventing motion and fretting.  I don't think there is enough difference between a 13/32" and 3/8" bolt to significantly change the cross sectional area of the rod and lead to a tensile failure outside of the bolt. 

I never ran into this particular situation when I was a Test Engineer with a small engine manufacturer - those engines used multi piece cranks with single piece rods pushed on during crank assembly.  Now I am working with HD Diesels, and 95% of what we do are all fractured rods these days, which obviously have a bit different clamping consideration.  Just a few old DT466's still come through the shop with "old style" rods.

[Henrysnephew]

Merc, are the new gen powdered iron "cracked" rods not rebuildable/resizable on their big end?  Randy M

[Jim-W]

Thanks very much for your opinion, especially from an engineering perspective. I just happen to have a set of big bolt rods reconditioned with ARP bolts. I’ll put them to good use!

[FrozenMerc]

Merc, are the new gen powdered iron "cracked" rods not rebuildable/resizable on their big end?  Randy M

We don't here.  We rebush the small end, but if the big end is damaged, throw them away, and get new.  Supposedly there are guys resizing Chebbie LS and some other automotive fractured rods, but with these big 13 to 15 Liter CAT, PACCAR, and Navistar motors, it is easier and more cost effective to just replace the rods.

The 13MX PACCAR's and A26 Navistar's even use a fractured main cap.  So if the motor spins a main bearing and walks a cap, the whole block is junk because once the fractured surface is deformed, the cap never mates up properly again.

[FrozenMerc]

Jim-W:  Just remember what Wernher Von Braun is often attributed with saying:

"One Test Result is worth a Thousand Expert Opinions"

That is why I was hoping someone with back-to-back experience would chime in.