Max RPM for 258

[swatson454]

Do you guys know about how many rpm's a new set of 258 rods with ARP fasteners can reliably handle in polished and balanced trim? I'll likely be using run-of-the-mill, .030 stock-replacement pistons and a heavy 258 crank. I tried the search function


Shawn

[amcinstaller]

with both the 258s in my amc cars, they really stopped pulling past 4500, cant see it being much past there

[swatson454]

I just mean the strength of the rod, not whether or not the engine combination can actually get it there or not. My software is showing a 4,750 rpm hp peak with the combo I'm currently considering but I'm not sure that I'm warm and fuzzy taking the rods that far.

Thanks!

[Cheromaniac]

Do you guys know about how many rpm's a new set of 258 rods with ARP fasteners can reliably handle in polished and balanced trim? I'll likely be using run-of-the-mill, .030 stock-replacement pistons and a heavy 258 crank. I tried the search function

I have the 12cwt 258 crank, 258 rods (balanced to within 1g), ARP rod bolts, and cast pistons. I've revved my engine to the 5200rpm limiter on several occasions without issue.

[IH 392]

There doesn't seem to be much issue with the rods, the issues is with the crank shafts, it's claimed that they experience "torsional vibration" at 5400-5500rpms and running them at this rpm will snap them, that's probably why they put the 5200rpm rev limiter on the 4.0's?

[Cheromaniac]

Yup. The crank journals can develop cracks at ~5500rpm and there's a harmonic at ~5600rpm that can break timing chains and throw the distributor out of the block. As IH 392 said, the stock rev limiter is there for a reason.

[swatson454]

Thanks guys. I've read about the torsional issue on the cranks and, thankfully, that's above my rpm range on this build (budget with a capitol B!). I'm not big on using OEM rods on builds but it seems like they'll be just fine for what I'm doing here. Thanks again for your input.


Shawn

[amcinstaller]

i notice that the question is about the max rpm of a 258, and although a stroker build tends to use a 258 crank and rods, id like to ask what youre doing? just as a point of curiousity it doesnt sound like youre building a stroker

[swatson454]

i notice that the question is about the max rpm of a 258, and although a stroker build tends to use a 258 crank and rods, id like to ask what youre doing? just as a point of curiousity it doesnt sound like youre building a stroker

Nope, I'm just going to build this old pig. It's mostly to satisfy my curiosity but I'm going to spend some quality time prepping the 258 head and intake plenum, use a well-suited cam and see what happens.

I might just be but like I said, I sure am curious.


Shawn