ARP vs OEM Bolt Tightening Torque...

[jondee86]

Starting this to get some input from the pro's who regularly use ARP fasteners to replace OEM stuff. Also because I see another thread where someone is questioning the ARP recommendations. Here is the ARP "General Information" chart which is headed up... Recommended Torque to Achieve Optimum Preload (Clamping Force) Using ARP Ultra-Torque® Fastener Assembly lubricant*



To make this relevant I bought some ARP flywheel bolts (M10 x 1.25) for a 4AGE, and the slip that came with the bolts said use blue loctite on the thread, ARP lube under the head and tighten to 70 ft-lbs. Spec for OEM bolts is 54 ft-lb with oil on the threads and under the head.

Thinking about this and using a conservative figure of 20% more torque due to using ARP lube instead of oil, I estimated that the ARP spec would apply 55% more tension to the bolts than the OEM spec. Now the ARP bolt might be OK with that, but what about the OEM threaded hole ?

Now the bolts that were supplied to me were "ARP PRO" which are rated as 200,000 (psi) items, and the chart for that spec says that they should be tightened to 62 ft-lbs, which will still give something like 37% more clamping load. So that is the figure I will use.

What do the pro's say ???

Cheers... jondee86

[jondee86]

Did a bit of reading about bolt tightening torques, and it seems that the rule of thumb is that the load required to shear the female thread should be about twice the tensile load required to break the bolt. This means that the bolt should snap before the thread strips... the preferred option.

Generally speaking, when the bolt and the threaded hole are made from the same material, 0.7-0.8 bolt diameter of thread engagement is the minimum requirement, but make it 1.0 bolt diameter to give a safety margin. However, when the bolt material is stronger than the threaded hole material, you need more thread engagement.

In the case of a OEM crankshaft the material will be around 100,000psi whereas the ARP bolt will have 200,000psi material. This suggests that a minimum of 1.5 bolt diameters of thread engagement is required, call it 2.0 to be on the safe side. Note that here we are talking about torquing the bolt up to breaking point. Since the specified tightening torque for the bolt will be less than 75% of the breaking load, in the real world 1.5 bolt diameters of thread engagement should be adequate for my flywheel bolts.

In the morning I will measure the thread engagement

EDIT: Bolt is 27mm long and flywheel is 12mm thick, giving 15mm or 1.5 bolt diameters of thread engagement. As this falls short of the estimates originally made above, like all good politicians, I have modified the theory to fit the facts.

Cheers... jondee86

[cervus]

i wouldn't worry much about the threads in the crank honestly, they're quite strong

the important bit is the right tension to the bolts! the extra clamping load is the added bonus of a higher tensile material, but this is achieved when the bolt/stud/material has reached the right amount of stretch, which if i'm not wrong it's usually around 70 to 80% of the yeld point, a lower tension applied to it, despite being higher than std oem fastener, it's like having it loose, as it is out of its design elastic range, therefore not providing adequate clamping load!

long story short: just follow the instruction supplied