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View Full Version : "Dry" Torque vs "Wet" Torque


Enigma Man
11-09-2008, 09:46 PM
So i have been doing a lot of research regarding torque specifications, different types of torque, grade 8 vs 5 bolts etc. for this engine rebuild that I am about to embark on. Thought it might be interesting to discuss / throw out there.


"Fatigue usually doesn’t play a big part in grade 8 or grade 5 fasteners since most steels are good for 2 million to 10 million cycles. Here is a quick point about fastener fatigue. Almost all fastener fatigue failures are the result of improper (almost always too low) torque. Too low a torque will cause the fastener to pick up more load more often and eventually cycle it to failure. Therefore, you want to make sure you torque your fasteners to the appropriate level using a torque wrench and make sure to torque dry, clean threads. Lubricated threads significantly change the actual preload on the fastener and you risk over torquing it."



Now, I can understand "cycling" a bolt before it fatigues and breaks, however would this be more of a concern on Connecting Rod Bolts and Crank Shaft Main bolts? I say this because of RPM "cycle" load on the bolts.


Also, Which bolts require a "wet" torque when assembling the block as this will drastically affect the actual torque specs?

Ryan Michelotti
11-09-2008, 10:51 PM
ARP will give a different Torque Spec depending what lube you are using. For Example Standard Motor Oil on a head Bolt/Stud will require a HIGHER torque spec, than if you were to use ARP's Thread Lube...Sometimes as much 25-30ft/lbs difference.

Ryan

83hurstguy
11-10-2008, 06:17 AM
ARP moly lube actually burnishes the threads and increases fastener tension every cycle you torque it. Joe Mondello was telling us that at this year's OCA nats... that is why you cycle the bolts 5 times when installing...

Enigma Man
11-10-2008, 09:34 AM
ARP moly lube actually burnishes the threads and increases fastener tension every cycle you torque it. Joe Mondello was telling us that at this year's OCA nats... that is why you cycle the bolts 5 times when installing...

Do you mean torque down the bolt and then loosen it to "cycle" the bolt 5 times?

Interesting stuff, good knowledge to know. Typically from what I have heard when putting together a motor is to use a Straight 30 weight oil and torque down the bolts. I just want to make sure that I get it done right the first time and not have bolts flying out of the engine lol

83hurstguy
11-10-2008, 09:51 AM
Do you mean torque down the bolt and then loosen it to "cycle" the bolt 5 times?

Interesting stuff, good knowledge to know. Typically from what I have heard when putting together a motor is to use a Straight 30 weight oil and torque down the bolts. I just want to make sure that I get it done right the first time and not have bolts flying out of the engine lol

I believe the new ARP main fasteners use that procedure for initial tightening. I'd recommend following the ARP directions when you buy new stuff...

joe_padavano
11-10-2008, 04:28 PM
So i have been doing a lot of research regarding torque specifications, different types of torque, grade 8 vs 5 bolts etc. for this engine rebuild that I am about to embark on. Thought it might be interesting to discuss / throw out there.


"Fatigue usually doesn’t play a big part in grade 8 or grade 5 fasteners since most steels are good for 2 million to 10 million cycles.

Well, let's do a little math. At 3,000 RPM that's 180,000 cycles in one hour, or 1.8 million cycles in ten hours. Yes, you don't run 3,000 RPM continuously, but it is not hard to reach this fatigue limit in a normal engine lifetime. Also, the fatigue life of metal is a function of both number of cycles and magnitude of the load. Lots of cycles at low load can have the same effect as fewer cycles at high load. Parts like main and rod bolts see higher load at higher RPM.

Here is a quick point about fastener fatigue. Almost all fastener fatigue failures are the result of improper (almost always too low) torque. Too low a torque will cause the fastener to pick up more load more often and eventually cycle it to failure.

Well, it depends. Under some loading conditions a loose fastener will cause slop in the system which leads to impact loads on the fastener and thus higher loads (and quicker fatigue failure). On the other hand, if one fastener in a pattern is loose, the others are forced to pick up the additional load, leading to premature failure of those fasteners instead of the loose one. Think of a loose lug nut, for example.

Therefore, you want to make sure you torque your fasteners to the appropriate level using a torque wrench and make sure to torque dry, clean threads. Lubricated threads significantly change the actual preload on the fastener and you risk over torquing it."

The selection of wet or dry is a function of how the bolted joint was designed. The Chassis Service Manual (for example) will list the proper conditions that go along with the specified torque. Some fasteners are intended to be installed clean and dry, others lubricated. Using lube on fasteners intended to be torqued dry will lead to overtorquing and likely failure. Not using lube on fasteners intended to be torqued wet leads to undertorque and loose fasteners. Also, pay attention to the underside of the head of the bolt and any washer. Much of the friction variation that caused incorrect torque readings can come from these points.

Enigma Man
11-10-2008, 11:37 PM
Interesting read Joe,

Thanks for the additional clarification. It seems to me like changing bolts (Ie: Mains, connecting rods etc.) is a good idea due to the fact that these bolts have exhausted their "cycle" duty over the life of the engine.

Not knowing the history of the motor or how it was "abused" It seems the safe bet would be to replace these bolts mainly for insurance purposes.

Where can I buy a ARP bolt kit for my motor?

Ryan Michelotti
11-13-2008, 04:01 PM
There are no ARP bolt kits for the Oldmobile Motors....

You just need to buy the proper ARP bolts for the part you are bolting on...

Mains (studs)
Rods
Heads (Studs)
Balancer (1 bolt, about $20...my daughters STILL make fun of me for spending $20 on 1 single bolt)
Oil Pan (Studs)
Valve Covers (Studs)
Intake
Carb (Studs)

It's a lot of bolts and studs/nuts....

Ryan

Enigma Man
11-15-2008, 04:16 PM
so ARP Bolts are about $20 each? Thats insane

IIIQuaZIII
11-15-2008, 05:20 PM
No, not all...just the harmonic balancer bolt. It's a big meaty custom bolt...that's probably why.

FE3X CLONE
11-15-2008, 07:11 PM
And if you want ARP bolts for just about any other part of the engine, check out www.allensfasteners.com

Every bolt on my engine is either a black oxide or stainless 12 pt bolt from them.