ARP rod bolt torque

[2eighTZ4me]

Have a set of 8mm ARP rod bolts for my stroker motor. Would like to have had the 9mm's but the jerk that sold me the pistons didn't bother to measure and I got ripped. Didn't figure this out until AFTER the pistons were pressed on and I went to start installing the new rod bolts. So I have to go with what I have. Anyways - the ARP instructions stated to torque the nuts to 28 ftl/lbs. twice and then a third and final torque at that spec. This stretches the rod bolts to their specs. FSM calls for 33-40lbs on stock bolts. Do I stick with what ARP says and leave them at 28, or do I go for what the FSM says? I did the first round to 28 lbs when plastigaging the motor - clearance was about .0015 to .0018 across the board. Seemed to be ok, but just don't want these things coming apart at 7K.

[BRAAP]

Have a set of 8mm ARP rod bolts for my stroker motor. Would like to have had the 9mm's but the jerk that sold me the pistons didn't bother to measure and I got ripped. Didn't figure this out until AFTER the pistons were pressed on and I went to start installing the new rod bolts. So I have to go with what I have. Anyways - the ARP instructions stated to torque the nuts to 28 ftl/lbs. twice and then a third and final torque at that spec. This stretches the rod bolts to their specs. FSM calls for 33-40lbs on stock bolts. Do I stick with what ARP says and leave them at 28, or do I go for what the FSM says? I did the first round to 28 lbs when plastigaging the motor - clearance was about .0015 to .0018 across the board. Seemed to be ok, but just don't want these things coming apart at 7K.

 

Torque specs are for the purpose of achieving a certain amount of bolt stretch, i.e. the spec is particular to that bolt being torqued, not the part/s you are clamping with said bolt or some other manufacturer of bolt. As such, do NOT second guess the bolt manufactures recommendations as it usually ends in disastrous results. For ARP Fasteners, follow ARP’s torque procedure TO THE LETTER!, including the exact lube for the torque spec they recommend.

[TrumpetRhapsody]

Is it ok that you torque'd it to spec to plastigauge, then took it back apart? I assume the three times they refer to is once and for all, not 3 separate times right?

[2eighTZ4me]

Nope - the sheet I got said to torque them 3x to 28 lbs. Loosen and re-torque is what I was to understand - I'll triple check the sheet this evening. Certainly, there must be some folks on this forum that have built radical high-perf motors and used the ARP products before. What process did you use? This is my first time with ARP stuff, so I'm asking the folks that have done this before with the ARP stuff. I shall follow Paul's advice to adhere to ARP's recommendation to the letter.

[TrumpetRhapsody]

Braap's word is law IMO, lol, I blindly follow!

 

I was just curious, haven't done a full rebuild yet.

[2eighTZ4me]

That has been my take on as many posts as I've read of his. Saving money now to have him port a head.

 

Curious though (and Paul - chime in if you can) Nothing was mentioned about main studs and re-torque. I've had "issues" with my bottom end such that I've had to remove the main stud nuts several times. No mention was made in any of the ARP literature about unbolting and re-bolting the main studs. I'm assuming since these are "race quality" parts, they're meant to be taken apart and put back together many times for service reasons?

[BRAAP]

Yes, they are able to be torqued, disassembled, and retorqued several times. I do not know of a cycling limit. Someone such as Tony-D GrumpyVette, JeffP 1fastz, may know and hopefully chime in, or you can contact ARP directly with that question.

[1 fast z]

As long as you NEVER EVER torque ABOVE the max torque spec, you can use as many times as you want. Many people think "hey I can torque my bolts a bit more than it says just for that extra tightening effect" WRONG. NEVER do that, you are going past the designed yeild point of the material. Always use some sort of moly graphite lube as well.

[jeffp]

All good answers guys. Here is some engineering behind their specifications.

I have taken my bottom end apart two times, the cylinder head three times. I use the ARP studs in both.

The ARP studs can be utilized a number of times with your engine. The bolts and the nuts are designed to do so.

The torque specification is what it is, meaning even though the torque spec is less then what Nissan did, really makes no comparison.

You will note that ARP recommends a specific lube when installing the nuts on the studs. Buy what they sell for their parts.

The lube they sell with their product is designed to work very well with their products.

The engineering aspect of the install is that with the lube they use, the torque is significantly more then the stock bolts. The reason is LOWER friction between the two threads because of the lube, so even though their specifications are less, there is in reality more torque on the contact surfaces due to the lower friction of the contact surfaces when tightening the bolt.

Over torqueing the stud will stretch the thread farther then it is capable of safely stretching, it will cause micro fractures in the thread and the part will fail prematurely.

I can tell you one thing from experience, the ARP studs are very hard, I bought some 10mm studs for my bell housing and had to cut and tap the ends of the studs. They wore out two hack saw blades and they were by far the most difficult thread I have ever cut. The steel is hard.

trust what ARP recommends and you will not have any problems. Over torqueing the bolts may or may not break the stud because every good engineer provides for headroom in their design. It would appear ARP has followed this design concept.

Normally the failure of a rod bolt/nut assembly is failure of the bolt, and not the thread and nut. That is the reason for better bolts in the first place.

Hope that helps

[Tony D]

I'm with the rest, do NOT torque them to above what ARP says, and that means using THEIR specified lubricant.

 

What you have detailed in effect is ARP's 'settling procedure'...torque, relax-settle, retorque, relax-settle, final torque. You are polishing the threads and making sure in those three steps that the torque you are reading is the actual torque of the fastener, and not some residual turning friction from the threads or the bolt head on the rod surface.

 

They are multiple0use fasteners, but to make them last, they must be torqued to 75% of ultimate yield. If the fastener has any false torque resulting in a lower torque than specified, it will allow movement and tensile cycling of the fastener. This can result in fatigue breaks, or more likely fretting of the bolted parts.

 

The procedure they are using is common in other applications where very precise torques are required due to specific yield points of the fasteners. You are basically 'bedding' the components, to allow for a more accurate torque when doing the final setup. The more they are worked as a matched set the more accurate a torque reading you will get on the fastener.

 

That means mating the specific nut to the stud and keeping them mated for their lifetime. This will result in a more and more accurate torque for each subsequent reassembly of the component as turning friction will be further and further minimized. If you swap a nut...then the "three torques" procedure will mate them to a mimimaly acceptable state where they will return a proper torque / clamping load on the bolted component.

 

You can see this in Impeller Nut Torques on Centrifugal Machinery. Due to balancing requirements they are match-marked, and if you torque to the same impeller nut after balancing, you almost always see the match mark misaligned. Try it with a simple stud and nut on a thick washer on the exhaust stud. Torque it to spec, make a mark, then relax it and torque it again. See where your match-marks on the nut ends up---chances are good each time you do this, the mark will go slightly further than the time before. This will reach a point where it stops moving for the most part. This is the same thing ARP is having you do.

[grumpyvette]

"For ARP Fasteners, follow ARP’s torque procedure TO THE LETTER!, including the exact lube for the torque spec they recommend."

 

"As long as you NEVER EVER torque ABOVE the max torque spec, you can use as many times as you want. Many people think "hey I can torque my bolts a bit more than it says just for that extra tightening effect" WRONG. NEVER do that, you are going past the designed yeild point of the material. Always use some sort of moly graphite lube as well."

 

 

 

 

youve gotten good advice in the above posts... and the others, follow the manufactures procedures, and youll be fine, several of my engines have been totally disassembled and reassembled 7-8 times and IM still useing the same ARP studs........now I will point out that there ARE TORQUE to yeild and torque plus angle fasteners but they also come with instuctions, and are plainly marked for single use

 

 

http://www.arp-bolts.com/FAQ/FAQ.html

 

http://www.digitalcorvettes.com/forums/showthread.php?t=55575

 

http://www.digitalcorvettes.com/forums/showthread.php?t=81323

 

IF your useing ARP studs or bolts USE ARP SPECS AND PROCEDURES

[2eighTZ4me]

Excellent advice from you all. ARP sent me the assembly lube with the rod bolt set, so I'm good there. I have only torqued once to plastigage, and I must say, I did not keep track of which nuts went where. I will make it a point to do so for the second and third torque sequences. Thanks for that tip Tony D.

 

Probably have to tear down the bottom end again. It looks like the machinist bored the #1 cylinder too little. I can barely fit the piston in there without rings, and is a very tight fit. With the rings installed, you can feel it binding along the cylinder wall (even thought it's soaked in oil) when you spin the motor over. The other 5 cylinders are fine. He says he bored each cylinder to each piston, but how could you screw up just one if that's the case? Wouldn't you use the same tolerances across the board for all 6?

 

anyways - thanks again guys. feel a LOT more confident about this now.

[BRAAP]

Those guys, JeffP, TonyD, 1fastZ, Grumpy, are our core engine building/tuner gurus and are always successfully pushing the performance envelope to the cutting edge.

 

Thank you guys for chiming in.

 

Take care,

Paul

[Ben's Z]

Sorry to bring up an old topic but I am about to venture down this path, and I want to make sure I understand everything. If I plastigage the rods and mains and I am using ARP fasteners, I should just torque each cap to the recommended value ONCE, and then back off the fasteners and cap and check the tolerances? Or torque it as many times as they recommend as if I was doing finally assembly for checking with a plastigage?

[Lazeum]

You can tighten them to check your plastigage and reinstall them again with no problem. Make sure you repect the torque requirement , don't forget the moly lube provided with the bolts and you'll be fine..

[Tony D]

Let me add to this as a reward for searching.

 

Why does ARP say to torque THREE TIMES?

 

When you torque a nut onto a bolt, by design the threads of the NUT are deformed.

The amount of friction that this takes changes due to an effect known as 'burnishing of the threads'---by torquing these fasteners three times, ARP is taking away the excess friction used by thread deformation and normalizing the friction.

 

If you were to torque that bolt to 10 inch-pounds (no plastigauge installed) and mark a reference point on the nut and rod, you would see that at your 28 ft-lbs torque point, on the first torque up the angle-turn (which is mechanically related to thread pitch, and therefore rod bolt elongation) would be in one position. On your second, it would be further, and on the third it would be further still. Subsequent torques MAY reveal that the threads are normalized and would stay relatively stable in terms of angle-turn/elongation of the rod bolt.

 

Then again, it may not, and ARP may specify replacement of nuts after each usage cycle. If they do, then the three turns is to a normalized point they know will relate to X amount of elongation/stretch.

 

Recapping forces in a nut-bolt, for a given lubricant used, 75% of your torque is friction under the head, 15% is friction on the threads, and 10% of your indicated torque actually goes towards fastener tensile elongation.

If on the first torque your thread friction is 25% (to pick a number) then that pie sum remains the same---it has to equal 100%, so you would actually have NO TENSILE ELONGATION of the fastener (no clamping) even though your TORQUE was "Correct"

 

If on the second torque now your thread friction is 20%, now you have gotten HALF your tensile elongation, even though your TORQUE indicated is STILL only 28 ft-lbs.

 

On the third torque, now your thread friction is 15% (as designed), you now get 10% tensile elongation, and at 28 ft-lbs of torque you have full tensile loading of the stud into the elastic range of tension (75-90% total tensile load of the fastener.)

 

Now, JUST SUPPOSE you re-use that nut, and the friction changes the same as it has on the prior three tightenings, that it thread burnishes and you drop your friction another 5%, so thread friction now is only 10%---everything still has to add up to 100% right? That means that your force for elongation went from 10% now to 15%....

 

Think about that---you have just increased tensile loading "only 5%" but in reality this is 50% more tensile loading! (15/10=1.5!) You now likely have moved the fastener from the elastic, recoverable, use it forever 75-90% range into PLASTIC deformation...meaning some time down the road through load cycling on the joint , the bolt will likely fail.

 

BUT YOU ARE STILL AT 28 Ft-LBS of Torque!!!

 

 

This is why Bryan's "Always use Moly-Graphite Lube" is not really correct, you have to use the Lubricant SPECIFIED. His suggestion applied to a fastener to use normal engine oil would lead to the very overstress he warns against avoiding! You can see that TORQUE from the above examples really is IRRELEVANT when the designed parameters of the fastening system is not followed. Four different tensile loadings all with EXACTLY the same torque, the last being to a failure point "without ever going ABOVE the max torque spec"! Lubricant changes friction on EVERTHING (As JeffP said), and underhead friction is the biggest contributor. Our machines have two types of thread lubricant (ARP consulted with us on fastener design)---these Aermet Studs specify a graphite lubricant, though a moly variant exists. The coeficient of friction difference between moly and graphite is on the order of 100%, your friction drops in half...

So that formula for stud elongation, using EXACTLY The same 'Torque' applied to the bolt head would change from 75-15-10, to 50-2-48! This is a demonstration I do for our class, and I actually make a big dramatic announcement (imagine that, me being dramatic...) to 'stand clear'! The tensile elongation of that fastener changes 480% JUST BY USING MOLY LUBE INSTEAD OF THE SPECIFIED GRAPHITE. I get to about 50% torque and the fastening apparatus goes BANG!

 

The whole nut head assembly literally blows off the end of the stud! We then examine the break and point out characteristic 'tensile failure' signs.

 

As Grumpy mentions, Torque is VERY inaccurate, with many critical fasteners now tightened to stretch dimension, or generically using "Angle Turn" where you take the angle and can calculate based on the number of degrees turned how 'far up the ramp' you go according to thread pitch. We also demonstrate this in the lab, using a stretching apparatus, as well as an angle-turn fastener and a dial indicator on the stud head. The students see exactly the correlation between Angle Turn, Hydraulic-Stretch, and the variation introduced with changing variables when using 'Torque'...

 

When they say follow EXACTLY, they mean it. The above is why!

 

I suggest anybody else that wants to know more about fasteners and why you do some things, or why they fail to go to the UK based "Bolt Science" website. Google them, they come right up. There is GREAT reading there, and I always tell the students to spend time watching the "Junkers Machine" video before they spout off on how great split lockwashers are for "keeping bolts tight"---there is TWO THINGS that keep bolts tight: PROPER TENSILE LOADING ON MAKEUP and PROPER JOINT DESIGN. Period.

 

Loctite is another matter best left for another time, other than to say if it says INSTALL DRY, Loctite IS a friction-changing substance and it CAN induce bolt failure (I can cite an expensive example!) Friction changes are all accountable through calculation, the thing to take away from this is not to think "Torque" gives a consistent result. If you change a variable, then you MUST recalculate the "Required Torque" based on the new frictional data for the joint.

 

I went on entirely too long on this, but I just repeated this daily for the past 6 weeks in Thailand, so it's all fresh in the mouldy brain! It's a little more in-depth on the mechanics of how it works. It's basically the same as what JeffP said, but more 'whys' are answered.

Edited October 6, 2012 by Tony D

[Lazeum]

it is a good reading

 

Do you have some curves showing burnishing/friction of a joint vs. number of tightening OP. by any chance?

 

I'm asking because I believe at some points, we should start to have an asymptote/stabilization of the burnishing? It should depend as well of the micro finish / hardness of the threads / corrosion protection layer as well. I believe with some nitro-carbonization or other treatment you could minimize burnishing with hard outer layer, not sure though if it would be compatible full heat treatment the bolt would require to get its full strength...

 

 

 

 

 

[Tony D]

Most bolting methodology for vehicles and HP Racing comes almost directly from the Airframe/Aircraft Industry.

There many bolts are phosphor coated, or Zinc coated, and are specified to be thrown away after one use. Nuts are cheap...

 

They know the friction of a new fastener with X coating, and design the torque around that.

 

As theorized above, the Torque Three Times is likely to get the nuts to a normalized state they could repeat. But unless you actually jig the fastener up, put a dial indicator on the stud head, and then tighten it, you won't know.

 

And really that is all there is to it. If you jig the bolt and test it (more likely testing 10, or better yet, 100) then you can get a number like you are looking for... or at least a sampling of what happens and then draw some conclusions.

 

The thing is NUTS are generally considered DISPOSABLE considering the consequences of a failure induced by their changes.

 

I would have to ask, given the extreme low cost of nuts, why NOT simply replace them each teardown? What REAL economy do you gain by checking for a theoretical burnishing limit?

 

Remember it is the nuts affect on the STUD that is important. By simply replacing the nuts, like Bryan said above "you can use it over and over and over again indefinitely"!

 

We change the nut, to protect from going into the stud's plastic state. Why risk it? Something falls out of the sky, or the rod detaches and wastes the engine.

 

Because someone didn't want to spend $15 on a new set of nuts?

 

Like the saying goes: Get some NUTS MAN!

[FricFrac]

Let me add to this as a reward for searching.

 

Why does ARP say to torque THREE TIMES?

 

When you torque a nut onto a bolt, by design the threads of the NUT are deformed.

The amount of friction that this takes changes due to an effect known as 'burnishing of the threads'---by torquing these fasteners three times, ARP is taking away the excess friction used by thread deformation and normalizing the friction.

 

If you were to torque that bolt to 10 inch-pounds (no plastigauge installed) and mark a reference point on the nut and rod, you would see that at your 28 ft-lbs torque point, on the first torque up the angle-turn (which is mechanically related to thread pitch, and therefore rod bolt elongation) would be in one position. On your second, it would be further, and on the third it would be further still. Subsequent torques MAY reveal that the threads are normalized and would stay relatively stable in terms of angle-turn/elongation of the rod bolt.

 

Then again, it may not, and ARP may specify replacement of nuts after each usage cycle. If they do, then the three turns is to a normalized point they know will relate to X amount of elongation/stretch.

 

Recapping forces in a nut-bolt, for a given lubricant used, 75% of your torque is friction under the head, 15% is friction on the threads, and 10% of your indicated torque actually goes towards fastener tensile elongation.

If on the first torque your thread friction is 25% (to pick a number) then that pie sum remains the same---it has to equal 100%, so you would actually have NO TENSILE ELONGATION of the fastener (no clamping) even though your TORQUE was "Correct"

 

If on the second torque now your thread friction is 20%, now you have gotten HALF your tensile elongation, even though your TORQUE indicated is STILL only 28 ft-lbs.

 

On the third torque, now your thread friction is 15% (as designed), you now get 10% tensile elongation, and at 28 ft-lbs of torque you have full tensile loading of the stud into the elastic range of tension (75-90% total tensile load of the fastener.)

 

Now, JUST SUPPOSE you re-use that nut, and the friction changes the same as it has on the prior three tightenings, that it thread burnishes and you drop your friction another 5%, so thread friction now is only 10%---everything still has to add up to 100% right? That means that your force for elongation went from 10% now to 15%....

 

Think about that---you have just increased tensile loading "only 5%" but in reality this is 50% more tensile loading! (15/10=1.5!) You now likely have moved the fastener from the elastic, recoverable, use it forever 75-90% range into PLASTIC deformation...meaning some time down the road through load cycling on the joint , the bolt will likely fail.

 

BUT YOU ARE STILL AT 28 Ft-LBS of Torque!!!

 

 

This is why Bryan's "Always use Moly-Graphite Lube" is not really correct, you have to use the Lubricant SPECIFIED. His suggestion applied to a fastener to use normal engine oil would lead to the very overstress he warns against avoiding! You can see that TORQUE from the above examples really is IRRELEVANT when the designed parameters of the fastening system is not followed. Four different tensile loadings all with EXACTLY the same torque, the last being to a failure point "without ever going ABOVE the max torque spec"! Lubricant changes friction on EVERTHING (As JeffP said), and underhead friction is the biggest contributor. Our machines have two types of thread lubricant (ARP consulted with us on fastener design)---these Aermet Studs specify a graphite lubricant, though a moly variant exists. The coeficient of friction difference between moly and graphite is on the order of 100%, your friction drops in half...

So that formula for stud elongation, using EXACTLY The same 'Torque' applied to the bolt head would change from 75-15-10, to 50-2-48! This is a demonstration I do for our class, and I actually make a big dramatic announcement (imagine that, me being dramatic...) to 'stand clear'! The tensile elongation of that fastener changes 480% JUST BY USING MOLY LUBE INSTEAD OF THE SPECIFIED GRAPHITE. I get to about 50% torque and the fastening apparatus goes BANG!

 

The whole nut head assembly literally blows off the end of the stud! We then examine the break and point out characteristic 'tensile failure' signs.

 

As Grumpy mentions, Torque is VERY inaccurate, with many critical fasteners now tightened to stretch dimension, or generically using "Angle Turn" where you take the angle and can calculate based on the number of degrees turned how 'far up the ramp' you go according to thread pitch. We also demonstrate this in the lab, using a stretching apparatus, as well as an angle-turn fastener and a dial indicator on the stud head. The students see exactly the correlation between Angle Turn, Hydraulic-Stretch, and the variation introduced with changing variables when using 'Torque'...

 

When they say follow EXACTLY, they mean it. The above is why!

 

I suggest anybody else that wants to know more about fasteners and why you do some things, or why they fail to go to the UK based "Bolt Science" website. Google them, they come right up. There is GREAT reading there, and I always tell the students to spend time watching the "Junkers Machine" video before they spout off on how great split lockwashers are for "keeping bolts tight"---there is TWO THINGS that keep bolts tight: PROPER TENSILE LOADING ON MAKEUP and PROPER JOINT DESIGN. Period.

 

Loctite is another matter best left for another time, other than to say if it says INSTALL DRY, Loctite IS a friction-changing substance and it CAN induce bolt failure (I can cite an expensive example!) Friction changes are all accountable through calculation, the thing to take away from this is not to think "Torque" gives a consistent result. If you change a variable, then you MUST recalculate the "Required Torque" based on the new frictional data for the joint.

 

I went on entirely too long on this, but I just repeated this daily for the past 6 weeks in Thailand, so it's all fresh in the mouldy brain! It's a little more in-depth on the mechanics of how it works. It's basically the same as what JeffP said, but more 'whys' are answered.

 

 

Thanks Tony for the info. I follow the rules for a reason but the most interesting part is knowing WHY the rules are there in the first place.