Cam Cap bolts pulled out / Why?

[zarati]

While reassembling the motor on the D2 S8 last night I looked up the torque spec for the Cam Caps (I had cams out to do tensioner pads & seals while doing the Timing Belt job. According to the factory manual which I found on line here:
http://workshop-manuals.com/audi/s8_qua ... -4.2l_(ays)/relays_and_modules/relays_and_modules_accessories_and_optional_equipment/alarm_module_(vehicle_antitheft)/component_information/locations/

The specified torque is either 5Nm or 10Nm (+ 90degrees). Its specifies it differently depending on which part of the manual you look in. My Torque wrench can't even go to 5NM so I did 10NM + 90degrees. When I got about 1/2 way through one pulled out.

"Camshaft Bearing Cap Tightening Sequence & Torque Camshaft Bearing Cap, Tightening Torque and Sequence; Refer to Camshaft, Engine, Service and Repair See: Service and Repair Bearing cap to Cylinder head ................. 10 Nm + 90 ° (7 ft.lb + 90 °)
Camshaft Adjuster To Cylinder Head ............................................. 10 + 90° (7 + 90°)
Camshaft Position (CMP) Sensor Rotor To Camshaft ............................. 23 Nm (17 ft. lbs.)
Camshaft Position (CMP) Sensor Housing To Cylinder Head .................... 10 Nm (7 ft. lbs.)
New ................................................................................... 0.05 - 0.15 mm (0.0020 - 0.0059 inch)
Wear Limit .......................................................................... 0.20 mm (0.0079 inch)"

Good news, The hole is captive "IE doesn't go down into an oil valley or anything." so I can tap it and put a heli coil in there.

BUT: Whats the consensus on torqueing these things. Doesn't seem to be tight enough to be a torque to yield bolt like the head bolts. That 90-degrees really didn't take that much torque, but it is only an M6x1.0 Thread bolt. I figure one of three things is going on.
1) Someone was in there before me and over torqued it.
2) Someone was in there before and torqued it right and the aluminum casting was just weak in that spot. Plus aluminum doesn't like multiple applications of steel bolts being torqued in anyways.
3) Bolts do stretch a little. They are not listed as single use though. For 2nd use maybe only go like 60-degrees???

I'd love some thoughts on this from those that build a lot of heads/engines.

[zarati]

I will also add that the bolt that stripped was NOT being used to pull the cam down. Cam was fully seated and all bolts for all cams had been cross torqued to 10Nm. This particualar bolt was to cap #4 which was added after seating the cam. I don't think it was a case of user error. ;-)

[loxxrider]

Did the bolt strip or the aluminum?

My guess is that someone over torqued it before or if you had oil on the threads, that would also do it. Anti seize was the reason I recently broke an exhaust stud in fact. Anything that lubricates the threads serves to multiply the torque being applied.

It not necessarily true that aluminum doesn't like steel bolts threaded in multiple times. If it was designed properly, it should withstand thousands of cycles of torquing. However, the way audi treats it's aluminum blocks as disposable items, I wouldn't be that surprised if it wasn't designed for more than a few cycles!

[zarati]

Chris, Thats a great point about oil on the thread.
Yes the aluminum is what stripped. When I got about 1/2 way through my 90-degree stroke I felt it get easier all of a sudden. I removed the bolt, inspected, looked ok so reinstalled and tried again. That time it never made torque, just pulled the aluminum threads right up.

Engine had been sitting for 6-months so I lubed all the cam surfaces prior to installing. Its possible I dribbled some oil down into the hole.

Any thought on the 90-degrees. Do you guys still go a full 90-degrees after the initial torque?

[loxxrider]

It could have had the same or similar effect if there was oil where the bolt head meets the metal or washer as well. I would definitely do the additional 90 degrees if that's what the manual specifies. That is the most accurate way to achieve proper torque once the initial torque reading is met.

[dana]

I've had those bolts pull the threads before when torquing down to the specified torque. I blamed it on my torque wrench being inaccurate at such low numbers, but I dont know for sure.

To be honest, I dont use a torque wrench on them anymore, and have never stripped one out or had one come loose/wear strangely. I think I have probably installed 10-15 cams with the "feel it out" method and had no issues. Obviously a highly accurate 1/4" drive torque wrench would be a wise buy...but

[PRY4SNO]

This is just a sticky point but I'm interested in what Chris said about thread dope multiplying torque.

My understanding is that a bolt that isn't doped will lose up to 90% of the torque being applied through friction. Thus, to be able to consistently achieve the desired torque, one needs to lubricate all the mating surfaces.

To put it another way, doping threads doesn't increase torque but maintains consistency.

This is my understanding coming from work, where we manually or pnuematically torque and stretch bolts on flanges with lines at high temps and pressures containing hydrogen or acid, etc, daily. Anywhere from 5/8" to 4" studs.

My guess is reusing TTY bolts or improperly calibrated wrench. Again, just a guess.

[loxxrider]

I'm not sure why anyone is talking about torque to yield bolts when it's the aluminum threads that pulled out. TtTY only matters for the fastener.

As far as the lubricated threads, either whoever told you that is mistaken or you might have interpreted what they said wrongly. I spent nearly an entire semester studying bolted joints and fasteners. Lubrication ABSOLUTELY does multiply the load applied to the fastener and it's mating part. The physics of it are very simple actually. Think about it. The torque that must be applied to the bolt to achieve a given tensile/compressive load depends on the friction factor between the threads of the fastener and mating part as well as how much tensile force is generated by the screwing action. That's pretty much it (there are other small details like angle of the threads, etc. But you get the idea.

You're right that lubricating the threads doesn't increase torque. In fact, it decreases it in a way! In other words, if you want to achieve 100 lbs of clamping force with a given bolt, it may require 10 ft lbs of torque unlubricated. However, when lubricated it may achieve that same claiming load when you apply only 3 ft lbs to it (I'm using arbitrary numbers here) to achieve the same clamping load because it's easier for you to turn it (the threads are slipping past eachother more easily than when unlubricated).

So torque specs are really meaningless without knowing what lubricant to use. Usually if it isn't specified though, it is to be torqued dry. I guarantee that the torque you guys use for those flanges would be specified to be much lower if you weren't using dope.

One thing that was correct about what you said is that putting a substance which yields a known friction factor on the threads will help with achieving consistent torque. However, it must be designed with that exact Lubrication in mind.

This is why ARP lists various torque values for their fasteners depending on whether you use their torquing lube or not.

So if you're torquing something to a specified value you must also know whether it was designed to be torqued dry or with a specific kind of lubricant. This is also why you often see the manufacturers coat their torque to yield bolts with something when you buy new ones. That substance is to fool proof the torquing process. It makes sure that the user uses the proper lubricant to torque the bolt (since it's already applied to it).

Also, it is harder to achieve an accurate clamping force with dry threads because of the excessive static friction which needs to be overcome before the bolt actually gets turning. That can lead the person doing the torquing to believe that they are applying more torque than they really until it gets moving again and dynamic friction takes over (which is what is used to figure the torque spec).

So... The 10 Nm that was applied to the bolt was probably correct. The problem is, that same 10 Nm can generate a much larger load on the threads (think of it as tensile force in the fastener) when lubricated with oil than it can dry. That is how you can pull threads out in a situation like this.

[loxxrider]

My post about multiplying the torque may have been misleading. I meant that it has the effect of multiplying the torque, not that it multiplies the torque that you must apply to achieve the same tensile load on the fastener. In other words, it multiplies the load, force, stress, or whatever you want to think of it as that the bolt can apply given the same torque applied to it by the user.

[PRY4SNO]

We mostly concur, Chris.

I may have been a bit quick to toss out TTY bolts in my comment, I was just guessing (as stated). I've deformed threads (in a 7A head) before and took it to be either a fault of the bolt, or the wrench. I see now it was the wrench lol. Simple Helicoil and I was back in business.

As for my comment about un/lubricated joints being torqued to spec, I stand by it. I'm not an engineer (though I do spend all day at work fixing their design fuckups -- not directed at you ) but my trade in my jurisdiction is only out-qualified by reputation & scope by my contemporaries in Germany. You're right to point out that each application is intended with a specific spec of dope, as, say, Teflon or molybdenum or copper or nickel dopes each have decidedly different properties. Which leads into your comment about ARP studs. Couldn't agree more. Bottom line is an unlubricated joint requires more force to get the same final torque as a lubricated joint. Hence the perceived effect of multiplication. The nuance is that the effort is lost to friction, which varies based on the shape/profile of the mating surfaces which is impractical to measure in detail each time.

Also, agreed that when there is no specified dope mentioned that it's best to torque dry. Engineers are fastidious folk and if they meant for something, they will say so in the specs.

[loxxrider]

Bottom line is an unlubricated joint requires more force to get the same final torque as a lubricated joint.

This is far from true, at least as you've written it... but I know what you mean! Torque is torque. You are not "losing" torque to anything. You do have the effect of losing torque to friction, but what you are really losing is preload/tensile stress/clamp force, or whatever else you want to call it.

If you re-wrote what you said to, "Bottom line is an unlubricated joint requires more torque to get the same final preload as a lubricated joint," then that would be absolutely true!

[audifreakjim]

My old head had 3 or 4 studs that stripped. Had a helicoil set on hand every time I would touch the cams. Helicoil it and move on:)

[PRY4SNO]

Bottom line is an unlubricated joint requires more force to get the same final torque as a lubricated joint.

This is far from true, at least as you've written it... but I know what you mean! Torque is torque. You are not "losing" torque to anything. You do have the effect of losing torque to friction, but what you are really losing is preload/tensile stress/clamp force, or whatever else you want to call it.

If you re-wrote what you said to, "Bottom line is an unlubricated joint requires more torque to get the same final preload as a lubricated joint," then that would be absolutely true!

Thanks for that... I spent a healthy amount of company time today googling these terms so I can understand the differences

[loxxrider]

We meant the same thing

[zarati]

Ok,
Head is now Helicoiled and seems fine. I"m bolting up the valve covers tonight, hopefully bolting up the driveline as well after the Tranny Rebuild. Tranny is in, just no driveshafts, etc yet. I want to drive it this weekend!

Thanks for the comments.