Lubing and torqueing studs

[belfastguzzi]

General, not specifically Guzzi, but I came across this and thought it was interesting:

 

> These studs are rated at 190,000 PSI (consider that Grade 8 is less than 150,000 PSI).

Included washers are hardened, and the 12 point type nuts are of the aircraft type...

.......30 cwt. engine oil, or the recommended ARP lube, part number L99 should always be used, and the different torque specifications for each type of lube followed.

The tension obtained in the fastener (which is what we need) is affected by how well the lubricant being used is coated evenly on the threads.

 

When using oil, and especially when using the ARP low friction lube, it is highly recommended that the following technique is followed:

- Coat the fastener with the lubricant, and especially with the lube, try and rub it into the threads as evenly as possible.

- Screw the nut down and pull up snugly with a wrench (spanner), repeat this three times.

- On the third go torque to the recommended torque setting.

 

All of the above might sound a little odd, but many carefully controlled tests have shown that this method distributes the lube evenly between the friction surfaces, so increasing the tension each time for a given torque.

The difference in tensile load between step #1 and step #3 can easily be well over 1000 pounds on a 3/8" stud or bolt… very significant!

Always run a tap down each threaded hole prior to installing the studs.

Use a bottom tap (called a plug or #3 tap in England).

Where possible we recommend using a 60 degree counter sink into the top of each stud hole. This will help stop the top threads being pulled up above the surface.

We don’t recommend using Loctite type compounds on studs or bolts, but if you feel you want to, then be sure to complete the job, including final torqueing before the compound 'sets up'.

Studs should not be screwed into the block very tightly, only about 4 ft lbs is all that is required (not much more than hand tight).

The torque figures we recommend will give a preload equal to 75% of the yield strength of the fastener.

 

 

........like many other things in life, sometimes when you get into it a bit, you might find there is a little more involved than just "screwing it together"!

[Ryland3210]

That's a very thorough approach, and reflects the substantial effect thread and bearing surface friction has on the results of using the torque method.

 

For long studs, bolts, and screws, the "turns" method is more reliable and accurate, but that doesn't work on short fasteners because the number of turns to reach 50 to 75% of yield becomes a matter of too few degrees to easily measure. For example on 4 foot long threaded rods, it takes in the area of 1.5 to 3 turns beyond "snug", depending on the amount of preload required. The rod is being stretched a defined amount as a spring, taking advantage of the relatively reliable spring constant of the alloy. In this case, the lubrication of the threads and bearing surfaces only affects the ease of obtaining the number of turns required. For example, it took about 3,000 foot pounds to tighten the nut on a 1.25" rod to the 1.75 required turns in a particular job I was doing. As long as the threads were free to move, lubrication was not critical, although we were careful about that anyway. We had three strong men hang on the end of a 10 foot pipe on a breaker bar, and then jump up and down to get the job done.

 

In the case of short fasteners, the basis for the torque spec. is the ratio of tension generated to torque determined by the thread pitch and diameter of the thread. It's analogous to driving a wedge. Any variation in friction affects the accuracy of the preload. Hardened washers with smooth surfaces are important for highly stressed fasteners. If the washers are too soft and gall during tightening, you can imagine how much that will increase the friction.

 

On large diameter, short screws where we need high preloads, when necessary, we will preheat the screws and torque them in as fast as practical. They contract as they cool down to get the preolad we need. Thirty years later, when attempted to rebuild a machine done this way, removing a fastener like this can be a real battle.

 

An item not mentioned in the article is the class of thread. It's amazing how complex the specifications are on fasteners for highly stressed applications, for good reason.

[Steve G.]

Of course, alot of this good information will be not needed on 'stretch bolts', seen with much higher occasion on automotive applications. I just helped a buddy put his head on his car, with specific instructions to torque to 50ib, then 1/4 turn, on new un-used bolts only.

Ciao, Steve G.

[Ryland3210]

Of course, alot of this good information will be not needed on 'stretch bolts', seen with much higher occasion on automotive applications. I just helped a buddy put his head on his car, with specific instructions to torque to 50ib, then 1/4 turn, on new un-used bolts only.

Ciao, Steve G.

 

That's an interesting combination of torque and turns I have not heard of before.

Were all the bolts the same length? If not, the 1/4 turn would result in quite different preloads.

[Ryland3210]

Of course, alot of this good information will be not needed on 'stretch bolts', seen with much higher occasion on automotive applications. I just helped a buddy put his head on his car, with specific instructions to torque to 50ib, then 1/4 turn, on new un-used bolts only.

Ciao, Steve G.

 

Hi Steve,

 

I just did some quick calculations. The increase in bolt stress due to 1/4 turn on top of the stress caused by your initial torque, as a function of screw length would be as follows:

 

Length Stress, psi

1 453,000

2 227,000

3 151,000

4 113,000

 

For the shorter screws, these stresses are far too high. Even at 3 inches, even the highest strength bolts would be questionable.

[callison]

I was down at the Diablo Canyon Nuclear Power Plant during it's construction. For the reactor vessel, a massive structure, the bolts securing the top of the vessel are torqued to something like 100,000 ft/lbs. To do this, each stud has a threaded bolt hole on the top that a hydraulic jack is threaded into with a lift frame. The nut is run down the stud by hand, the jacking assembly is attached, then the stud stretched to the torque specification, the nut run down the rest of the way by hand and then the jack released. The scale of these parts is staggering, but the process is fascinating.

[belfastguzzi]

Fascinating stuff alright.

 

Back to domestic scale, Ryland's info makes me wonder again about the head bolts on the car I'm driving. It was weeping oil so I tightened the bolts. I didn't want to go through a whole head loosening and tightening procedure. I put a torque wrench on but it clicked pretty much straight away. Not sure how much I could trust the click I tightened the bolts, some of which took more than others. I was very worried about pulling the threads out of the aluminium block. Tightening by feel isn't the best thing in this application, but it did the job and the block is still clean and oil free.

 

– How much danger is there of stripping the threads in an aluminium engine block and when, roughly, is the danger point approached? Presumably oil in the threads makes that danger more likely.

– How accurately can the original bolts that are in place be torqued to a good tension, with out starting again from scratch, sequencing-up with new bolts and clean threads? Is there any point in using a torque wrench at all in a situation like this?

– In the real world could the sort of uneven and possibly over-tightening that I have done be causing a problem for the longer term?

[Ryland3210]

I was down at the Diablo Canyon Nuclear Power Plant during it's construction. For the reactor vessel, a massive structure, the bolts securing the top of the vessel are torqued to something like 100,000 ft/lbs. To do this, each stud has a threaded bolt hole on the top that a hydraulic jack is threaded into with a lift frame. The nut is run down the stud by hand, the jacking assembly is attached, then the stud stretched to the torque specification, the nut run down the rest of the way by hand and then the jack released. The scale of these parts is staggering, but the process is fascinating.

 

Fascinating indeed.

 

I suspect the spec. on the reactor stud preload was more likely in pounds rather than foot-pounds, since they were being stretched rather than the nuts being used to tighten to preload. The result is equivalent to using the turns method.

 

In another application, my objective was to preload 4 rods to 50 tons each. I didn't have the resources of a power company at my disposal, so I used a torque mulltiplier on the the 3.5 inch nuts and a five ton overhead crane to pull the end of its bar.

[Ryland3210]

Fascinating stuff alright.

 

Back to domestic scale, Ryland's info makes me wonder again about the head bolts on the car I'm driving. It was weeping oil so I tightened the bolts. I didn't want to go through a whole head loosening and tightening procedure. I put a torque wrench on but it clicked pretty much straight away. Not sure how much I could trust the click I tightened the bolts, some of which took more than others. I was very worried about pulling the threads out of the aluminium block. Tightening by feel isn't the best thing in this application, but it did the job and the block is still clean and oil free.

 

– How much danger is there of stripping the threads in an aluminium engine block and when, roughly, is the danger point approached? Presumably oil in the threads makes that danger more likely.

– How accurately can the original bolts that are in place be torqued to a good tension, with out starting again from scratch, sequencing-up with new bolts and clean threads? Is there any point in using a torque wrench at all in a situation like this?

– In the real world could the sort of uneven and possibly over-tightening that I have done be causing a problem for the longer term?

 

I believe it is quite easy to strip the threads in an aluminum block because the torque/preload specs are going to be far lower than with ductile iron blocks. It better to have studs in the block and nuts with washers on top of the head to tighten. That way, the studs can be threaded as far as practical into the block first.

There is a saying in mechanical engineering circles that you can tighten to failure, but you cannot tighten a screw to fail. In other words, the fastener snaps or the threads strip during the tightening procedure before the fastener can be loaded enough to cause failure in service.

As long as the bolts were orginally torqued to spec, I believe you could safely tighten them to the torque spec. for your specific motor, PROVIDED you follow the proper sequence, which will be specified in the service manual. If that isn't available, usally a very thorough but reliable sequence is as follow: Do not loosen any screws. Start from the middle of the head, tighten to 30% of the the torque specification. Of course, some of the screws will already be tighter than that. If so, move to the next. Alternate from top to bottom, while working your way out to the ends of the block. Same again at 50%, 80% and 100%.

 

The pattern would look something like this on a ten screw pattern:

 

8 4 2 6 10

 

7 3 1 5 9

 

This procedure reduces the chance of warping the head. When installing a new gasket,, the screws should be checked again after a few hours of normal driving. This is rarely done on new cars these days. In the old days, gaskets would compress a good deal, so this was considered good practice. Even today, I just replaced all the seals and gaskets in '57 Ford engine. Even with very careful tightening, I found that after one 50 mile trip, a number of screws took another good 1/8 to 1/4 turn to return the preload to the spec. Because it has solid lifters, that meant checking the valve clearance all over again. Taking care of this now, will virtually guarantee my head gaskets will never leak, and I don't have to worry about the valve clearance for a very long time.

 

You probably know much of this, but I didn't want to take the chance of leaving anything out, not knowing your level of knowledge.

 

If all else fails, and threads are stripped in the process, you could install heli coil inserts, which will dramatically strengthen the block's holding power, but that would require removing the heads.

 

Good Luck!

[belfastguzzi]

Thanks. If I understand right, you answered the main query with: There is a saying in mechanical engineering circles that you can tighten to failure, but you cannot tighten a screw to fail. So if it's in place and didn't cause damage while tightening in, things will be ok, as far as the threads are concerned anyway.

 

I'm aware of the warping danger, but tend to think that it's less consequential if the head is tight down even if not perfectly balanced, than in its prior state when it was at the original torque settings but allowing oil out somewhere.

 

BTW, as it happens, my next car job at home is to get a broken exhaust manifold stud out of a VW Polo motor.

[Martin Barrett]

BTW, as it happens, my next car job at home is to get a broken exhaust manifold stud out of a VW Polo motor.

 

Stud extractors (eazee out) tools of the devil - good luck

[Ryland3210]

Thanks. If I understand right, you answered the main query with: There is a saying in mechanical engineering circles that you can tighten to failure, but you cannot tighten a screw to fail. So if it's in place and didn't cause damage while tightening in, things will be ok, as far as the threads are concerned anyway.

 

I'm aware of the warping danger, but tend to think that it's less consequential if the head is tight down even if not perfectly balanced, than in its prior state when it was at the original torque settings but allowing oil out somewhere.

 

BTW, as it happens, my next car job at home is to get a broken exhaust manifold stud out of a VW Polo motor.

 

Things will be all right as far as the threads are concerned, and the screw will not fail either.

 

The worst case on tightening would be the case of installing the head from scratch, and tightening one of the screws at one end all the way, which compresses the gasket at that point more than normal, then tightening the screw at the oppossite end. That will use the head as a lever against the gasket near the first screw, and have the tendency to deform the head at that point. Starting at the middle and working to the ends avoids that lever effect. Tightening them all in stages keeps the head's pressure on the entire surface of the gasket more uniform as it compresses a little more with each stage.

 

If there is any part of the stud you can grab onto, after applying penetrating oil and leaving it be for a few hours, I've used stout vice grip pliers, got them as tight as possible and tried that. If necessary, I file or grind flats to give the vice grips a better grip. If the stud is threaded into aluminum, it can really get stuck hard due to the dissimilar metals combined with heat that eventually burns off any lubricant that might have been on the stud when installed.

 

As Martin says, then it's easy out city. In my case, that's only worked about half the time, but worth trying before you resort to drilling out the stud. When I have to do that, I center punch the stud as close to dead center as possible, to help the drill bit to stay in the center. Then comes the fun of getting the remaining pieces of stud remaining out. I apply "Never Sieze" to studs like that when installing them. Then lock washers and brass nuts on the steel stud to make later repairs easier.

[belfastguzzi]

brass nuts on the steel stud to make later repairs easier.

There's an idea. Must seek some out.

[Ryland3210]

There's an idea. Must seek some out.

 

Go for it!

 

Studs usually have coarse thread for the block end and fine thread for the nut end. The best kind of brass nuts are fine thread and extra long to make up the lower strength than steel. If they are long enough so the stud is flush with the end of the nut, there is no steel proud of the nut to rust.

[mike wilson]

 

The pattern would look something like this on a ten screw pattern:

 

8 4 2 6 10

 

7 3 1 5 9

 

 

My preferred pattern would be:

 

8 6 2 4 10

 

9 3 1 5 7

 

Hair splitting?