Proper torque on the BMW motorcycle
Plus, tips on removing those stripped out fasteners.
The main reason for a spec on tightness is to prevent over-tightening, not under tightening. In the case of multiple fasteners on one part, it is also to prevent warping.
Carefully check for the torque on the part that you are tightening or loosening. One of the BMW shop manuals had incorrect torque settings in it. Some of the aftermarket books have not always gotten it correct. Use common sense and double check it.
Aluminum is especially sensitive, as it has some odd characteristics. I am not any kind of expert in the area of metallurgy, but I have learned a few things. For example, BMW hand levers can be straightened with gentle slow pressure in a warm room. This is due to a characteristic called “cold flow.” The Japanese levers usually break when straightened. The cases where they do straighten one is followed with stories of hearing a “clink” and then finding the piece on the floor. The BMW lever (Magura) is (was) the best around. I have straightened them when they were bent double. Has it changed in the last 25 years?
A major aluminum problem for BMW was the heads on the /2 in the starting in the early to the mid-60s.
The valve cover center stud is subject to over tightening too. It should only be tight enough that it doesn’t fall off. Just beyond finger tight. Over tightening pulls the stud out of the head and that means some rethreading procedure must be used. It is far better to lose the nut and washer than stripe out the head. Then you will lose the nut, washer, and stud. Then it is time to use a Heli-coil or other thread repair procedure. Loose is better than too tight.
Axle nuts should be the necessary amount, which is seldom the amount in the book. Read my page on wheel bearings.
When I consider the amount of money that I collected, caused by incorrect tightness, I find it shocking. We fixed problems caused by over and under tightness. Expenses for over tightness were at least 10 times the amount of under tightness expenses. Safety, due to over tightening, was also jeopardized by a factor of maybe 5. I didn’t keep exact records, but it wasn’t even close. Over tight is bad, very bad. See my page on parts that fall off of a BMW.
If you must err, do it on the “under tight” side. Head torque should be 25 ft lbs. I would rather my BMW be 15 ft lbs, than 30 ft lbs.
Repairs caused by incompetence, ignorance, and inattention were many times greater than those caused by poor design, lack of inspection, cheapness and just plain bad luck.
Learn how to use a torque wrench. The threads must be “clean” and able to be run all of the ways down with my finger. I have seen a head bolt that was so jammed into the head that it took 15 lbs just to turn it against the sides of the hole when it should have fallen out, but I had to hammer it out. It would be impossible to torque that head bolt to anything meaningful. Learn to use slow tightening as opposed to fast.
I prefer to use my beam torque wrench, but have to check it from time to time to ensure accuracy. The beam type is much better for removing a fastener as it will show where it breaks loose, which is a sometimes useful number. The clicker type is attractive due to the cool factor, but in most cases, they are far from ideal. They lose accuracy easily too.
This torque wrench may not be the best one for use on a BMW:-)
Removing a stripped out bolt
This is a very old trick that I learned as a teenager from my grandfather. When you find a bolt, nut or screw that seems questionable, take a second to think about this well-proven method.
This is an example of a stripped Phillips head screw on an oil cover of a /6 engine. It looks hopeless, but it can be removed quite easily. I would use my hand impact tool. Make absolutely sure that you have the correct sized bit. Then dip the tip of the Phillips screwdriver tip into valve grinding paste. The paste is made up of ground-up silicate or known as common sand. The small particles will take up a bit of space, but mostly they will really grab both the Phillips tip and the seemingly ruined screw. Hold the impact tool tightly and give it small taps at first to see that the tip seats as well as it possibly can into the screw head. Hold the impact in the direction of unscrewing or counterclockwise for most fasteners. Now give it a hit.
This is the impact tool that I have used for more than 40 years. It is currently fitted with a Phillips tip. This trick with valve grinding paste will also work on slot screws and hex head bolts.
This should be the last word on torque
I was a fastener Procurement Agent with Boeing – yup, I ran the Nut Desk. I sat on a panel in a 7-year study – yup, 7 years – which studied not only the run-on torque but also the force needed to break the nut loose from the bolt. One of the large nuts on the 6 bolts holding the fin on a 767 backed off, which is nick-named a ‘spinner’ by aircraft maintenance folks. It was more common than desired to find spinners during one of the major maintenance checks on planes. Often enough to initiate a study. Most of the major manufacturers and specialty houses participated in the testing.
Yes, lube is important. The majority of fasteners used on aircraft in critical torque installations have a dry lube on them. There has been a lot of research on the type and amount to achieve consistent run-on torque to spec. The installation guns are pre-set for each size/type of fastener – not only for the torque – but the speed the gun turns, which generates heat. Some coatings are metals like silver, zinc, and cadmium (now replaced). There are many, many bolts/nuts which have a cycle limit spec’d – or not reusable.
Here are some things discovered to affect both Run-on Torque and Break-away Torque:
Temperature: of the parts, the gun, the bolts, the nuts, the room (at time of installation).
The speed of run-on to torque.
The number of prior cycles.
Amount and type of lube – and how applied (sprayed, dipped, chemical, electrical)
Length and size of bolt and nut.
Type of metal or alloy for both bolt and nut.
Pitch of threads for both nut and bolt.
Type of thread: J-thread, cold rolled thread, hot rolled thread, cut thread (speed of cutting and metallurgy)
It was found the exact same spec bolt or nut made by two different manufacturers had different results.
It was found that when all of the above were exactly the same for multiple tests – it had different results.
It was found that if a test jig that measured the torque was vertical or horizontal it got different results.
It was found a full moon, low tide, Monday, height, and weight of person testing, and leap years got different results.
There was such a huge amount of test data to be correlated Boeing dedicated one of their Super Cray-2 computers to analyze the variables. It ran for 6 weeks.
The results were – there were no definitive results. It was determined there were so many variables that affected the outcome they could only agree to specified ‘norms’ and create an industry standard accordingly. The rest was up to God.
A 747 is about 6 million parts flying in close formation, mostly held together by fasteners. You should learn to pray.