Torque Setting Numbers

[15523]

I have two Haynes manuals one for 2-valve BMW's the other motorcycle maintenance techbook. The 2-valve manual states when there are two number settings (15-25ft-lb) you should set the torque in between the two numbers (20ft-lb). Now the techbook states that you torque down to the first number (15ft-lb) and then torque it down to the second number (25ft-lb). My question is which one is correct? Any help would be greatly appreciated. The torque setting was an example not a real setting, I should have said that. The bike is a '78 R80/7. Just wanted to know which way to do it was right. Sorry , I was not clear with this.

 

[omega man]

Welcome to the forum!
If you can edit your post as to the bike(s) you are working on - year and model, I will move this to the appropriate forum section.
Good luck.
OM

 

[GTRider]

A range of 10 lb-ft of torque on a fitting carrying a max of 25 lb-ft would be huge and unusual, and would likely lead to a fitting coming loose. And, it depends upon the fitting being torqued. If you’re talking head bolts on an airhead, for example, then torquing first to 15 then to 25 would be the correct procedure. A well-written service manual should have staged torque values clearly spelled out. If you are questioning your written materials I’d suggest looking at the YouTube procedure explanations provided by reputable techs like Boxerworks or Boxflyer.

Best,
DeVern

 

[robsryder]

I believe that an ideal torque is about 1/4 to 1/2 turn before the threads strip. This testing method can involve lots of drilling and tapping of new threads (and maybe use of a thread repair device). This seems to have been my favored method when I was in Jr. High School. Eventually I discovered torque wrenches.

The specified torque should be such that the clamping force keeps the parts together during use and the fastener won't come loose; use of a thread locking compound might be useful in some instances. Aluminum parts will strip much more easily than steel, some folks have learned this lesson the hard way.

Presumably those specifying a torque value have taken such factors into consideration. Then there is also the matter of the accuracy of a torque wrench or other torque measuring device. If one chooses to apply some lubricant or anti-seize compound to the threads it may be necessary to reduce the applied torque. I have seen recommendations to reduce the torque for lubricated threads by about 10%.

An interesting observation that I made some while back was a systematic error in a BMW factory manual in converting from N-m to ft-lbs (the ft-lbs values were all incorrect!) And a particular pet peeve of mine is use of senseless units, i.e., kg-cm is NOT a torque unit!!

The late Snowbum had some thoughts about these matters -

Torque values for bolts and nuts, etc., for BMW Airhead motorcycles

torque wrench values and settings for BMW Airhead and slash 2, /2, motorcycles

bmwmotorcycletech.info

Various suppliers of fasteners have provided specifications for a variety of fasteners. Some links follow -

Metric Thread - Bolt Torque table

Recommended metric bolt torque M2 to M68, metric bolt torque table

www.metric-bolt-torque.com

Torque Specifications for Metric Bolts: Maximum Values & Tables

Recommended thightening torque metric bolts.

www.engineeringtoolbox.com

https://www.fastenal.com/content/feds/pdf/Torque-Tension%20Chart%20for%20Metric%20Fasteners.pdf

Metric Bolt Torque Chart – Class 8.8, Class 10.9, Class 12.9 - Mechanicology

These metric bolt torque charts show the ideal tightening torque for class 8.8, 10.9, and 12.9 bolts for both wet and dry tightening. The bolt’s class indicates its material strength, with higher numbers meaning higher tensile and yield strength. The classes are set by the International...

mechanicology.com

 

[15523]

Welcome to the forum!
If you can edit your post as to the bike(s) you are working on - year and model, I will move this to the appropriate forum section.
Good luck.
OM

‘78 R80/7

 

[20774]

Snowbum's guidance on head bolt torques...sounds like that's what you're talking about. It's in Section 11 on this page:

- three stages of initial torquing - 10, then 18, finally 25.
- it is a moving torque reading...read the value as you apply the torque. Technically not supposed to just try 25 ft-lbs on these bolts/nuts and conclude if they don't move, you're OK. You're supposed to loosen them by about 1/4 turn then retighten. This is for checking the final torque.

 

[kbasa]

Probably get more views if this was in Airheads, eh?

 

[omega man]

Probably get more views if this was in Airheads, eh?

That is where it is. When a new member posts in an area not specific to get an answer, we tend to not move a thread too fast that may cause confusion for the member- new to the forum. Once a new member has been back to their first post a few times, and hopefully greeted and notified that a move was upcoming, a thread move is made.
OM