Torque Wrench Discussion, Types, Conversions, BMW mistakes, etc.   BMW Airhead Motorcycles especially.

I have had numerous private postings to me from embarrassed owners who have made some expensive mistakes due to over-tightening things. While most of these problems occurred due to the owners not having a proper feeling for parts…AND…not using torque wrenches, …..some have occurred due to BMW published information.

MOST nuts, bolts, screws, etc., on our BMW’s have a published tightening specification. Some do not, such as the oil sender and neutral sender switches….at least in most literature.  Do NOT use BMW’s numbers for footpounds.   Convert the BMW Nm numbers to footpounds yourself  ….pay attention to this caution, the reasons are below!

 

BMW made some serious mistakes many years ago in converting metric to English values in various of their publications. They admitted this. In BMW Motorcycle Dealer Bulletin (Vol II, No 23,  dated 3/1982) BMW SAID NOT TO USE ANY PUBLISHED BMW CONVERSIONS FOUND IN BRACKETS IMMEDIATELY BEHIND THE MILLIMETER FIGURES, IN ALL SERVICE LITERATURE, INCLUDING RIDERS MANUALS, SHOP MANUALS, etc.   This means that BMW does NOT you to use their published foot-pounds of torque, or any other torque value other than the metric.   You can almost always TRUST BMW’s figures in Nm….although I many times recommend less torque, for such as spark plugs; wheel preload, ATU nut, alternator rotor bolt, and a few other places, with some special cautions on the flywheel (clutch carrier) bolts. I specify my recommendations.

In general, the BMW specified metric specifications are correct, “so long as one knows about later updates and cautions”; and, you pay attention to this author’s details on torque figures in his various tech articles, including the ones specifically for torque listings.

NOTE:   Many times various non-BMW publications will use BMW’s own …and wrong or out of date  …ftlbs figures in their literature. BEWARE!!!

Those of you that have torque wrenches (I hope all of you) MIGHT have them scaled in ONLY foot pounds, although many are now sold with other scales also included, particularly in Newton-Meters (Nm) .

IF your torque wrench does not show the settings you need:

Nm x 0.7378 = Foot-pounds
Nm x 8.85 = Inch-pounds
Inch-ounces = 141.6 x Nm
Foot-pounds x 1.356 = Nm
1 Ncm = 0.0885 inch-pounds
Foot-pounds x 192 = inch-ounces

Foot-pounds, ftlbs, & pound-feet,  mean the same thing.

The best and most accurate torque wrenches are the industrial-quality beam types with DIAL readouts.  They are $$$.   The relatively inexpensive plain beam-bar types withOUT the dials are very usable for NON-critical items. The types most of us use are that last type ….AND/OR the round tubular type often called ‘clickers’.  I recommend AGAINST the old-fashioned low cost plain beam-bar type, except for use at the 27 mm locking nuts at the rear swing arm, the exhaust nuts (I peronally don’t use any torque wrench there), transmission output flange center nut, input nut to rear drive, and a few other high torque but relatively non-critical places.

Torque wrenches are a NECESSARY part of any owners tools if he maintains his own machine. Tubular clickers are not very expensive, unless you insist on Snap-On brand. I suggest you obtain yours in a 3/8 drive size, or 1/2 inch drive, depending on what your metric sockets and Allen socket wrenches already are.  I have found the 3/8″ drive types more versatile for Airheads. I suggest your first purchase of a torque wrench be one that has a range of zero (or, more typically, near zero) to 75 ftlbs, which will cover most of the BMW torque settings for our Airheads. If you get one with a higher range, say 100 ftlbs or more, it will not be as useful, due to inaccuracy and difficulty in hearing or feeling the click at settings lower than 12% or so of maximum setting.  Return the setting on any clicker torque wrench to somewhat ABOVE minimum/zero after each use, so the calibration remains stable.   This is considered unnecessary by some folks…I do it, however.   Your second torque wrench purchase should be an inch-pound type, which is needed only for a few places, such as pan bolts and the ATU nut.  You may be able to pick up, cheaply, a high range pointer type of torque wrench, that, perhaps, goes as high as 150 or 200 ftlbs.  Those come in handy once in awhile.

You can roughly check the torque wrench calibration yourself with a string and weight hanging from the center of the handle, the drive clamped in your vise, handle horizontal.

Don’t use torque wrenches for loosening; UNLESS it is the inexpensive beam type with a zero-center scale. 

Tubular clicker torque wrenches are rather variable in quality and accuracy.  Almost all of them are less and less accurate as settings get closer to their minimum settings.   Thus the 75  ftlb maximum setting torque wrench of the tubular clicker type is NOT very accurate near the very lower part of its range, and the click may be difficult to feel there, so avoid using it below ~ 6 ftlbs (I don’t use a 75 ftlb wrench below 9 or 10 ftlbs). FIND OUT what your ‘click’ feels like at the setting you are going to use if you have no experience with that setting. DO NOT use an inaccurate torque wrench for places like pan bolts or the ATU nut.

Be very cautious about using a 75 or 100 footpound wrench for pan bolts. I HIGHLY recommend you do NOT use such a torque wrench there.   You can use an INCH-ounce or INCH-pounds torque wrench.  Use about about 72 inch pounds (= 6 footpounds), tightened in a cross-pattern, and very evenly; begin with low value, and then progressively increase to about 72 inch-pounds…again, do this in stages, to avoid bunching-up the gasket..  I use a very short wrench, not a torque wrench, and I do it by feel.  If your ‘feel’ is lousy, use an inch-pound wrench.  See my article:  https://bmwmotorcycletech.info/pangasket.htm

Be ESPECIALLY cautious about the M5 nut at the nose of the pre-1979 camshaft, where the ignition automatic advance resides….I do them by feel.  4 ftlbs maximum at that ATU nose nut, which, on an inch-pound wrench is 48. Not sure about your ‘feel’? >>>use a torque wrench.  If you snap off the tip of the cam due to overtorquing, your day …and more ….is going to be RUINED!

A torque wrench is also a necessity for tightening cylinder head stud nuts.   I recommend an accurate 25 footpounds for ALL years of ALL Airheads.  DO NOT torque higher than 26 ftlbs!!

BMW’s are fairly tolerant to MILD over-torqueing, but there are some places you want to torque fittings quite accurately and carefully:
Cylinder stud nuts.
Center acorn nut on the valve cover.
Banjo bolts at oil canister cover on oil-cooler models.
Nut holding the automatic advance on the 1978 and prior models.

Be reasonably careful about the torque you use here:
Flywheel, clutch and possibly u-joint bolts …and probably some I have forgotten to mention.

Consider purchase of an INCH-pound type for lower readings.

The author’s website has three articles on torque settings, torque wrenches, calibration, etc.
https://bmwmotorcycletech.info/index.html

Snowbum

Rev:
Updated for clarity/emphasis on 05/18/2020
01/17/2021:  Minor clarifications & emphasis