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Sunday, August 1, 2010

Something to be torqued about

"If I had four hours to chop down a tree,
I would spend three hours sharpening my axe."
- Abraham Lincoln
Too many of us jump into a job without thinking about the basics. I just took apart an engine that was apparently assembled by King Kong. The aluminum drain plug gasket was mashed down to a mushroom shape from being over torqued. The valve covers took a 6 point socket and an 18" breaker bar to loosen. Even the valve adjustment nuts were miserable. What was this person thinking?

One of the first things they discussed in Honda School was the correct tightness of fasteners. They had a variety of nuts and bolts on a test board and we were asked to tighten them down as we thought best. Then they took a dial torque wrench and showed us how far off we were. In every case, myself included, they had all been over-tightened.

Obviously this leads to stripped threads but, more subtly, it leads to parts failure because of excess stress. At the school, the next hour was spent tightening all sorts of things with a torque wrench to get the proper feel for each of the bolt sizes. Nobody gets out a torque wrench to tighten side cover bolts but a few minutes with a torque wrench can be very revealing. I do it about once a year as a reminder of what I already "know". Funny how what I "know" seems to drift with time.

For the record the standard values for each size is listed below. Just as a reminder, the listed size refers to the thread size, not the wrench size. A 6mm bolt generally takes a 10mm wrench to tighten it, 8mm bolt is 12mm or 14mm wrench size, and 10mm is 14mm or 17mm.

Standard torque values from the Honda manual:
(most 3/8" torque wrenches are unreliable under 20 ft/lbs. A 1/4" drive torque wrench is necessary for accurate work)
  • 5mm - 3.6 ft/lb - 43 in/lb
  • 6mm - 7 ft/lb - 84 in/lb
  • 8mm - 16 ft/lb - 192 in/lb
  • 10mm - 25 ft/lb - 300 in/lb
  • 12mm - 40 ft/lb - 480 in/lb
  • 6mm flange head - 9 ft/lb - 108 in/lb
  • 8mm flange head - 20 ft/lb - 240 in/lb
  • 10mm flange head - 29 ft/lb - 348 in/lb

    A handy torque conversion calculator can be found here.
If you are tempted to tighten something more to be sure it doesn't loosen consider that if you cause the bolt to stretch it will loosen even faster! Possibly even failing completely!

Loctite - 242 Blue Threadlocker Medium Strength, 6 ml.<br><font color=red>On Sale</font> (37418)
If you are worried about something vibrating loose use Loctite, don't overtighten.