16 October 2015

Public Service Announcement: Differential and Transmission Plug Torques

TIS specifies the following torque values for the Z3 M coupe differential plugs:

There are some plugs that use a crush washer (spec = 65 Nm) and some that have an integrated O-ring seal (spec = 60 Nm).  These are very high torque values.  Owners have cracked their differential cover installing plugs.  The trick is the oil on the threads...  The oil makes torque wrench readings unreliable.  I never go this high.  Maybe when the differentials are filled and assembled with dry threads this value would be OK.  I use the later plugs with integrated O-rings (33117525064, $6.17r each, one use only) and I torque to 45 Nm.  The pressure built up inside the differential is low and 45 Nm is plenty to compress the o-ring.

The transmission plugs on our cars have tapered threads.  There are no crush washers or integrated o-ring seals.  As such, I always use a little pipe sealant on them.  Most owners see the plugs weeping.  This won't happen if you use teflon tape or sealant.  Make sure the threads are clean and the sealant or tape is on the very end so it isn't up in the fluid bath.  The spec on these plugs is 50 Nm, again very high.  I use less.  The transmission housing is aluminum and it will crack too.

This is the transmission drain plug on my car.  Typical weeping shown here.  Fixed with pipe sealant or teflon tape during the next oil change.

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