At 04:08 PM 6/17/2010, John Reynolds wrote:
>John -
>Who has has been involved in testing and design of "few" seat belt
>bolts for a few decades now.

Dear John,

Since when do UNF bolts come with grade 8.8 stamped on the bolt end?

>I wrote:
>>Bolt is M11x1.25  grade 8.8 (which grade most bolts in the Vanagon are).
>>
>>Starting torque was 50 N-m, about 37 lb-ft.  Removal effort was low
>>but very quickly increased to 15 and then 20 N-m as the slightly
>>corroded end of the bolt entered the hole.  The bolt was warm when
>>it came out, reflecting the work done to remove it.
>>
>>Measured OD was .430" against a nominal 11 mm which is .433 (7/16 is .4375).
>>
>>More to come.
>
>The M11x1.25 bolt thread was chased with 7/16-20 (moderately
>interesting pics to follow).  A 7/16-20 hole was made in mild steel
>(75% engagement depth) and tapped with a Craftsman carbon steel
>tap.  The bolt was tried in the hole and entered freely.  It could
>be spun in and out with great ease but did not feel sloppy.
>
>The bolt was then inserted to the same depth as the depth of the
>hole in the Vanagon, and torqued to failure.  At 70 N-m (~52 lb-ft)
>the fastener pulled out of the steel.  There was some thread
>deformation and rounding over on the bolt, but the threads in the
>steel were pulled out complete and could be unscrewed from the bolt.
>
>Conclusion: For this application it is acceptable and safe to use
>the existing M11x1.25 grade 8.8 bolt in a hole that has been chased
>with 7/16-20 tap, even though the thread forms are slightly
>different and the tap is just a touch large.  The VW-specified
>nominal torque is 30 lb-ft.  However an M-series grade 8.8 bolt
>corresponds to a UN (Unified National) grade 5 bolt, with three
>lines on the head.  Using a grade UN grade 8 bolt will require more
>tightening torque to avoid potential fatigue failure (remote-to-zero
>possibility in this application) and the hole in the van might
>possibly not be strong enough for the increased torque.
>
>
>d

Yours,
David