Date:         Fri, 26 Aug 2011 21:50:16 -0400
Reply-To:     Mike <mbucchino@CHARTER.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Mike <mbucchino@CHARTER.NET>
Subject:      Re: Choosing a case sealer
In-Reply-To:  <20110826075145.01JJO.1502049.imail@eastrmwml44>
Content-Type: text/plain; format=flowed; charset="utf-8"; reply-type=original

Quoted from one of the articles;

"Silk Thread: Incorrect use can lead to disaster

In order to achieve the desired preload, the engineers take the preload that
is required and convert that into a torque value to use when torquing the
bolt. When figuring out this torque, they take all of the factors like the
bolt size, material, and friction into account. Varying any of these factors
will affect the effective preload that is being applied. For example, if the
procedure calls for lubricating a bolt before torquing and it is not,
although the torque wrench may indicate that the desired torque was
achieved, the actual preload will be lower because of the added friction
when torquing. In addition, damaged or dirty threads can reduce the amount
of preload applied.

In the same manner, the bolted joint preload for the thru-studs is a
critical factor. When torquing these thru-studs, if gasket material and silk
thread are added to the thru-studs, although the indicated torque may be
reached on the torque wrench, the effective preload will be lower. Instead
of a firm preload being applied to the part, the gasket material acts like
microscopic springs. The result is an effective loss of preload on the
assembly.

What happens next?
So what is the result of inadequate preload? Basically, failure can occur
because either the undertorqued bolt or nut backs out or because of a stress
fracture. A stress fracture can propagate on the bolt because it is
subjected to stresses that it otherwise wouldn't if the proper preload was
applied. It starts off as a small crack at a localized area of the bolt,
usually around the collar. With each successive operation, the crack
propagates slightly due to the cyclic stress it is subjected to. Over time,
the crack will be large enough that the bolt completely fractures.

Tubbs offers the following engine failure reports where the failures are all
similar:

    The through-studs failed and exhibit fatigue fractures just under a
cylinder hold down nut.
    The cylinder hold down short studs either fail in fatigue, or the nuts
back off.
    The engine quits when the cylinders move far enough out from the
crankcase to sever the intake system or from piston structural failure.
    Upon teardown, the crankcase halves show considerable fretting and
evidence of sealer and/or thread or other material between the bearing
bosses.

Tubbs says that many of these failures are attributed to improper torque
when the engine is assembled. Obviously, torque variations during assembly
can cause these problems. However, accusations of improper torquing are
erroneous more often than not.

As shown, even if proper torquing procedures are followed, the gasket
changes the dynamics of the bolted joint and leads to an inadequate preload
situation. Failure is almost inevitable. This practice, no matter how
harmless it may seem, can be detrimental to the life of the engine."

http://www.amtonline.com/publication/article.jsp?pubId=1&id=1281

I never use silk thread on a VW engine, as it's not engineered-in by the
manufacturer.

Mike B.