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Re: Grade 5 vs. 8

To: <LBC286@aol.com>, <shop-talk@autox.team.net>
Subject: Re: Grade 5 vs. 8
From: "Tim Mullen" <Tim.Mullen@trw.com>
Date: Tue, 25 Sep 2001 10:15:01 -0400
First the second part, I know nothing about "Bowmalloy bolts" sounds 
like some kind of brand name instead of something that meets a standard...

Now the First part:

The "preference" for Grade 8 bolts is the premise that "Bigger is Better", 
and "Stronger is Better".  Unfortunately, it isn't always true, as there are 
always trade offs.

In simplified terms:

Grade 8 bolts have higher tensile strength, but tend to be more brittle than 
Grade 5 bolts.  

Grade 5s tend to yield (stretch) before failure which relieves some of the 
load before breaking (things tend to get loose).

Grade 8s tend to break without yielding when over loaded (although it 
takes more to break them, when they do fail, things tend to fall off).

Under dynamic loads (think suspension), Grade 8 will have more of a 
tendency to break than Grade 5 due to fatigue caused by repeated cyclic 
loads.

Most OEM bolts used on cars are high quality Grade 5 bolts.  And 
they don't just use such bolts just because they are cheaper.

If you really want stronger bolts, use aircraft bolts (AN series) as they 
are stronger, and more ductile.  A desired failure mode on aircraft is to 
have bolts stretch, and not fail (think wing twisting instead of falling off).  
The only downside to aircraft bolts is the cost.

On a side note, I once (25+ years ago) had car that kept breaking the 
Grade 5 bolts on the air conditioner mount.  After the third time, I installed 
a Grade 8 bolt.  Instead of the head of the bolt breaking off, the mounting 
boss on the block snapped off.  I had to buy a new short block...

Tim Mullen

>>> <LBC286@aol.com> 9/25/2001 9:41:44 AM >>>
Grade 5 vs. 8

OK, here goes the bolt strength thread (no pun intended) again.
On another list there is a heated discussion about using all grade 8 bolts on 
restoring a car. Can anyone give me a simple explanation as to why it may or 
may not be a good idea to replace grade 5 with grade 8?
Part deux: What about Bowmalloy bolts, from Bowman? They are higher 
strength than grade 8, according to Bowman. Anybody know about these?

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