<VV> oil filter and alternator mount bolts...

[JVHRoberts at aol.com]

Keep in mind, that bolts are elastic. The number of threads picking up the  
load will be whatever it takes. In steel, if you torque to yield, it's more 
than  three threads. If only three threads picked up the load in a torque to 
yield  application, most bolts would start to strip! So, clearly there's more 
going on  here. The bolt will stretch and deform, either elastically or 
plastically to  equalize the load as needed. 
 
In a message dated 7/31/2005 10:37:56 P.M. Eastern Standard Time,  
kirbyasmith at gwi.net writes:

I hope  no one misunderstood me; I was not promoting cheap nor do I have 
grade 2  CRES bolts holding down my alternator.  In fact, I'm a fan of  
overkill.  I'm reasonably sure (without looking) that my alternator  is 
using the OEM bolts.  However, I was in doubt about grade 2 bolts  
breaking in that application.

The three threads point I made was  not to assert that more threads are 
not needed, but that with steel on  steel most of the force is taken up 
by the area of only three threads, no  matter how many are engaged, or so 
I have inferred in the past from the  MEs I interact with at work.  I 
understood that to be due to standard  tolerances in thread forming, but 
I will ask.  I don't believe they  intended to imply that the engaged 
surface area was continuous so that  only three threads had to be 
engaged, but I will ask about that  also.

kirby


JVHRoberts at aol.com wrote:
> Well, the  aluminum is also lower in modulus than steel, so load 
> spreading by  all the threads happens with little or no permanent 
> deformation of  the aluminum part. So, the bolt is the weakest link. 
> Grade 5 is what  GM uses for all automotive fasteners as a minimum. Going 
> less than  that is flirting with failure. GRade 2 is FAR less fatigue 
> resistant  than Grade 5, Grade 2 is FAR softer than Grade 5, and I just 
> won't  use anything less for ANY automotive fasteners. Add to that the 
>  thermal cycling, and that aluminum has nearly 3 times the CTE of steel,  
> and you could slowly stretch bolts that way. Given that the oil  filter 
> mount carries the load of the alternator (which, if you'll  notice, uses 
> Grade 8 bolts from the factory, no doubt due to the high  bending moment 
> on those!) and the idler pulley, it's just not worth  it to go cheap.
>  
> John