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

[JVHRoberts at aol.com]

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
 
In a message dated 7/31/2005 5:20:11 P.M. Eastern Standard Time,  
kirbyasmith at gwi.net writes:

Interesting.  I think this implies that the aluminum also deforms,  as it 
was my understanding that only 3 threads are normally actually  engaged 
in threaded fasteners (at least for steel on steel).  So I'm  imagining 
that the aluminum deforms until a sufficient number of threads  are 
engaged to make the engagement area larger than the root area of the  
bolt.  Then the bolt becomes the weak link.

Independent of my  imaginings, however, isn't the real question whether 
the strength of a  bolt is sufficient for its purpose, i.e., holding the 
alternator down  against g loads and resisting turning moments and shear 
from the fan belt  tension?  Or, given that the bolts are supposed to be 
torqued, isn't  the question whether the bolts are sufficiently strong 
for the specified  torque?

kirby

JVHRoberts at aol.com wrote:
> You will, in  fact, break the bolt long before you pull the threads from 
> the  aluminum. This isn't determined by the material strength, but by the 
>  thread engagement. Rule of thumb for aluminum using steel bolts is to 
>  make the threaded portion at least twice as long as the major diameter  
> of the threads. On Corvair engines, the threaded holes have even more  
> thread engagement than this. And if the holes are in good shape and  the 
> bolts are the right length, the bolts WILL yield before the  threads do.
>  
>  
> In a message dated 7/31/2005  11:46:32 AM Eastern Standard Time, 
> kirbyasmith at gwi.net  writes:
> 
>     Aren't these bolts threaded into  aluminum?  (Sorry, but its been a
>      while
>     shice I built my motor.)  I'd be  surprised if the aluminum is higher
>     tensile  strength than any reputable 18-8 stainless bolt, even grade 2.
>  
>     kirby
> 
> 
>      JVHRoberts at aol.com wrote:
>      > 
>   > I would agree, except get Grade 5, and nothing less. SS  bolts
>     tend to be 
>      >  Grade 4 at best.
>      > Grade 5 have three lines on  the head. Typical hardware  store
>     stuff is  Grade
>      > 2, and may stretch at the required  torque.