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

[Kirby Smith]

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
>  
> 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. 
> 
>