Stud grip length vs joint strength 3

[Jsgarage]

Guys, I have a very basic question: in a bolted joint, does using a threaded stud beyond a certain length gain me anything in strength? Specifically, I have an aluminum racing engine block with threaded stud holes 2.7" deep for 1/2-13 SAE studs holding the aluminum heads. ARP doesn't make such a stud off-the-shelf, and I wonder if all that threaded length is necessary. The block mfgr is out of business so can't ask him directly. Thanks-

 

[TrackRat]

Yves-

The original question was would he gain any thread strength by using a longer thread length than necessary. He asked this because no one currently offers the stud he needs with 2.7" of thread. He's asking can he use a shorter stud/thread that may be available. The answer is obviously yes as discussed above. Loctite can be used if he desires to fix the stud in the block at the correct height, though it's not necessary but acceptable in most applications.

My concern would be determining the correct tensioning of the new studs with many unknowns regarding the original stud/joint clamp force, old stud tensile and elasticity, the block material and tensile, thermal growth, existing thread fatigue, etc.

 

[ornerynorsk]

For all practical purposes, nearly 3 inches of thread length to install a stud is laughable at best. I have to agree with TrackRat, 2.5-3X is considered normal in cast Al. I have spent more than a fair piece of time in the machine shop over the past 28 years, and if you need all of that thread length to keep the thing from coming unglued, there are deeper design problems.

Helicoils are definitely worth considering if you think that you'll ever be servicing that block again in the near future, especially since you mentioned that the OEM is no longer in business.

Just my 2 cents.

 

[Tmoose]

http://members.fortunecity.com/alicat1/ccfwa-c.jpg

http://studiotimecapsule.com/yahoo_site_admin/assets/images/P1010015.251172249_std.JPG

This coventry climax engine tends to crack its aluminum block, with cracks originating at the end of the stud, in the long stroke version when raced with mods requiring high revs.

I don't have info on stud length, but the bosses seem to end a little beyond the main bearing boss, at which point the casting also get pretty thin, pretty quick. I envisioned it as a double or triple shot of stress concentration at a point without the benefit of bolt preload's innoculation against fatigue.

 

[jimwolf]

I would agree with all of the good advice above, but presented with an unknown aluminum block to head fastening system, there are a few more constrains then just the tensile strength of the threads.

First, determine if the head gaskets you intend to use are expecting torque to yield fasteners (TTY) as in the case of most metal laminate steel (MLS) gaskets. MLS gaskets typically use TTY fasteners in AL to AL engines, as the thermal expansion is better handled when tension is normalized throughout the operating thermal cycle. TTY tension will be different than on a non-TTY fastener.

If you’re using shouldered studs (pre-tensioned), they will tension differently than bolts or non-shouldered studs (pre-compressed), as the threads will be pre- tensioned between the shoulder and the threads in the block before the heads are torqued on. This more evenly spreads the tension across the threads, loading the bottom threads more effectively.

Extending the threads deeper in the hole then is necessary for adequate tension, may also help reduce block distortion, as the goal is to vector the head stud tension to the crank bed-plate when ever possible.