studs vs. screws

How do they say ,a screw is an accountants decision and a stud is an engineers . Its got to do with the forces in the casing .If you take a stud and bottom it out on it's threads it creates a compresive force in the casing then you come along and put the outer casing on with a washer and nut creating a tensile load on the stud ,equaliesing the froces in the casing .If I remember correctly I think that's how it works ? A coarser thread should always be used in softer material (aluminium ),but I notice Ferari does not always do this ,that why most of the threads are Helicoiled . Don't know maybe they come from the factory like that ? Maybe someone else has a better explanation .

 

Typically, a bolt will transfer the bulk of its load into a case/ nut etc primarily via the 'first few threads' as the metal stretches.

If a stud is bottomed out, the load goes into the 'first few threads' that are now at the BOTTOM of the hole. When the cover is installed, the load going into the stud now pulls on the 'first few threads' at the TOP of the hole. The end result: the entire length of thread in the case is now more evenly loaded.

The earlier point about protecting threads from overuse and damage is also valid. The FCar al engine castings are surprisingly soft. Fcar loves Helicoils.

Cheers,
Vince

 

Most studs are into aluminum or magnesium - rarely steel. Repeated removal and installation of bolts into light alloys increase the risk of cross-threading, destroying a casting or housing. Once a stud is installed, the risk disappears. It's too bad there is no similar solution for spark plugs into aluminum heads. I've never considered the thread loading issue but it's an interesting analysis.
Tom Kizer

 

After reading my post again I must just clarify .When I said bottom out the thread it was not into the bottom of the threaded hole ,but against the end of the thread on the stud and the outer casing .The stud has a "shoulder " were the thread ends this is turned in until it reaches the casing ,stopping the stud going any further .

 

If the shoulder is intended to 'cram' into the top threads before the stud bottoms... it totally negates the theory I had proposed.

My TR water pump cover's stud backed out upon removal. I just measured the hole depth versus the stud length... you are correct, the stud hole is **significantly** deeper than the stud length. So much for my theory.

This leads me to ask about installation technique for these studs. The stud in question is a small 6mmx1.0 stud. I had intended to double nut the stud and install it with locktite red. I had planned on locktite blue on the nut that secures the cover to the stud.

Assuming the red/ blue locktite is correct... how much torque should one use when double nut installing the stud? If it is intended to 'cram' the shoulder of the stud into the top threads... it would seem that the torque value would be minimal. Perhaps only two or three ft-lbsf is warranted. (this is a small stud!)

Given that the stud 'crams' into the top threads with very little tensile load... the theory of even distribution of that load thru pre-loading is apparently false. Then it seems that the studs are primarily for base metal thread protection and ease of cover/ gasket mounting.

Some studs have base collars mounted as shown in the photo.... compare the distributor mounting stud to the adjacent valve cover stud. why the collar? Better sealing?

Interesting & helpful thread -thanks!

Rgds,
Vince
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OK straight out the textbook
"When a stud is torqued up ,the shank bottoms out on the chamfer .The stud's thread pull on the shank , inducing compressive stress in the surrounding base metal .An upward force applied to the stud pulls the compressive stress back to zero before inducing tensile stress ,resulting in lower net tensile stress ."
I would not use red locktite on any aluminium thread .If you ever needed to remove that stud the ally thread will come with it . If it were me I would not use any locktite ,but you could use a tiny drop of blue .
Studs with a collar are usually for locating the outer casing as per a dowl .

 

Good info... it makes sense, thanks! If you don't mind me asking, what is the book you reference? Sounds like good reading.

The 'inducing compressive stress into the surrounding base metal' I read as helping to maintain a flat and true sealing surface. It helps eliminate the 'bulge' of the base metal surrounding a bolt. Note that the stud shank will still have the same tensile loads - regardless.

Very good info.

Fcar does not appear to have used any form of locktite type agents. I am surprised to hear that red will pull threads. Since there is no 'downside' to a stud backing out upon disassembly.... a tiny bit of blue will do.

It makes sense that the collars are on the housing that secures a rotating shaft & seal assembly. They must need the collar to precisely locate the cam/ distributor seal.

Thanks for the expertice!

Rgds,
Vince

 

There's also the "ease of assembly" aspect. Ferrari didn't use many pins / bosses / flanges, etc. in their mating parts. So, the studs aid assembly by serving as guide pins during mating. Two good examples are the oil sump and gear box covers on the 308 engine. It's much easier to line these covers up during installation by getting all the projecting studs captured in the clearance holes, then sliding the cover into place. This also helps to ensure that any gaskets or sealant on the parting joints doesn't get smeared or gouged from the parts first coming together mis-aligned and being shifted under contact.

 

As for the "locking" of the studs...

Most studs used are "set-up" studs. These studs have a different thread profile than a standard screw, which is profiled for repeated removal and re-insertion. On a set-up stud, the profile of the threaded end which goes into the "fixed" part is somewhat self-locking due to interference between the internal and external thread profiles --- this is why you don't want to remove and re-install studs if you can avoid it --- they are designed to be left in place. The other (exposed) end of the stud has a standard (2A or 3A) class thread which allows repeated removal / installation of the nut.

Using a nut with a mechanical locking feature, such as a nylon insert, or a swage, is a good way to "lock" things up onto a set-up stud. You'll notice Ferrari uses this configuration on many of it's fastened joints. If you want more security, you may use loc-tite with regular (free-running) nuts.

 

Stud are also much easier on the threads in the block or whatever and cause less distortion to or the part when torqued. Just all around better.

 

Thanks again for the expertice. I did not know they used different thread specs. The pitch is the same(?) so presumably the spec is a 'tighter' stud to hole tolerance...

Since the nylocks have been on and off a couple of times before, the nylon is geting significantly weaker... that is why I intend to supplement it with a dab of blue. I'll keep it minimal.

Good thread!

Thanks for the time & help,
Vince

 

FYI ---

Most nylocs are only good (ability to lock) for about (2-3) R&R's --- BTW, only one-time if the nut has been exposed to heat (like on the engine) as nylon creeps and deforms like crazy at temperature. It's generally recommended not to reuse them with loc-tite, as a nyloc nut usually will have a shorter threaded length than a free-running nut. So, you don't get as good of thread engagement for sufficient locking. And, the loc-tite will not bond to the nylon --- so you get nothing from that area in terms of any locking help.

I always replace nylocs with new ones when I take something apart ---- most metirc sizes and pitches that Ferrari uses are readily available. Plus, they're inexpensive and "pretty, shiny, new".

If you can't find the nyloc you need at the local hardware store or auto parts store ---- go to McMaster-Carr's website.

http://www.mcmaster.com/

 

Yes, the thread size and pitch are the same of course. Sometimes the threads are cut / formed to different minor and major dimensions which mildly interfere with those in the tapped hole. And, sometimes the thread cross-section has a different shape (triangular profile) which creates the interference.

Do a google search on "thread classes" if you want to know more detail...

 

As a 40 year A&P mechanic, offer this. Never use Nylocks on hot engines/ parts, allways use steel or metal lock nuts.
Saves airplanes and lives.