TheSamba.com :: View topic - Does replacing hardware with stainless change torque specs?

Sat Jun 11, 2016 11:32 am

Just as the title says. Seems to me like it would...

Sat Jun 11, 2016 11:45 am

why would it?

torque is designed to put a certain axial stress (or is it strain??) on a fastener

true, some of the difference is thread & top washer friction but that should not change much, nor would ... um axial plasticity...

Sat Jun 11, 2016 12:07 pm

I thought the axial stress came from the deflection of the threads. Change the type of metal the threads are made out of, and the same torque potentially deflects the threads less. Taken to the extreme, let's say a bolt and nut has absolutely no elasticity. You could put 100 nm of torque on it, but have no tension held in the system. Theoretically, it would still be loose. Now let's say we're putting this inflexible bolt in an aluminum nut. Now, the nut would carry all of the thread deflection, potentiality enough to strip it out.

Sat Jun 11, 2016 12:22 pm

If threadlockers are being used than my opinion is Yes.
BTW, there are stainless steel specific threadlockers.
I use Wurth products manly.
Very rarely do I see the stainless specific lockers at basic FLAPs.

Searching the Permatex, Henkles and Loctite sites will get you part numbers of these products.

Sat Jun 11, 2016 12:25 pm

jkmcrg wrote: I thought the axial stress came from the deflection of the threads. Change the type of metal the threads are made out of, and the same torque potentially deflects the threads less. Taken to the extreme, let's say a bolt and nut has absolutely no elasticity. You could put 100 nm of torque on it, but have no tension held in the system. Theoretically, it would still be loose. Now let's say we're putting this inflexible bolt in an aluminum nut. Now, the nut would carry all of the thread deflection, potentiality enough to strip it out.

the threads dont need to deflect to allow torque to be made.

the consideration with a differanf material is the coeffecient of friction differance if any of the new material bolt, which is a function of thread shape, surface finish, and material etc.....

if the stainless bolt is more slippery than the replaced bolt, then for the same torque placed on that bolt will result in a tighter attachment. (ie higher stress on the bolt) if you install an oiled bolt verses a dry one for instance for a given torque, the tightenss or stress on the bolt will be different.

for most of the application on the van, the differance is probably not a big deal, however on critical items like rod bolts, you want to get it right. (of course bolt stretch, creep and other factors come into play, that is why you probably want to stick with the correct bolt there) for the bolt that holds on the bumper, no big deal, it will be close enough to work with no problems.

Sat Jun 11, 2016 2:12 pm

Both the tension and friction characteristics of stainless are different than mild steel, so the torque Will change, too.

Sat Jun 11, 2016 3:22 pm

sure, but likely not significantly - esp. since torque wrenches are not the most precise instruments out there

he would also need to know _which_ SS alloy he has

Sat Jun 11, 2016 4:24 pm

Easy there cowboy!

Stainless is inherently WEAKER than ferrous steel. You cannot simply replace critical hardened steel fasteners with stainless products!

Catastrophe may be written all over it.

Torque sucks honestly. It is a poor way to measure fastener stretch. Sadly, while other means of measuring stretch are available, they aren't affordable for mass use. Because of this, we hang onto torque values.
Torque angle is far better but still falls short.

What fasteners are you switching out?

Stainless fasteners size for size to ferrous grade can take far less torque.
A 1/4-20 ferrous ASTM A574 can take 10-14 FOOT POUNDS

Stainless 316 1/4-20 can take max 78.8 INCH POUNDS
78.8 inch pounds is 6 1/2 foot pounds.

Dave

Sat Jun 11, 2016 4:39 pm

A 302 or 304 (A2 for you Europeans) st-stl bolt has a tensile strength of approximately 90,000 psi while a property class 10.9 bolt is about 150,000 psi (1040MPa).

So ignoring friction briefly, if the torque spec was to load the a PC 10.9 bolt to approx 80% of proof load, which would be typical for a structural joint, then would would start to fail, or fail, the st-stl bolt.

Now, back to friction. It has THE most influence on the torque setting for the joint. A galvanized bolt could need two times the torque to match the same tensile stress for an modern finish with a lubricated top-coat. So a finish change can alter the torque by 100%.

Now, having said all that, how do you make a suitable educated adjustment? Basically, you really can't.

One key item to remember - standard 300 series stainless steel is going be weaker than a PC 8.8, 9.8 or 10.9 bolt, so if the bolt is doing structural work, St-Stl should not be substituted. Some locations it is certainly fine, most of those location will not have torque specs.

Sat Jun 11, 2016 5:45 pm

While it is not specifically on topic, I have to ask WHY? Why are you considering SS fasteners in locations that require a specific torque and are spec'd for a fastener of a different material? What locations are you considering?

Sat Jun 11, 2016 6:43 pm

My ignorance shines again! I wanted to go stainless because it seemed like it would last longer... Not sure where I got this notion. Basically I'm doing a full engine rebuild right now and have everything in baggies, I thought before I put it all back together I would index all the hardware, take a list to a hardware store and replace em all, just to reset the clock on em.

Sat Jun 11, 2016 7:26 pm

no - don't do it

you ....could... replace certain fasteners with SS, and replace certain fasteners with Ti, but only with a team of engineers doing some testing beforehand

not something for a Vanagon - I've seen this done on two cars, both specialty Porsches being developed by race teams and adv. vehicle development teams; also Lotus maybe; McLaren yes, etc.

Sat Jun 11, 2016 7:27 pm

jkmcrg wrote: My ignorance shines again! I wanted to go stainless because it seemed like it would last longer... Not sure where I got this notion. Basically I'm doing a full engine rebuild right now and have everything in baggies, I thought before I put it all back together I would index all the hardware, take a list to a hardware store and replace em all, just to reset the clock on em.

Don't.

Replace with new like grade zinc coated steel, but do step away from the stainless idea.

Dave

Sat Jun 11, 2016 7:29 pm

jkmcrg wrote: My ignorance shines again! I wanted to go stainless because it seemed like it would last longer... Not sure where I got this notion. Basically I'm doing a full engine rebuild right now and have everything in baggies, I thought before I put it all back together I would index all the hardware, take a list to a hardware store and replace em all, just to reset the clock on em.

unless there are rusted, stripe or bent, reuse the old ones in most locations. you dont need to reset the clock on them, waste of moeny and bolts. one use stretch bolts are a one use item, other than that they can be reused unless specifically specified otherwise.

Original hardware is a plus in a restoration so long as it is in good shape.

your original bolts are probably european made, and meet VW's quality control specification, the ones in the hardware store could be made anyplace, and I am sure the typical haedware store does not have a quality control program for them.

some wiz bang geneous at CalTrans (california dept of transportation) used the wrong kind ofmbolt on the san fransisco bay bridge rebuild, they are snapping and that is a problem some or 30 geet long and inbedded in concrete! some failed during torque, no quality control. the chinese maker lost the heat treatment rr3cord, and hence did not know which batch had been heat treated or not, so they heat treated all of them, the second heat treatment ruined them that got it twice, no fucking records and caltrans signed off on it anyway

Sat Jun 11, 2016 7:41 pm

yes, clean each one carefully and inspect the threads and the wrenching fixture on the top - if you have old eyes, use a loupe or magnifier

if a fastener is one time use (most are not) then replace with a high quality new one

- don't beat yourself up on the question - you were smart enough to ASK the question

Sat Jun 11, 2016 8:00 pm

Haha oh boy okay seems like a unanimous vote. I guess I'll just count the last 4 hours of caliper and tap & die measuring as my daily meditation :lol: . I'll clean up the current hardware and replace with zinc coated steel if it's non essential seeming (alternator bracket, throttle body screws) or in bad structural shape. Sound good?

Sat Jun 11, 2016 8:33 pm

jkmcrg wrote: Haha oh boy okay seems like a unanimous vote. I guess I'll just count the last 4 hours of caliper and tap & die measuring as my daily meditation :lol: . I'll clean up the current hardware and replace with zinc coated steel if it's non essential seeming (alternator bracket, throttle body screws) or in bad structural shape. Sound good?

yeah sound like a sound plan.

I did use stainless on the exhaust parts like the bolts that hold together the cat, muffler and such. the nuts used where copper plated locking types and spring lock washer dont do too well with the high heat (they lose teir spring, and go flat, no longer lock. the stainless bolts I used have held up very well and have not rusted on the exhaust as the plain steel has

Sat Jun 11, 2016 8:40 pm

djkeev wrote: Easy there cowboy!

Stainless is inherently WEAKER than ferrous steel. You cannot simply replace critical hardened steel fasteners with stainless products!

Catastrophe may be written all over it.

Torque sucks honestly. It is a poor way to measure fastener stretch. Sadly, while other means of measuring stretch are available, they aren't affordable for mass use. Because of this, we hang onto torque values.
Torque angle is far better but still falls short.

What fasteners are you switching out?

Stainless fasteners size for size to ferrous grade can take far less torque.
A 1/4-20 ferrous ASTM A574 can take 10-14 FOOT POUNDS

Stainless 316 1/4-20 can take max 78.8 INCH POUNDS
78.8 inch pounds is 6 1/2 foot pounds.

Dave

Note: all steel by definition is ferrous. no such thing as non ferrous steel. Ferrous means iron containing, all steels are iron based, thus all steels are ferrous

now you can have ferritic stainless or austenitic stainless even martanistic stainless, but all are a ferrous metal as they contain iron so even stainless steel is ferrous.

Sun Jun 12, 2016 2:02 am

Ok..... Chrome containing steel aka stainless (which is a misnomer anyway).

Know how to read a bolt head before replacing hardware!

The markings on a bolt are very very important.

Do NOT simply grab bolts locally from places like Home Depot or Tractor Supply. They stock largely unmonitored cheap garbage! You want a quality paper trail assuring the bolts you buy meet standards.

Buy known quality bolts from a known quality supplier. Bolt quality varies widely and there is a lot of inferior trash on the stores shelves!

Fastenal, Grainger, Bell Metric, etc are good sources.
Quality hardware isn't cheap $ either.

Dave

Sun Jun 12, 2016 5:52 am

For a lot of my metric hardware needs I use these guys.

http://www.metricmcc.com/catalog/catalog.aspx

I have no experience with their mail order operation,since they are between my home and job, so I just stop in.