Are there any fasteners or parts you'd recommend against using stainless steel? Any fasteners with "mission critical strength" requirements?

I have been told SS bolts are not really strong so anything that holds the wheels like a spindle I would shy away from although I think most parts on the Nortons are overdesigned size wise because they were made when high strenght steels were either not available or too costly. The front axle I think is way too big for it's needs since it was made in the 40s and probably had a legacy design from the 20ies. The only thing I have found bad about SS bolts and nuts is that some lubricant must be used on the threads if both parts are SS because they can "gall" (also called cold weld) and lock up solid. So far I have had three fasteners do that without lube and it's not easy to undo.

Jean

I will not use stainless for the rear axle. I had a rear axle snap while tightening it long ago and if SS is weaker then that's a bad thing.

The other visible fasteners on a Norton aren't torqued anywhere near their limits, so no problem. But as Jean pointed out you need to be aware of galling, which I found does happen with stainless head bolts (the 4 that are visible). A bit of anti-sieze compound seems to prevent that.

I've broken SS bolts before from galling on other bikes. Little anti-seize and it's good to go.

There are a lot of different alloying "recipes" for stainless steel (properly called CRES for "orrosion resistant steel). The cheaper versions in many consumer products are indeed considerable less strong and less "springy" than many conventional steel alloys.

In the airplane business, there are many high-strength requirements where CRES fasteners have to be used, such as joining aluminum to composites or to titanium. There are some very specialised CRES versions for that.

CRES is also often used for high pressure containers. When I was at NASA, we had some air tanks, about 80 ft high and 15 ft diameter, that held compressed air at 8000 psi. They were welded structures entirely in a CRES alloy called 4340.

The food industry uses CRES in all its production machinery, usually because regulations require it. There are lots of high-strength alloys for those applications too. A freind who worked in the facilities design group at the Coors brewery was a moto-cross rider. He built an all-CRES 2-bike trailer for his bikes, using scrap pieces from equipment construction.

The only generalisation I'd make is that the cheap "stainless steel" that you can get at Home Depot is probably not as strong as other steel alloys.

As a matter of interest, a "proper" fastener installation shoudn't have any of the thread inside the hole. The grip length (from under the head to the beginning of the thread) should be the same length as the assembled pieces you're bolting together and a washer under the nut provides the final tightening clearance.

ludwig wrote:It's not just a matter of strength , but also of elacticity .
In a way a bolt is like a spring , and it is the stretching of the bolt that gives the required tension to keep things together .
I would not use SS bolts on anything that matters , defenitely not on cylinder heads and barrels .

Which frame fasteners would you also avoid? I'm not too concerned with engine much except for the mounting bolts.

I would not use SS bolts on anything that matters

There's a fifty-seven story building in Minneapolis that has all it's curtain wall (the entire 57 story outside wall) fastened with 5/8" stainless steel bolts. I would venture to say the right stainless steel alloy bolt is far superior to a cheap mild steel bolt. I have replaced just about every fastener on my Commando with stainless, including axles and head bolts, with no problems for almost ten years. It is true that stainless to stainless does indeed gall. You definitely need to use an anti-seize lubricant when fastening with stainless. Torque reading may differ for stainless, but I really don't know that for sure. The torque reading we use is actually a resistance to rotation. The actual function, in most applications, of a fastener is to apply tension. Most times the torque value is a relative indication of the clamping force (tension). Change the resistance to rotation, with dirt, rust or bad threads and the tension will be much less than required, but have the correct torque value. That's why some manufactures specify turn-of-the-nut method rather than torque specification.

swooshdave wrote:

ludwig wrote:It's not just a matter of strength , but also of elacticity .
In a way a bolt is like a spring , and it is the stretching of the bolt that gives the required tension to keep things together .
I would not use SS bolts on anything that matters , defenitely not on cylinder heads and barrels .

Which frame fasteners would you also avoid? I'm not too concerned with engine much except for the mounting bolts.

The NOC Roadholder mag had an interview with a highly regarded engineer/mechanic. He advised against using the then stylish polished stainless headbolts because, as Ludig points out, tension depends on elasticity. And in the case of the repeated heat cycled growth-shrink of aluminum , you're just asking for it.

Engine mounting bolts are a perfect example of using the right "grip length" bolt. Anything in double shear can't ride on threads. In this case that would be the cases and the cradle. This is especially important in this instance where you can't just tighten the hell out of the thing and hope for the best. Aircraft Spruce and Caroll Smith's book on fasteners are great on this subject. AS as a source for hardware and Smith's book on how to use it. There are plenty of more than adequate SS fasteners in the AS catalog, by the way.

One of the problems with stainless stell for use as a bolt is not so much strebgth, as hardness.
The problem with many grades of stainless is they have a high percentage of chrome in there.
What happens on a good steel thread is when you tighten it the threads deform slightly to get full thread contact between the male and female threads, therefore giving good shear strength accross the thread (more surface area in contact resisting the shear).
The problem with many cheap grade stainless hardware is the stainless is too hard, and therefore the threads do not deform to match each other, so the aread resiting the shear is dramatically reduced. This is the reason why folk say 'it just seemed to carry on turning, never felt like it got fully tight'. It was shaering across the thread gradually, instead of deforming.

I would advize against stainless on head bolts, head steady bolts, and stress important bolts, like axles.
Use on engine covers (where the ali can deform) and minor fittings where lock washers can be used, maybe with a dab of locktite.

Stu

I guess the biggest problem is you rarely know what grade of stainless you are getting. I have seen stainless headbolts yield and neck (obviously not an alloy that was too hard) and I have seen two stainless rear axles snap, both at the beginning of the threads. Obviously the head bolts and axles were made from a different alloy.

I have also built engines with stainless headbolts (at the owners request) that torqued easily and are still in use after several years. At least one of those has had the head removed and the bolts re-used.

The headbolts that stretched came from a supplier in England. The headbolts supplied by Stan Smith (Rocky Point) have never failed.

Coincidence? Maybe.

I suppose another way to look at it is how do you know what quality the non-SS stuff you get is? Or is it less finicky than SS to produce?

Correspondence in 'MotorCycle Sport' between Middleton and Molnar put me off ever using stainless wheel spindles as they disagreed about what was suitable. My head bolts appear to be stainless-ish and were on the bike whzen I bought it. They have never given any trouble.

I must say that I never liked the feel of tightening stainless engine mounting bolts and have now replaced them.

I stuffed my 850 into a car at 60mph and the stainless isolastic stud and bolt showed no signs of damage. The engine bolts were distorted. The rear iso stud has now become footrest studs for the 16H (talk about risky !)

It would depend upon the grade of the stainless. The SS fasteners like axles, head bolts, tranny mount bolts ect are probably a grade 303, good for threaded rod, nuts, bolts ect.

http://www.aircraftspruce.com/catalog/h ... nbolts.php

Quality nuts and bolts aren't expensive. They just don't sell'em at Home Depot. There are only a few bolts on a Commando where they would come in handy. Cradle bolts and trans mounting bolts come to mind. Is it true that Kenny Dreer sleeved trans bosses to get a slip fit?