stainless steel - acceptable uses on Laverda's?

well, within the spirit of my original premise of doing a mostly stock rebuild this time for this 82 1200, so far I have cleaned, re plated, etc all the original fasteners, except the yellow nylocs which have gotten a good cleaning in GUNK HydroSeal 2, then several trips through the ultra sonic, then a quick brushing with a brass wire wheel. No stainless steel at all, yet.

for the mounting of the calipers I am going to use the new Grade 8 steel metric bolts I have already sourced. I have even salvaged  all the engine case mounting bolts, the Allan heads with the knurling, they cleaned up really well.

I suspect I will likely replace the valve cover bolts with stainless, as the ones that came on the bike were un plated black fasteners, and so rusty as not be salvageable even after soaking for a few days in Evapo Rust which cleaned them up but exposed all the pitting.

I am going to carefully measure up the stainless steel axles I have and compare them to stock before making any final decisions.

Paul LeClair
 
I have been using stainless steel fasteners for caliper mounting on both my 79 Laverda 1200 (since 1999) and on my 84 RGS Executive (since 2004) without any issues
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Hi, Paul.
The stainless steel bolts (A2) usually have a tensile strength of 700 N/mm2 min
(symbolised by a 70 on the head), compared to 800 of the 8.8 steel bolts.
In Germany it is forbidden to use them on brakes. Depending on the engineer
who inspects your bike they might deny you the mot.

Ernesto
 
I have always used S/S fittings on my Jota, including disk and calipers, and have never had a problem.
The only bolts I was advised against using S/S on was the engine mounting bolts which are still H/T steel.
Just remember to use thread lock where required or copper slip.
Have a nice day.
 
Ernesto said:
Hi, Paul.
The stainless steel bolts (A2) usually have a tensile strength of 700 N/mm2 min
(symbolised by a 70 on the head), compared to 800 of the 8.8 steel bolts.
In Germany it is forbidden to use them on brakes. Depending on the engineer
who inspects your bike they might deny you the mot.

Ernesto
Click to expand...

This is my view, take it or leave it!

For me the ultimate tensile strength of a fastener tells only part of the story.

Sure, if you keep the load of the spindle/bolt/stud below the yield point and don't heat and cool it too much then you have years of faithful service with stainless as some have testified.
However if you do exceed the yield point through mechanical or thermal stress then you are into work-hardening territory. With mild to medium carbon steels this is not necessarily a problem (depends how hot though) as there is a reserve of toughness, but some stainless materials work harden and become brittle if this process is repeated - cyclic loading!
A very good example of this is the stainless exhaust systems that you all moan about, after months or years the material has been through so many heating and cooling cycles, vibration, expansion and contraction etc. that it has had enough, becomes brittle and cracks or shatters. Funnily enough, currently, chromed mild steel is all the rage for this application!

Kept within safe parameters I am fairly sure stainless is just fine.
 
And why it is important to apply anti-seize grease.


Galling analysis by fastener expert Joe Greenslade http://www.estainlesssteel.com/gallingofstainless.html
Interesting reading - if you are into such things of course.


'Thread galling seems to be the most prevalent with fasteners made of stainless steel, aluminum, titanium, and other alloys which self-generate an oxide surface film for corrosion protection. During fastener tightening, as pressure builds between the contacting and sliding thread surfaces, protective oxides are broken, possibly wiped off, and interface metal high points shear or lock together. This cumulative clogging-shearing-locking action causes increasing adhesion. In the extreme, galling leads to seizing - the actual freezing together of the threads. If tightening is continued, the fastener can be twisted off or its threads ripped out'.

 
when I rebuilt my Brembos 10 years ago the rebuild kit included allen screws to fasten the calipers to the forks. Is this no longer the case? Why not use an OEM spec screw that was designed to handle the tension instead of a screw that delivers a lot of bling, but may fail and cause a lot of harm :-X

Gerald
 
You're right, I stand corrected. But I still think that a screw of correct tensile strength should be used....
 
Paul LeClair said:
I suspect I will likely replace the valve cover bolts with stainless, as the ones that came on the bike were un plated black fasteners, and so rusty as not be salvageable even after soaking for a few days in Evapo Rust which cleaned them up but exposed all the pitting.
Click to expand...

Paul,

These are perhaps the only fasteners that I would NOT replace with stainless items, next to the caliper joining bolts.

If you closely inspect your rusty items, you will find they are of 12.9 grade, much higher strength than the rest of the engine fasteners.  The factory used them for a reason.  I have come across several that had simply popped off their heads, these were in every case not the stock screws.  Apparently, the expansion rates at the cam cover play havoc with the screws.  If bling is required, they are also available zinc plated.  During a re-build, I renew these if incorrect screws were fitted prior.

Why not re-plate your yellow nylocs?  They come out like new, just require a drop of Loctite if you are anal about the insert not being up to the job and potential loosening resulting...

Engine mounting bolts should be fine-pitch, stainless or otherwise, to avoid loosening through vibration.  Higher tightening torques are possible.

piet
 
sfcpiet said:
Paul,

These are perhaps the only fasteners that I would NOT replace with stainless items, next to the caliper joining bolts.

If you closely inspect your rusty items, you will find they are of 12.9 grade, much higher strength than the rest of the engine fasteners.  The factory used them for a reason.  I have come across several that had simply popped off their heads, these were in every case not the stock screws.  Apparently, the expansion rates at the cam cover play havoc with the screws.  If bling is required, they are also available zinc plated.  During a re-build, I renew these if incorrect screws were fitted prior.

Why not re-plate your yellow nylocs?  They come out like new, just require a drop of Loctite if you are anal about the insert not being up to the job and potential loosening resulting...

Engine mounting bolts should be fine-pitch, stainless or otherwise, to avoid loosening through vibration.  Higher tightening torques are possible.

piet
Click to expand...

Hi Piet

I had no idea the valve cover bolts are likely the highest strength fasteners on the entire bike.... :o

Thanks for the info, I'll see if I can find or order in some zinc plated 12.9's for the valve cover.

Paul LeClair
 
To follow up Paul, the early 180? works racers had M8 bolts in place of the stock M6...

As there is nothing else fastened to the cam cover, not even in the later space-frame racers, I guess the factory came across some difficulties and up-graded.  Perhaps this caused them to specify the stronger bolts, don't know what was specified on the early production triples as the earliest triple parts list I have dates from 1974.

I have plated 12.9 Allen head bolts in stock if you can't find them locally.

piet
 
sfcpiet said:
To follow up Paul, the early 180? works racers had M8 bolts in place of the stock M6...

As there is nothing else fastened to the cam cover, not even in the later space-frame racers, I guess the factory came across some difficulties and up-graded.  Perhaps this caused them to specify the stronger bolts, don't know what was specified on the early production triples as the earliest triple parts list I have dates from 1974.
Click to expand...

12.9 bolts? Do these go into an alloy engine case? If so what is the point?

You need to look at the weakest point. That would be the alloy threads. Easy to understand why they went from M6 to M8 as this gives a greater thread area in the alloy but 12.9 bolts?

I don?t think I have a bike that uses 12.9. 10?s are the hardest and one place used: they link the crank to the flywheel not something low stress like a valve cover.

I was taught that nothing over 8.8 should be re-used. The material is so hard it isn?t suitable for attaining adequate thread stretch more than once.

I can?t even imagine how you could get a 12.9 to stretch in a threaded alloy hole, even with an insert.

Just the way I look at it.

Rod, confused.
 
The name "stainless" is a little misleading. The main alloy elements in stainless steel are iron and chromium (along with a few other things). Chromium is actually more chemically reactive than the iron. The reason stainless steel stays shiny is that it the chromium reacts with atmospheric oxygen and forms thin oxide layer. That oxide layer forms a protective barrier that stops further oxidation.

Something keep in mind when bolting anything together is crevice corrosion, otherwise known as oxygen concentration corrosion. Water in contact with the metal surface will normally contain dissolved oxygen. An oxygen corrosion cell can develop at any point where the oxygen in the air is not allowed to diffuse uniformly into the solution, thereby creating a difference in oxygen concentration between two points. Corrosion will occur at the area of low-oxygen concentration which is anodic.

It has nothing to do with dissimilar metals, and can happen even when there is a more anodic metal (aluminium) in contact with the bolt. Stainless steel is more susceptible to oxygen concentration corrosion than non-stainless steels because the chromium is more reactive than iron. The oxide layer that normally protects exposed chromium is not present in this case because oxygen is excluded from the anodic area.

In the case of a stainless bolt, it's the part of the bolt that's hidden inside the bolted component (not exposed to oxygen) that will corrode - the shank and thread. So you won't see it until you remove the bolt. You might have stainless bolts that look nice and bling on the outside, but they could be rotting from the inside. I bet most of us have removed a stainless bolt or screw from something, and found it to be "necked" by corrosion. Boat owners especially will have seen this.

It only tends to be a problem in damp enviroments. A classic example is when stainless screws are used on wooden boats because wood tends to hold moisture. But it can happen in vehicles if they are often exposed to damp environments. If you wash your bike every week, then you may have crevices that never dry out.

The solution is to not let water get into the crevices around bolt shanks and threads. It's easy enough to protect threads with anti seize grease, but not so easy to protect the shank of the bolt where it passes through a non-threaded hole.

If you use stainless bolts in critical applications (like brake calipers) that often get wet. It would be a good idea to take them out maybe once each year to check that they aren't pitted around their necks.

Google "oxygen concentration corrosion" if you want to get technical.

Cam
 
Rod,

If you look closely at the stock cam bearing blocks to which the cam cover is attached, you will find steel nuts cast in them.  As the cover is tightened down solid onto the bearings, it is the cast-in nuts that are stressed, not a thread tapped in cast alloy. 

Guess it is the expansion rate of the cam cover popping off the heads of lower grade bolts.

Reckon the cam cover isn't as low-stress as it may seem... :D

piet
 
I don?t have a triple. I?m a twin guy but Paul?s first post referred to ?Valve Cover? and on my twin lots of the valve cover bolts go into the alloy. Triples may be different.

Cam cover? Valve cover? I?m still not convinced. Even if the nut is cast into alloy then it still will be pressing into a softer material, albeit with a greater surface area.

One reason a lot of high tensile nuts/bolts come loose is someone has used a stainless or low grade washer. It all torques up just fine but over time the washer compresses and relieves the tension. I see no difference if a mainly alloy block is used. The whole system from head to bolt need to be the same grade.

A chain is only as strong as its weakest link.

If something is specified then I need to understand why. Why tells you everything and allows you to think around issues. What and how can be done by a robot.

Just the way I was taught. I?d get a kick in the arse if I just followed the instructions if they didn?t make sense to me. I was blessed with great mentors.

Happy to learn when someone can explain to me why.

Rod
 
Hi Rod

I am a "why" kind of guy as well, but for valve cover bolts I had NEVER given that area of the motorcycle any thought. For my various previous (non standard) Laverda restorations I have just as a matter of course replaced every fastener I could with stainless steel, liberally coated with copper anti seize paste.

this current Laverda triple build, I went in completely the opposite direction and have tried to use all stock original fasteners, also liberally coated with copper anti seize paste.

this current Laverda build is the first one I have tackled that was really rusty. The bike sat outside unused from 1999 to 2013, and most of the fasteners (as well as lots of other bits) were rusty.

the valve/cam cover fasteners were among the worst, they weren't even plated, just black, or should I say RUST.... I did manage to get them out, and I did soak them in Evapo Rust to remove the rust, but they were too far gone to re use. So, without knowing that the valve/cam cover fasteners from the factory were apparently special 12.9, I innocently mention I am going to use stainless, and Piet kindly informs me what the factory spec was. In fact, if I can't find 12.9 plated fasteners locally, I will likely order them from Piet, just to remain factory correct with this particular build.

But, I don't know why Laverda used 12.9 hardness bolts. They must have had a reason, and I believe Piet when he mentions he has seen heads broken off valve/cam cover fasteners.

As another example of Laverda engine fasteners, for the small upper case front case studs that stick down into and through the lower case, with nuts and washers on the outside, every Laverda triple motor I have ever taken apart has those studs heavily corroded, including this engine. This engine, I cleaned those studs with a wire brush, sprayed them with galvanizing paint, slathered them with copper anti seize, and then used copper washers to try and seal the case passages they pass through. Next time this motor comes apart, I guess someone will see if that approach worked to prevent corrosion.......

so, for cam/valve covers, I have had stainless steel fasteners in there on my 79 1200 for more than a decade without a problem, and in my 1984 RGS for almost as long, both without problems, but this current motor is going to get Laverda correct 12.9 fasteners in the valve/cam cover just because all the other engine bits and fasteners are going to be correct...

are there any metallurgists out there who can explain why Laverda would have used 12.9 in the cam/valve cover?

Paul LeClair
 
Perhaps Massimos' ghost could explain... don't know if anybody of the old design dept is still around, doubt that Piero could offer an in-depth explanation.

The bolt threads into a steel insert/nut and is tensioned by the rise in temperature of the alloy cover which expands further than the bolt.  The nut is cast quite deep in the cam bearing, so there is also a couple of mm of aluminium there that will also expand and further tension the bolt.  The expansion rate apparently exceeds/overcomes the tensile strength of a normal M6 bolt, hence the use of the high-tensile bolt.  Couldn't really be bothered to dig out all my study books and manuals, sure to be the appropriate formulae in there somewhere. :D

What I know for sure, I have seen head-less bolts in there with the cover spewing oil all over the place.  The original bolts have always been 12.9 grade, the racers had M8 bolts instead of M6... must be a reason.  Gijs van Dijk went to great lengths to convert to M8 on his endurance racers, and he has quite some experience with snapping studs and bolts during races!

I have found lots of bolts loose or missing from the covers, usually down to not properly tightening and only compressing the rubber gasket partially.  Once this settles a bit, an incorrectly torqued bolt will quickly disappear.

With hindsight, there's not much wrong with the original Laverda designs.  The Breganze boys didn't bullshit around, and on a whole, they knew what they were doing.

piet

 
Paul LeClair said:
but this current motor is going to get Laverda correct 12.9 fasteners in the valve/cam cover just because all the other engine bits and fasteners are going to be correct...
Click to expand...

You should use unplated ones then Paul.  I'll be marking you down for that at Wolfgang's next impromtu concourse.  :D
 
Gulp! The cam cover is one of the few spots I put stainless bolts on!!!!!

However, perhaps I accidentally got it right?
Coefficient of linear expansion (10 to power of -6 m/m K):

Aluminium = 22.2
A2 (304) stainless = 17.3
Steel = 13

So the difference in thermal expansion of my stainless bolts to aluminium is not so bad. On the other hand a steel bolt is going to get stretched more as it does not expand so much, therefore it needs to be high tensile. 
 
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