What do we see here?

[Bryanj]

ANY lubricant on surface or thread allows the nut to turn easier on the thread and this alters the torque setting when the nut stops turning thereby altering the clamping force applied to the items in question.
You shouls always use the manufacturers setting in which should be specified dry, oiled or grease and if so which type.
Honda do specify molycoate on some of the GL1000 bolt settings

[K2-K6]

As Bryan says above.

Fundamental problem is that (for convenience) the assembly uses a torque method to measure rotating resistance (friction is changed with lubricant type) to carry out setting of tensile (stretching the cylinder stud) loading.
There's conversion between the two, controlled by friction, which can be changed with the lubricant used.

Make the nut more slippery and for the SAME torque number, it will pull more tensile load on the stud, clamping the cylinder joint tighter.

Ultimate (and how the original engineering design sets the torque) is to put a dial guage on the end of the stud, measure elongation, and compare that to tensile load specification for material used. Example maybe, you get 0.5mm elongation for 10lb/ft torque.

Think of the stud as a elastic "bungy" cord.

[Oddjob]

I wonder how that would affect stainless steel cylinder head studs Nigel. I mean it's not really practical to measure the stretch and as they are stainless you need grease to prevent galling so how would you know the correct torque after using say Nickel grease.

[K2-K6]

In reality the stretch and calculations would be done for a representative fixing to give a torque number that's suitable and not by individual stud in situ. So measuring and practicalities involved to give usable working figure, same as any stated torque setting.

There's a more fundamental issue, in that generally available stainless steel has nowhere near the tensile load capability of these std studs. In other words it lacks the ability to clamp the structure to the same force needed to keep the head and barrels on during use.

If the tensile load could be matched, then it would likely be a very similar figure.

The stud is working (the bolts on the engine are steel that's "quenched and tempered" ) with a minimum tensile capability to give the clamping force the designers intended. The effect on changing to ss that didn't match this would be like not doing the bolts up sufficiently,  the same rotary torque would likely stretch the ss into yeald and not clamp the structure.

These studs may look ordinary, but there's alot of interesting features in them, probably a topic on it's own.

[Oddjob]

I've got an engineer making me some stainless steel studs for the 500/550. I find they rust through the barrels. So I sent him one and asked him to match it, the first bolt failed the stretch test, at 16ftlbs it stretched too much due to the waisted look of the old bolts, he's now reducing the waist to see if he can get one that works whilst sort of keeping the original look. I'll also be fitting stainless steel shouldered head nuts, hence the question re the torque.

[K2-K6]

It's quite an involved subject, changing material in relatively high stress application. 

The shape,  waisting, of the stud is not cosmetic but coincidental to it's function.  To fit in the space of the design it's an advantage to be smallest possible, that brings the stress it's asked to take closer to the material design limitations hence the higher tensile heat treatment and correct metallurgical composition to accommodate this.

The stud is then designed with a waist such that the root diameter is consistent from one end to the other with the threads sitting outside this. What this does is to allow the highest tension to be applied for the space available by evenly distributed stretching across each successive mm and consistent from one end to the other. If you increase the waist diameter that equilibrium starts to shift as you can't increase the root diameter of the threaded portion of the stud.
Now as you load it, the centre won't stretch (because the cross section is larger) to the same degree as the thinnest part, this accumulated difference is greatest as the diameter is forced to change going into the threaded section. This effectively overloads (all the stretch has to happen here) the material at that point and the tensile load capability can even go down as you've produced a failure point (shear coupling? ) with all of the forces directly focused at those two transition. Focusing it there can simply be more than that section will ever see ordinarily as the rest of the increased stud no longer shares the stretching.

What needs to happen is to increase the tensile specification of the stainless (loops back to material properties again) rather than change the cross section.

This is why bolts generally fail at the thread juncture if over torqued, the shape of them.

[Trigger]

A good wright up K2-K6.
I don't think any stainless with do this job, even if you torque up to 18 ftlb and re-torque the compression will just stretch stainless. You have 4 hammers (pistons ) at 170 psi trying to push the head off. I think Stainless is just asking for trouble    

[Oddjob]

It's odd that the 400 cylinder stud doesn't appear to be waisted Nigel. I wonder why that is?

[taysidedragon]

It's odd that the 400 cylinder stud doesn't appear to be waisted Nigel. I wonder why that is?

I'm pretty sure mine were.

[K2-K6]

If the 400 doesn't use that waisted type it would likely be because the loading required is further within the capacity of the material for the 400. That smaller percussive effect of the cylinders takes a fair amount of load off the overall design compared to the larger engine.

I feel the stainless studs would creep too, think of it in structural terms as more like toffee  sustained load on it gradually deforms over time. The quenched and tempered steel very specifically produces a mild spring like property that is maintained even under loading.

Additional to all of the above (common I think to most of the bolts used on these bikes) the threads are rolled not cut with die or screwcut on a lathe. The effect of this is to effectively forge the threads into the steel structure such that the peaks and troughs are formed leaving no sharp cut into the material surface. This is much stronger and difficult to match by other methods, particularly critical at the thread lead out into the straight stud section.

They really did throw alot of pure engineering theory at these bikes, more from aircraft sphere than other fields (Sochiro was originally in that discipline I believe) which can be seen in the concept of these components.

Also why many "heavy duty" studs are just funny, they include all the faults Honda avoided in original design. Most, all? go with thicker is better claim, avoiding answering the material spec completely.

[Trigger]

All engine threads are rolled on SOHC's.
Julie has just reminded me that i did a CB350 four that the owner had rebuilt with stainless head studs . There were so many warped mating surfaces that were beyond repair, the customer found it cheaper to buy another engine and i rebuilt that 

A Die cut thread does not match a rolled thread. Found this out on building a sand cast 750 many moons ago. We here having trouble finding some of the original correct number 8 crank case bolt. The customer had found some longer number 8 bolts to be cut down and a new thread die cut. I have put the crank cases together but, when i went to tighten up the bolts they just spun on the threads and then i remembered that all the threads were rolled. Crank cases spilt, inserted V-coils and that was the only way for the die cut threads to take grip in the threads and take the torque settings     

[taysidedragon]

These are the engine studs on my 400f. Main shaft of the stud is smaller diameter than the threaded sections. [ Guests cannot view attachments ]

[deltarider]

Forgive me... what is a rolled thread?

[Bryanj]

It is a thread formed by rolling plain bar inside rollers with thread form on them, no metal is removed and threaded bit ends up bigger than plain bit, most common on wheel spokes

[Oddjob]

Cheers Nigel. Well that looks to be a busted idea, shame but that's sometimes the way things go. I'll have to either buy new ones or plate the old ones after a good polish up to get rid of the pitting etc.