Engineering compromises leading to exhaust threads stripped (2012)

dobba99 said:

My spec for torquing the zorst
C spanner and two foot of inch pipe. Bike on center stand and enough umpty to start to lift/tilt the bike (obviously don't tip the bike off its stand !)
Click to expand...

That was my point, thank you.
 
With my Commando/Featherbed I just tighten the rose up and it stays tight have never done it while hot or any other way in over 38 years and have never had my flang come lose yet, a lot of problems with Commandos is everything is rubber mounted, well that's my opinion anyway and the balance pipe wasn't a good idea as well and having my headers made for my Commando/Featherbed also made a big difference to how the exhaust fits.

Ashley
 
I do the majority of my tune/balance by ear so.....that balance pipe abortion was the first thing to vanish when I got my baby. Besides I just didn't like the damned thing, but the remainder of the bike I can live with.... Too late to decide otherwise after all these years. It must be love.
 
concours said:
What is the torque specification for the nut?
Click to expand...

No figure mentioned in the workshop manual. Thread size 1-15/16"-14 UNS according to DynoDave in another thread.

Using the coarse formula T = K*Fpr*D , where K = 0.2 (dry friction steel/aluminum), dm = 1.87 in medium diameter (figure for 16 tpi, as I didn't find the figure for 14 tpi), di = 42.4mm (inner diameter of the nut), and prestress using aluminum 6061T6 (Fullauto head), ultimate stress = 290 MPa,

Fpr = 0.7*290MPa*pi*((D/2)^2- (42.4mm/2)^2) = 73069 N yields T = 694 Nm.

This is a very coarse figure of course, and assumes a bolt length of about 0.8*D. I believe this is not the case for exhaust nuts.
A torque reduction of 50% for poorer alloy and shorter thread length may give you an indication of the thread's load capacity.

-Knut
 
Last edited:
Tornado said:
I believe this OldBritts Tech Article showing how to run safety wire on the roses states 70-80 ft-lbs to be used on the exhaust rose nuts:

https://www.oldbritts.com/exh_nut.html
Click to expand...
Note also the angle of the safety wire, that acts in a way directly preventing & counter against loosening/unscrewing,

also going to another bolt that acts in tension, all typically used in aviation

https://aviation.stackexchange.com/...re-on-aircraft-and-the-reasoning-behind-the-w
this is done in a way such that the loosening of the bolt is counteracted by the tension in the wire i.e. in case the bolts loosen, the locking wire becomes taut and prevents further movement of the bolt.
Click to expand...
 
Last edited:
mdt-son said:
No figure mentioned in the workshop manual. Thread size 1-15/16"-14 UNS according to DynoDave in another thread.

Using the coarse formula T = K*Fpr*D , where K = 0.2 (dry friction steel/aluminum), dm = 1.87 in medium diameter (figure for 16 tpi, as I didn't find the figure for 14 tpi), and prestress using aluminum 6061T6 (Fullauto head), ultimate stress = 290 MPa,
Fpr = 0.7*290MPa*pi*(D/2)pow2 = 1130 kN yields T = 10.79 kNm.

This is a very coarse figure of course, and assumes a bolt length of about 0.8*D. I believe this is not the case for exhaust nuts.
A torque reduction of 50% for poorer alloy and shorter thread length may give you an indication of the thread's load capacity.

-Knut
Click to expand...
I asked that tongue in cheek, based on another post about my mention of the SOP (Standard Operating Procedure) of OVER tightening.
10.79 kNm.... ?
Is that 10.79 KILO Newton Meters? I’m not familiar with that expression.

Also, I already found the thread’s load capacity, thanks.
 
84ok said:
Note also the angle of the safety wire, that acts in a way directly preventing & counter against loosening/unscrewing,

also going to another bolt that acts in tension, all typically used in aviation

https://aviation.stackexchange.com/...re-on-aircraft-and-the-reasoning-behind-the-w
Click to expand...
Question on safety wiring: Isn't there going to be some amount of tension take-up on the twisted wire as the fixture begins to un-thread, due to nature of the twists and even some wire stretch? If so the fixture can undo slightly....isn't this an issue particularly with our exhaust roses, leading to thread damage from header flange movement?
 
Tornado said:
Question on safety wiring: Isn't there going to be some amount of tension take-up on the twisted wire as the fixture begins to un-thread, due to nature of the twists and even some wire stretch? If so the fixture can undo slightly....
Click to expand...

Not if done correctly no.

If done correctly safety wire keeps the fastener it is wired to under tension in the direction of tightening. It is therefore (as far as I am aware) practically impossible for a correctly wired fastener to vibrate loose.

However, what can happen is the faces being compressed by the torque continue to compress, when this happens, even though the fastener has not undone any, it is now loose.

With the exhaust nuts on Commandos, clearly this is a possibility, if the nuts have not been tightened enough to fully crush the soft sealing washers, they can crush further. Personally, I believe this is why they come loose, vibration causes movement of the header, which causes further crushing of an inadequately crushed washer.

Those soft washers need crushing. Hence the need for them to be RFT!
 
Safety wiring just keeps stuff from falling off. This application it’s a poor choice, you may not know it got loose, until the threads are vibration pounded out.

JMWO
 
To clarify, I’m not advocating use of lockwiring for ex nuts either. I was attempting to answer Tornado’s question and offer an opinion on why they do come loose.

As I’ve said before RFT on a hot running engine with a fork stanchion cheater bar is my preferred style !
 
concours said:
I’ve been called an idiot before.
So, all are idiots who’ve had to deal with those problem?
Of course.
I fix stuff.
Lots of it.
Wildly complex & expensive stuff sometimes.
Do/say what you will.
I offered to share for free an idea.
Seams it’s working well.
Low cost.
Many bikes will never need the solution, because, well, their owners aren’t idiots.
And would NEVER ride them fast or far, never mind BOTH at once. :p
Like my oil-tight primary fix, clearly, non-idiot owners don’t need it either.:cool:
Merry Christmas:D
Click to expand...
I never said anyone was an idiot, I said " not making something idiot-proof is NOT an engineering compromise" And I always thought the solution to a leaky primary was called a belt drive.
 
Nothing wrong with the stanchion 'torque enhancer' in my view because I happen to use a 24" pipe wrench having tired of the loose exhaust header drama long ago. I never minded scratched up rosettes because they're painted, and also began wiring them down about the same time.
 
concours said:
I asked that tongue in cheek, based on another post about my mention of the SOP (Standard Operating Procedure) of OVER tightening.
10.79 kNm.... ?
Is that 10.79 KILO Newton Meters? I’m not familiar with that expression.

Also, I already found the thread’s load capacity, thanks.
Click to expand...

Yes. Newton and meter are SI units, used in every civilized country ;-)

694 N*m equals 512 ft-lb

-Knut
 
Last edited:
mdt-son said:
Yes. Newton and meter are SI units, used in every civilized country ;-)

10.79 kN*m equals 7958.30 ft-lb

-Knut
Click to expand...
Sounds good!

:D
:confused:
:rolleyes:
o_O
:p

I have a 1” drive, 4:1 torque multiplier, still not sure if I could get there.
:cool:
 
mdt-son said:
Yes. Newton and meter are SI units, used in every civilized country ;-)

10.79 kN*m equals 7958.30 ft-lb

-Knut
Click to expand...

If that figure is meant to be the yield strength of exhaust threads then I can sleep well knowing my technique of giving the c spanner a few good hits with a small sledge is not doing harm.
At 10k NM's, id think the head or block bolts would be shearing off.
 
You must log in or register to reply here.
Back
Top