Date: Wed, 6 Mar 1996 12:49:12 -0800 (PST)
From: Colin Bryant
>Chuck Kichline Said:
>I've never owned a Commando, but I have been around since it was more
normal
>for a motorcycle to vibrate your arms off then vibrate it's exhaust
pipes
>out.
>The thing I don't see in any of the postings about Commando exhaust
nuts is
>"tightening order". I remember that the process was to loosen all
the
>fasteners in the entire exhaust chain, then tighten the head nuts
first, and
>follow back along the pipe tightening each in order. The idea here
was that
>it was very possible to put additional stress into the fitting by
working
>against the rest of the chain. Sorta makes sense? Or just a 1968 Old
Wife's
>tale?
The key point with the Commando is that both the powertrain and exhaust system are rubber mounted. The exhaust, however, is several feet long and only attaches to the source of the vibration at one end. The exhaust therefore tends not to shake in good time with the motor. Movement happens at the exhaust pipe/cylinder head junction. Because of this movement the gasket wears/gets crushed and the nut becomes loose. Beginning tightening at the head is a good idea, but doesn't solve this built in engineering problem of everything not shaking in syncopation.
My personal technique modifies the appearance by drilling a hole in each nut and winding a long spring between them so that they are tightened as the sealing ring is crushed. I've been using it on all of our Commandos (currently 3) since getting my first in 1983. I've never had a nut come loose on the road yet.
Colin Bryant
From: Mike Taglieri
Date: Tue, 18 Jun 1996 00:49:36 -0400
>As I was giving it the obligatory full-weight-while-holding-the-bike-
>upright press, I could feel that awful sigh/give feeling as the port
threads
>gave out.
I hate to make you feel even worse, but for the benefit of others, a torque-it-as-much-as-you-can pull on this nut is NOT "obligatory" on a Norton exhaust nut, and that's why these things strip so much. We're bedding down an exhaust pipe gasket by turning a thread in aluminium with half a dozen threads engaged at best, and heavy torque is dangerous. The way I do it could be called "firmly snug." After your first ride, the gaskets will bed down a little bit, so do it again firmly snug. This is the same way the head has to be retorqued, yet no one suggests hundreds of pounds of torque on those nuts to avoid having to repeat it. After 3-4 rides, the gasket will have assumed its final degree of crush, and the nuts will not loosen again, with the torque never having to rise very high.
I suppose now someone will quote the old factory stories that Norton torqued exhaust nuts with a 6-foot pipe so they'd stay tight. We've been through this before, and it does not apply to us even if true because:
Mike Taglieri
Raul -- '72 Commando Interstate
Date: Tue, 18 Jun 1996 15:49:19 -0400
From: Robert K. Bell
At 08:37 AM 6/17/96 -0700, you wrote:
>Well, I found out why the exhaust pipe on my 750 Commando came
>loose last raceday (other than being a bonehead and not wiring
>it). As I was giving it the obligatory full-weight... press,
>I could feel that awful sigh/give feeling as the port threads
>gave out.
I wouldn't say that you were a "bonehead" for not wiring your Norton exhaust nuts, as that simply will not keep the beggars in place. However, as pointed out by others the full-weight press isn't the answer either.
It is more often than not helpful to understand why a problem exists before trying to remedy that problem. At times understanding the problem saves a lot of fixing.
Dominators, Manxman, SS650's, Atlases and Commandos all enjoy the same exhaust port configuration. That is an approximately 3/4 inch deep bore, this varies from head to head, with 1 15/16 x 14TPI threads. While they all have the same ports, only the Commandos shed their threads. It is seldom that you will see any of the others with exhaust port problems. It seems fair to ask why this is so.
One of the fundamentals of thread design, a rule of thumb, is that it takes thread depth of 1.5 times (1.5D) thread diameter to affect a secure mechanical attachment. This tells us that a 1 15/16 thread needs thread engagement of 2.90 inches to gain a secure mechanical attachment. This is of course just a guideline and in many situations much less thread engagement will do just fine. Less than a quarter of that called for by the "rule" works on the Domi, Manxman and Atlas, and doesn't on the Commando.
The reason is pretty simple. The exhaust pipes and silencer on the Domi, Manxman, SS650 and Atlas are bolted solidly to the frame as are the engines. They all vibrate at about the same frequency. The silencer and pipe cannot work like a wreaking bar on the port threads.
With the introduction of "Isolastic" engine mounting the exhaust nut problems began.
Think about it. First we have a thread with insufficient engagement, aggravated by the fact that the male thread is hard steel and the female is soft alloy. Next we attach the exhaust pipe, a very efficient lever, and anchor it to the frame as far as possible from the exhaust port with a semiflexible mount. Next we add the silencers to provide a counter weight for our lever, the exhaust pipe. Now when the engine is started its "Isolastic" mounts allow it to vibrate freely without transmitting any of it the frame and rider. Meanwhile the exhaust pipe vibrates with the engine at the one end and is held semi stationary by its mount to the frame at the other end. Now with the engine running and vibrating at its own rate, and the frame at another we set off on a ride. As we ride the bike encounters irregularities in the road and the suspension does its thing and so does the counter weight (silencer) and lever (exhaust pipe). All this leads to continuous working of the pipe at its joint with the head which loosens the nut, by crushing the underlying metal no matter how tight or well glued in place. Once loose the nut is free to hammer the exhaust port threads to oblivion. The wreaking bar at work.
After repairing a few hundred Commando exhaust port threads and having the opportunity to see near as many attempts to keep the nuts tight with set screws, safety wire, lock nuts, studs and flanges, lock washers and other sundry dodges, its become apparent that there is but one way to keep the threads in a Commando head. It's simple: use solid copper exhaust washers, and gently tighten the nuts after each ride. It is wise to replace the washers every few thousand miles. Even with constant attention to the nuts, the washers take a beating and compress over time. As they compress, and work harden, they no longer protect the alloy matting surface in the port. In time this will cause deformation of the head itself and just add to the problem.
I hope this will save somebody a little trouble. While I will admit to having made a modest amount of money repairing Norton exhaust port threads, and don't mind the business, it has always seemed a problem that could be avoided.
Ken Bell
Bell Engineering
Colesville, MD
From: D.J. Walker
Date: Wed, 28 Aug 1996 21:00:10 +0100 (BST)
Russell asks us:
> Just to confirm, are
> you saying that if one doesn't fit the balance pipe you don't need
the
> collet? or that earlier systems had a different fitting altogether?
Both. The early systems had no balance pipe stubs on the downpipes. Thus, you could thread the exhaust rose along the downpipe from the silencer end of things. The flange bit on the cylinderhead-end of the pipe was wider, and the rose simply pressed against this, as it was wound in:
THIS FLANGEY-BIT IS
WIDE ENOUGH TO TAKE
THE PRESSURE FROM
THE RIM OF THE ROSE
\
____
\ _ CRUSHABLE GASKETY THING. I USE
|____|,,,,,
.. | | / SUZUKI GS-SERIES GASKETS, HERE
- ----| ||||||-----|| | |
/ THEY'RE FAR BETTER THAN NORTON
|----||||||
|| | |/ ORIGINALS, AND SAVE ME A 16MILE ROUND
|
|||||| || | | TRIP TO "SUPREME'S".
SIMPLY GO INTO
_ ____|----||||||_____|| | |
LOCAL "MOTORCYCLE-BASED LEISRURE
|____|"""""
\ || |_| ACTIVITIES EMPORIUM" (IN LEICESTER
|____|
\ THIS IS KNOWN AS
"MCA") AND ASK THE
/
\ UGLOID BEHIND COUNTER
FOR TWO
/
END OF DOWNPIPE VT3003 GASKETS (THEY HATE IT, IF
EXHAUST ROSE
YOU CAN QUOTE THE PART NUMBER AT 'EM).
> On my current pipes (MkIII reverse cones), there's a conical seat
that
> sits 'deepest' in the head, then the flare on the exhaust pipe,
> then there's a collet round the pipe then the exhaust nut.
Like this?
SEE, THIS FLANGE-BIT,
HERE, IS NARROW ENOUGH
FOR THE ROSE TO GO ON
COLLET THIS ( <----<<<
) WAY,
\ NOW.
____
\ / _
|____|,,,,,|`___
/ | |
- ----| ||||||-----|| | |
|----||||||
|| | |
|
|||||| || | |
_ ____|----||||||_____|| | |
|____|"""""|.---`'
|_|
|____|
\
/
COLLETS TRANSFER THE PRESSURE FROM
EXHAUST ROSE THE ROSE
TO THE NARROW FLANGEY BIT.
Now take a look at the 850 downpipe:
_
________/_\
/ _________ )
/ / \_/
/ \
/ /\ \<----.
| | \O \
| | SEE, THIS STUB
OF PIPE THAT TAKES YOUR
| | BALANCE PIPE,
WOULD PREVENT YOU FROM
| | THREADING THE
ROSE ON FROM THE BOTTOM...
\ \_________________________________
\__________________________________) <----<<<<<
.....(THIS DIRECTION)
> are you saying that without a balance pipe one can just consign (B)
to
> the cardboard box that contains all the half useless bits that one
just
> can't bear to throw away?
Yes (I've got one of those). The exhaust roses you use on balanced systems are different, too. There's less length of thread on them (since they didn't need as much, you see, with the collets there, in the way). If you use the 850-system roses on an uncolleted 750 system, you'll find they've wound all the way into the head before they've taken up the slack. The 850 roses look more impressive, because there's more cooling-fin material on them, but this is because there was always less head/exhaust-rose contact surface on the later systems, so heat transference wasn't as good. You can use two of the VT3003 gaskets, that I describe above, to make up the difference, while you're saving up for some new exhaust roses (the roses are about 10 or 12 quid each, in the UK, I think). The extra thickness of gasket makes up for the lack of collets.
LITTLE FINS BIG FINS
\ \
____
____----
|____|
|____----|
| ||||||||
|________||||||
|----||||||||
| ||||||
| ||||||||
|--------||||||
|----||||||||
|____ ||||||
|____|""""""'
| ----|"""""
|____| \
`----____| \
Oh, Goody!
Oh, Bum! Hardly
Lots of
lovely any thread, at all,
thread,
here! on this one!
TWO COMMANDO EXHAUST ROSES. (THE 750 ONE IS ON THE LEFT,THE 850 ONE IS ON THE RIGHT.)
Alternative uses for unused exhaust roses include forming part of the thruster system, on the spaceship, in your realistic 1/24th scale Dr Who & The Daleks scale-model diorama (or is that just me?).
Bendy Dan
djw12@leicester.ac.uk
17.3 Exhaust Pipe Mounting Brackets.
Date: Thu, 5 Sep 1996 14:04:08 +0100 (BST)
From: D.J. Walker
Subject: Exhausted
Dear All,
The exhaust hangers on my bike, which permit a 750 upswept reverse
cone system to be fitted to an 850 Interstate are identical to standard
Roadster systems, except for one of the brackets.
061720, the backplate, is as standard, 061722 is, also, but the external plate, which holds the silencer itself, and fastens to the others via the rubber things, is triangular (as opposed to the lozenge-shaped one in the parts book).
_____ 061720
|| _ \ ___ 061722
Foot peg-------||()) \ || _\
hanger hole || \
||())|----Foot peg hanger hole
|| \ ||
|
_______________||___ \ ||
\ ___________
____________________\_ | ||
| __ || O O -` TRIANGULAR
\ ()| || ()|(_() ||() _/ PLATE
hole
2---()| | ||() / / __ ||
_/
hole 1---()_/
()| ||__/ | (_() ||()_/
_______------- \___|
| / ||_/
------- \
Rubber things
\
Tailend of Z-plate
Hopefully, that's clear enough. You're sort of looking obliquely backwards at it all. And its exploded. The plates 061720 and 061722 sandwich the back end of the Z-plate between them, via the holes, 1 and 2, and corresponding holes in the two plates (on 061722, these are the two "()" signs, towards the bottom of the plate). The pillion footpeg goes through the two big holes, at the top of each plate ("())" in the picture). A cylindrical spacer fits between the plates and the bolt-end of the footpeg goes through the lot.
: :
| | Pillion peg (folded)
_____ | |/
|||| _\| |
||||(__)_|)--bolt bit of peg
|||| |\
|||| | \
____________
_______________|||| \ \
|| O O -'
_________________|| | |
__ || _/
|| |()---HOLE (_() ||()
_/
|| ()| |
__ || _/
||() / ()---HOLE (_() ||()_/
_______---||__/\___|
/ ||_/
-------
\ \ Rubber
thing
\ \
061720 061722
The triangular plate mounts to 061720, via the two rubber things, that go through the "HOLE"s in the rear edge of the plate, and through the corresponding holes in the front edge of the triangular plate. The two holes in the top edge take the bolts or studs that fasten the silencers on.
Dimensions? I didn't take the plate off, to measure exactly, because I wanted to ride the bike, but the two silencer-mounting holes are about 1.25 inches apart. there's a 1 inch gap between these, and the holes that take the rubber things, and the rubber things are about 2 inch apart.
1.25"
|<1">|< >|
_______________
| O O |
-------|( )
_/
/\ |
_/
2" |
_/
\/ |
_/
-------|( ) _/
|____/
It's not a right-angled triangle, though, and the two silencer holes are inclined upwards towards the back.
I can see no reason why this setup is different from the standard Roadster setup, though, nor why a standard Roadster system can't be used...
Dan
If God wanted cowards to hurtle about the roads in sportscars, he'd have made pedestrians out of concrete. God DID NOT give us Aston Martins.