Exhaust Pipe Finned Nuts
("spider nuts" per BMW)
© 2012, R. Fleischer
exhaustnuts.htm
article #46
There are two basic styles, both finned. Early nut models
had some deep dimples/holes in the flat area closest to the head,
and that was for the tool-kit lug wrench (spanner). Later models
do not have those holes.
The official BMW wrench was a 18-600. Other
finned wrenches may be used, if they fit the fins OK.
Many versions of the FIN wrenches have
been made and I recommend the type that
engages a goodly number of the fins accurately. In an emergency, you can use some sort of 'strap-wrench',
but this needs to be done very carefully.
The exhaust nuts tend to seize up over the years if not removed
and the threads cleaned and recoated with an antiseize compound.
I recommend this be done on a scheduled YEARLY basis.
To remove the exhaust pipe, should you want to do that for some
purpose (never necessary for JUST the yearly finned nut antiseize
coating work)... it is not necessary to remove the nut, only loosen it
~1/4th turn.
YEARLY (Recommended!) SERVICING of the finned nuts:
It is necessary to totally unscrew the finned nut to
clean the internal rings (one or two depending on motorcycle model) and coat the threads (I
do the rings too) with anti-seize
compound. You need not remove the pipes, nor do you
need to remove the nut from the pipe. You need only to
slide it down the pipe some. Regular servicing of
the threads is highly
recommended.
I used to think that every 2 years or 20K would be adequate, but some
have been known to seize sooner, so I am now recommending every year. If you fail to do this, you
likely WILL be sawing and
replacing nuts...or, worse, repairing threads (which can be an expensive repair job),
and the nuts are not cheap (cheap to saw and replace, rather than
ruin the port threads however).
DO NOT over-tighten the finned nuts! DO NOT FAIL TO USE ANTISEIZE COMPOUND.
The finned nuts do have specifications on tightening. I am sure other
folks have tried
this. For
the models before 1981, the value in SOME literature is 101-130 foot pounds, and from 1981 it is 145-159 foot
pounds in SOME literature. I HEARTILY suggest you do
NOT
tighten them anything even close to that!!
That recommendation includes
not paying attention to the factory manual where in one place it shows a nominal 160 Nm,
and also says 118 +-15 foot pounds (which is a proper conversion
of Nm to Ftlbs).
NOTE!....some
confusion can be seen in the various literature due to
BMW...and others'... wrongly converting Nm to foot-pounds, and compounding the error backwards.
The Nm figures are correct! AGAIN:::: I do
NOT
tighten these finned nuts as tight as BMW says...I think that
WILL promote problems. It is NOT necessary to use a hammer
on the wrench when tightening; just a good grunt but NOT hardly
your full weight, assuming a reasonably long wrench (roughly 12"). The internal split ring does a fine job of tightening on the pipe.
I like the parts nice and clean of deposits and other crud before I use the anti-seize
compound. That means I actually clean the
threads, rings, etc. I use a medium-fine-small wire brush for
that purpose. You can use an old toothbrush too.
The best anti-seize I know of is the "Pure Nickel Special" of the Never-Seez brand. That is good for
2600 degrees. The standard Never-Seez is also excellent, and is the better buy. The Permatex brand commonly seen (1600
degrees) is
adequate. WHAT YOU MUST NOT EVER DO... is lubricate those
threads with WD40 or any common oil.....DO NOT DO THAT, as most oils carbonize!
It is, of course, acceptable to oil lubricate the nut threads with anything
if you are having problems getting one off/unscrewed.
PROBLEMS:
The "problem", if you have one.... comes when trying to remove or
unscrew the nuts. Sometimes those nuts have been over-tightened, or have self-tightened up really well. I
often use
an old piece of 2 x 4 (wood absorbs some shock) on the finned long wrench
(I have even used a light weight (7 or 8 pound) sledge hammer
or a brass hammer on the wrench).
The loosening direction is COUNTER-clockwise, in other words, to the left, just like any standard fastener,
as you face the nut, from in
front of the bike. It is NOT too unusual for the nut to be very tight, then break loose and after 1/8th turn, loosen up rapidly.
Do NOT go overboard with the sledge or wood piece!!! If the
nut loosens a small amount, maybe 1/10th turn,
AND then seizes, you can try
using a large flame oxy-acet or propane torch on the NUT, getting it fairly
hot, then try tightening/loosening again, sometimes that works.
If it does not
loosen, run the engine to heat the head and port and finned nut (maybe 2
minutes),
then turn off the engine and squirt penetrating oil onto the threads
as
the engine is cooling. Do this a
number of times as it cools. Let sit
overnight, then try loosening back and forth. If you do not have to ride
the bike soon, do the soaking of penetrant over and over for a week!
A very good penetrating oil
mixture that beats all others (at any price) is to mix ATF red oil with acetone.
I suggest maybe 1 part oil to two parts acetone.
****IF that finned nut, loosened a
bit, and NOW using your hand on the wrench (with only MODERATE
pressure),
suddenly
gets VERY stiff again, you are
hereby warned NOT to go farther at this
point....EXCEPT, that you can apply penetrating oil and wait
overnight. DO NOT force the nut if it re-seizes!!!
Seizure, if it happens, is usually at roughly 1/4+ turn most of the time when there is a problem...but this does not mean ALWAYS. A properly loosening nut will loosen QUITE adequately in a fraction of a turn, and then be hand-removable (usually no finned wrench needed)...or nearly so. Try it hot too!...how hot? 2 minutes of engine warming hot!
If the finned nut will not loosen or loosen enough without seizing, you MUST cut the nut off! The slow method is to use a hacksaw blade holder (see hardware store) that holds a short piece of 32 pitch hardened hacksaw blade. A common electric motor powered (Dremel?) tool can be used which is faster, but be careful! No matter what tool.... for the best safety in trying to NOT damage the port's threads, hold the tool at an angle, that is, tilted towards the shiny exhaust pipe somewhat. This lets you make a cut that will go through the nut and stop the cutting on the very hard steel rings inside. You need only make ONE cut, in the same direction as the fins, that is, one place, front to rear.
Do NOT cut into the PORT threads. When you are nearly to the PORT threads, try using a pry bar or very broad angled chisel and hammer to split the nut at the cut area. Again, do NOT overdo this. DO NOT injure the port threads. The purpose of the cut slot is to weaken the nut so you can force the nut to break and widen at the cut slot, and then the nut WILL BE easily un-screwable. Nuts are cheap compared to repairing the port threads! If you make a minor cut into the port threads, you can repair them with a thread chaser, or possibly the old nut!
You have TWO choices now...make a second cut at roughly 180°, and the nut comes off the pipe in halves.......or, avoid the second cut by removing the header pipe, so the old nut slips off.
Do NOT throw the old nut away...you can possibly modify it to be a thread chaser, even though it is rather soft! It also is valuable to see just how deep your cut can safely be, if you ever do this job in the future....before injuring the threads.
There is actually a die
available,
very pricey too, for the 52 mm port threads. They are available at such as
metric supply companies...like MSC. I don't know anyone,
including me, that owns one.
When installing the nut, whether a new one or old one, the threads
must be clean first on the male and
female portions, and THEN AMPLY APPLY anti-seize compound. I use a relatively fine BRASS brush, but a
steel brush will work OK for the CLEANING. Toothbrushes can be used, but
are generally not stiff enough for a proper job of cleaning, but are OK for
applying antiseize. Also apply anti-seize to the rings. If
you have any other parts of the exhaust system ever apart, clean them and apply anti-seize
(I do this to the
pipe junctions and small screws threads, etc.). Antiseize is a lubricant and really helps with nuts, screws, the
pipes, spark plugs etc., where they get hot...but, since it is a lubricant, you
do not want to over-tighten things.
The big problem with the exhaust
nuts is over-tightening, and of course carbon formation and steel to aluminum galling
with failure to unscrew and clean and recoat regularly.
If you do NOT over-tighten, and if you remove the finned nut and apply fresh anti-seize once a year, you will never have a problem.
Hints:
(1) The exhaust system should be installed with antiseize compound at each and
every joining point.
(2) For
some airheads there is BOTH a split ring and a solid collar, that fit on the pipe, inside the finned nut. To avoid you having to figure out how to assemble
them, the split ring goes next to the head, with the FLAT
END towards the head. The solid collar flat end faces the nut.
This will put the two slanted surfaces
towards each
other.
(3) If you used an oil for
loosening the nut, etc....be sure to use a good solvent and wash
it all away before using antiseize.
(4) Do not mix up left and right side nuts, they are the same, but may have worn the threads differently.
You won't have this problem unless you remove the pipes, as the
nut is captive without the pipes being removed.
(5) Various folks repair bad exhaust port threads by various
means.
I prefer real welding and new threads, but sleeves HAVE worked
OK, even though the heat trapped is a lot, theoretically.
Revisions:
04/10/2003: revised for clarity; add .htm title
06/30/2003: final revision; expand information ...additional clarity.
02/23/2005: Revise slightly for additional clarity
03/01/2005: add Ed Korn
03/27/2005: emphasis and minor typos
01/09/2009: Updated, mostly clarifications
10/30/2010: clarified a few details, improved on hints.
04/26/2011: minor clarifications
05/27/2011: Clean up article
© 2012, R. Fleischer