BMW exhaust repair
Repairing the BMW exhaust
I had problems with my K100RS-16v headers cracking on the join to the collector a couple of months ago, and everyone on the BMW-tech mailing list was very helpful on what appears to be a common problem with this model and the K1100. I welded them up so I could get to work, and pass the warrant of fitness inspection, but as everyone said, they cracked again, and in fact one broke almost all the way through and had a piece the size of a small fingernail missing on the way home from work before Christmas. I took the bus into work while I stole the ideas given previously by someone on the list about using stubs of pipe as sockets. Out with the calipers and the stainless steel suppliers handbook. I thought some of the others might find it useful to see what I did. Excuse the mess and welding - I don't do this stuff for a living...
First problem is that the steel suppliers stocked 6m lengths, and were closed for the Christmas break, and worst of all, the cheeky sods wanted money for it as well - a bit of an issue at present... Fortunately my Father in law had a 300mm long piece the right size. The standard header pipes are 28.4mm OD, and the next size up stainless tubing is 31.8mm in 0.9, 1.2 and 1.5mm nominal wall thickness. My piece is 31.8mm x 1.1-1.2mm wall, which gave just enough clearance without being really sloppy.
Tools required:
Old tig welder with SS filler wire
Angle grinder with grinding and cutting wheels.
Belt sander linisher
Air (die) grinder with grinding wheels and rotary burr
Hacksaw or Cut off saw
Misc clamps, vice grips, hammer, spanners, etc
Maniseal manifold cement.
Leg of an old pair of jeans to protect the shiny muffler.
I removed the exhaust, with one header pipe hanging on by a thread.
The other headers were starting to crack again - they are very thin - the K100 doesn't get it's porky weight from the front half of the exhaust...
The remaining headers were cut off with a 4" cutting wheel in an angle grinder - nothing like taking to your expensive and irreplaceable exhaust with a grinder to make sure you are awake! Clean the end up with a grinding wheel and belt sander so it is flat.
The remains of the headers can be seen in the end of the exhaust - they are not round but have been squared up in the manufacture of the exhaust. The new stubs didn't line up well at all - they may be better if the cut could have been made further out. These pipes butt up to a baffle divider that makes the pipe effectively a 4-2-1 exhaust header. I cut out the remains of the headers in the exhaust body with a small grinding wheel and hacksaw and cleaned up the remains with an air die grinder. I welded in a 20mm wide, 3mm thick piece of mild steel with an appropriate wiggle to match the baffle to extend the baffle to come flush with the end of the muffler body as per the pic. I didn't have any stainless, and by the time mild steel rusts out, I won't care. It does mean my headers are 20mm shorter than original with the secondary pipe 20mm longer. I don't know what effect this will have, but it has to be better than the broken pipe.
I tacked the stubs together into pairs and lined them up on the exhaust by eyeball, clamped and tacked them together, but not onto the exhaust yet. Next step was to fill in the 2 triangle pieces where 3 pipes meet, using a bit of SS rod, ground to fit.the hole. Once the end is welded fully, grind off flat and offer it up to the end of the muffler and make sure it is lined up, level and with minimum of gaps so you can get a neat weld when it is fitted.
Tack in place and try it on the bike.
If everything is good, weld it all the way around the outside, making sure to get into the V where 2 pipes meet.
As the stubs don't match perfectly with the remainder of the header or the outside of the muffler, I laid into with with the air grinder and a rotary burr to match as best as possible.
Gave everything a polish, and washed 15 years of dirt off the inaccessible bits of exhaust
.
I had to wait until the following morning to go and get some manifold cement to seal the joins. When you fit the exhaust, having a helper is easier, otherwise it is like jelly wrestling an armful of snakes. I found it easier to mount the exhaust in place then coat the end of the header pipe with cement and fit it up, starting from the back.
Clean off any excess with a wet rag and let the bike idle for 5 minutes to set the cement and check for leaks.
All in all , it probably took the best part of a day, although if I did it again, I am sure I could do it a bit faster. The final result appears to be solid and an excellent repair and the bike runs noticeably better and quieter without the cracked headers.