https://www.gumtree.com.au/s-ad/ingleside/motorcycle-scooter-accessories/bmw-k100-exhaust/1175810732
Thought I best put the pic up so it can be seen in future

Having recently purchased a fairly low mileage '91 K100RS with the exhaust in (so far) uncracked condition and hearing of all the cracked headers I have a couple of questions.

First, where do the cracks appear?   Are they in the header pipes themselves or in the weld at the collector?  

Second, does anyone have a good idea of what causes the cracks?  Is it vibration, thermal expansion, or some issue related to the installation of the exhaust system on the bike like a misadjusted or missing mount.  My old '92 K100RS had three OEM exhaust systems fail in 4 years back in the early '90's before a Remus system was installed.  Not knowing the history of the new bike I am wondering if there is a problem in the future.

I have been through 3 systems on the K1100LT but that system has been modified with a front section of a K100 8V system put onto the K1100 rear section. You can see the clamps that come from a K100.  An interesting mod that I'd not even considered.

I noticed that as well.  My reason for asking was to determine if it would make sense to modify my system to head off the problem before it occurs. 

Right now I have an early model K100 exhaust with a damaged muffler that could provide the header pipes and clamps to do the mod.  My reason for asking whether the problem is vibration or thermal expansion related is that cutting the headers and clamping them would be a good way to relieve the stresses of unequal expansion in the header pipes. 

Not sure how to control vibration, I did see the recent posting about putting hose clamps around the header pipe bundle and have done that, but there is no proof that it will really help.

The hose clamp around the header pipes originated with police bikes. Apparently it changed the frequency of the vibration, not sure what state instigated it but I believe it was effective.
Regards Martin.

RicK G wrote:I have been through 3 systems on the K1100LT but that system has been modified with a front section of a K100 8V system put onto the K1100 rear section. You can see the clamps that come from a K100.  An interesting mod that I'd not even considered.

yes , looks like a possible solution on a standard 16 valve system rick

I'm thinking that if I was to do the mod before the pipes cracked, it would be possible to just cut the header pipes and install some replacements with clamps from an early K100 exaust.

I have some header pipes from the old exhaust I had on my bike.  Have to go out this morning and take some measurements.

I have a complete, un-cracked header/muffler from a '93 K1100RS with oxygen sensor and one small ding in the muffler. Seventy five green bucks plus postage to whomever needs it. Located near Tacoma, Washington.

Point-Seven-five wrote:I'm thinking that if I was to do the mod before the pipes cracked, it would be possible to just cut the header pipes and install some replacements with clamps from an early K100 exaust.

I have some header pipes from the old exhaust I had on my bike.  Have to go out this morning and take some measurements.

The pipes probably don't have enough space to do that. The K100 8V pipes are spaced a bit wider to allow for the clamps also I think the #4 pipe wont have enough straight section as it enters the muffler.

From what I gather it is a resonant frequency plus age that makes them crack. On my recently retired K1100 I welded a strap around all 4 pipes about 3 inches forward of the collector and haven't had any problem since but haven't ridden it for 6 months.

RickG, I find it interesting that you say that age is a factor.  The service history on my '92 K100RS indicated that three exhaust systems cracked on it in it's first four years.  I doubt any of them was more than a couple years old.

I am putting hose clamps around the pipes on my "new" '92 K100RS as was mentioned as the "police" fix.  One is about 4" or so in front of the collector, and the other is around the headers for cylinders 1,2,&3 and is between cylinders 3 and 4.  If vibration is a factor, I want to make sure the two longest pipes are damped as much as possible.

Vibration and resonance is a well known issue with stainless steel.

Unfortunately the same issue also affected stainless steel exhausts in cars which is one of the reasons they still use mild steel.

Splitting the silencer and manifold certainly helps because the 2v engines don't suffer the problem.

I looked at a lot of options including frequent replacement of the rubber damper mountings but that still doesn't guarantee a cure.

Plan B was to take it off and split it and get a collector and silencer. I wanted to eliminate the problem altogether.

However before I got that far a complete Remus for the K1100 presented itself on eBay at a buy it now price not to be refused as it was much less than the cost of a collector and silencer. Maybe I should say muffler as the Remus isn't exactly a silencer but its darn nice.

Would I go down the road of welding it up? No because it will keep happening.

92KK 84WW Olaf wrote:Unfortunately the same issue also affected stainless steel exhausts in cars which is one of the reasons they still use mild steel.

Not had a mild steel exhaust on a car for years! Think that you will find that they have been using stainless for about 20 years now. Stainless is not just one thing, there are many different specs of stainless steel, you should know that!

I'm with Bob; all of my Guzzis have had stainless sytems as has LFB and the Blandit. The only one that has given trouble and needed replacing was the Suzuki but that, by owner consensus, was down to poor design where the collector box meets the silencer pipe. One Guzzi exhaust system is now over thirty years old. Looking out of the window, it's now obvious that the wee Nissan Note sitting in the drive also has a stainless system.

I've had cars with stainless exhaust since the mid '80's and never had an issue with cracking. 

My '92 K100 had a Remus exhaust on it when I got it.  Took it off after a year because it was rather loud with a sharp bark to the exhaust note that I found embarrassing when riding through town.

My wee cars have mild steel.......German and French......

Maybe they send the cheap ones over here or maybe its because they are diesel........

92KK 84WW Olaf wrote:My wee cars have mild steel.......German and French......

Maybe they send the cheap ones over here or maybe its because they are diesel........

Forgot to mention that all my cars have been diesels since 1981 and all German or French.