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Double-wall exhaust headers: modification question

Discussion in 'Modifications and Projects' started by mattb, Dec 5, 2012.

  1. #1 mattb, Dec 5, 2012
    Last edited: Dec 5, 2012
    G'day All.

    I want to cut off the welded-on mufflers on my Kwaka W650 in order to fit some slip-on mufflers. I am trying to do this cheaply and with my own very limited skills and tools. The challenge is that the header pipes are double-walled (two pipes, one inside the other). I will not cut them until I am ready to complete the project on the same day because it's a high use bike. My assumption is that after I cut each header pipe I will have to secure the inner-pipe against the outer-pipe otherwise the inner one will move about and even vibrate against the outer pipe. Also there is the issue that exhuast gas could leak into the space between both pipes.

    Somebody on the internet has measured the specs as: the OD of the inner pipe is 32mm, the ID of the outer pipe is 38mm (though there is discussion about a possible error and that it might be 39mm). I have found a spacer that matches those first specs, here.

    But I am wondering whether there is not a more flexible means of securing the pipes in place and sealing them. I know nothing of such things so I am looking for ideas. Could I use some sort of silicone, or rubber, or something else, that I can generously place in between the two pipes to seal and secure them?

  2. Silicone or rubber are unlikely to survive given the temperatures involved.

    Bronze spacer's not a bad idea since it'll expand with temp more than steel and should create a tight seal, but will transfer any vibration from the inner pipe to the outer (though plenty of bikes cope just fine with a single walled exhaust pipe).
  3. What ate the pipes made of? Why don't you just bolt up after the header and avoid cutting???
  4. Because his mufflers are welded on
  5. You could use something like a manifold cement, it expands & hardens when you heat it, but I expect the brass collar would be best option. Maybe both?
  6. First off, it's worth getting a handle on why double skinned headers are used. TThe primary reason is to prevent the chrome finish from blueing from the heat. The secondary reason, which has only become important in recent years, is that it reduces noise breakout through the pipe wall. There is also an argument for retaining heeat in the exhaust gases for performance reasons, but we can safely discount that on a bike like the W650.

    Bearing that in mind, a small leak from inner pipe to outer is of no significance whatsoever. There will be almost no actual circulation of gas in the air gap and so the temperature of the outer tube shouldn't rise by much. There may be a slightly increase risk of inside-out corrosion but, again, I wouldn't get too excited about it on a bike used regularly in Australia.

    The bronze bush is good, but it's probably overkill. Easy, though, assuming the dimensions are correct. If you want to go with a solid, metal spacer, I can spin you up a couple of mild steel ones on the lathe for the price of postage. I'd just need the dimensions confirming.

    I wouldn't trust exhaust putty as anything more than a seal. In my experience, if you try to fill anything large with it, it's just too brittle and breaks up fairly quickly with the expansions and contractions of the underlying metal.

    Silicone rubber type stuff may or may not work. As jd said, the temperature may be a problem. OTOH I know of a number of Eastern Bloc two strokes which use a rubber ring to seal their silencers to their downpipes, which works fine and quite reliably. They can be a bastard to get into place though, resulting in mine using silicone RTV gasket sealant for the same purpose, which was also fine.

    Good luck, and let me know if you're interested in the steel adaptors.
  7. #7 mattb, Dec 6, 2012
    Last edited: Dec 7, 2012
    Thanks everybody for the responses.

    Pat, that would be brilliant if you could make those spacers! I was checking out my dimensions on the W650 Yahoo group and somebody mentioned he received header spacers with some expensive aftermarket Japanese mufflers he purchased. From what I can tell he never used them in the header, so they weren't tested by him, but he did measure the spacers themselves at the time - see below. I see that the ID and OD differ from the suggested specs which I posted above (taken from somebody else - I see there is a debate that suggests the OD might be 39 rather than 38, and that concurs with these spacer specs) but I would assume that the popular aftermarket supplier (Posh-Faith) got it right.

  8. #8 mattb, Dec 7, 2012
    Last edited: Dec 7, 2012
    Looking at those spacer specifications, if I am reading it right theylist the thickness as 3.6mm, and yet an ID 31.75mm and OD of 39mm equals, rather, 7.25mm of thickness. I assume it is likely that the ID/OD specs are correct and that the thickness measurement is the one in error, but I have emailed the fellow to notify him of his error and see if he has the same opinion.
  9. OK, I'll have a look in the shed tomorrow and see if I've got some suitable steel (I'm pretty sure I have). Looking at those drawings, they're a straightforward turning job so it shouldn't be a big sweat and it'll give me an excuse to fire up the Shun Shin for the first time in ages. Lrt me know when youget final confirmation and I'll start cutting metal.
    • Like Like x 2
  10. You're a legend Pat! Thank you. Will do.
  11. When the Yamaha XS1 was first released, we had a customer bike which didn't breathe well on one cylinder. After much head scratching we found one header blocked because the factory had welded the inner to the outer at BOTH ends. The exhaust heat & nowhere to expand caused the inner to buckle in & block the pipe. Just thought you'd like to know.
  12. Hmm. Might be worth drilling a small hole at the lowest point in the outer of each downpipe. It'll let any condensation drain too.
  13. What about I a hacksaw a 1 or 2mm-deep slit in the spacers to combat that possibility - would that be enough?

    When I get home tonight I'll open a new thread about mufflers - a few ideas, including a good option with a non-contactable seller - I'll bounce them around with you guys.

    Thanks again, Pat, for the spacers.
  14. It would, but it means you have an opening between the exhaust and the air gap in the double wall. Whilst, as noted above, I don't think this would be a problem, I; personally, would prefer to vent the gap to atmosphere instead. If you can't drill it (and I know that drilling a shiny tube can be a sod to do with hand tools) a small saw cut in the bottom of the tube outer just ahead of the spacer would serve.