Anything to watch out for in repairing a convoluted exhaust ?

Sooo, been a long time, stayed away from PM on purpose, just had too much electric engineering and general life related stuff to take care of to be flooding my mind with really cool but less important mechanical knowledge

I have a Honda 750, the last version of that legendary series, the CB 'Sevenfifty' RC42. The same engine was sold in the US in a slightly different frame and called the CB 750 Nighthawk ,i believe.

Exhaust system is this monster:
https://www.totalparts.nl/picture/motorsloop/000000053364-1.jpg

Unfortunately that usually looks like this:

https://www.dd6vsk.de/fotos/0515-exhaust2.jpg

The crome plate on that weird collector/interference manifold dies and the thing declares itself holy....ehhh...holey .

Sounds kinda nice but impedes engine performance and i don't want a cop magnet for an everyday ride. Used exhausts are crazy expensive because they all die and aftermarket is even worse because the rc 42 is rather rare and German vehicle inspection rules are insane in 5 dozen different ways. Making it commercially unfeasible to offer good solutions to most engine related problems unless there is sufficient volume to pay for all the legal nonsense.

So, i will weld that thing or have it welded. At least 2 confirmed holes, both currently plugged with glass fibre mat, annealed sheet copper and a bunch of strong stainless hose clamps - which works well enough to be rideable but still has the exhaust pop and the engine braking behave jerky at low revs due to the carbs being not quite in sync with the unsymmetric, leaky system.
Probably more pinholes or weakspots at the very least.

I have access to: Stick(*chuckle*) and oxyfuel.

Very little experience with oxyfuel welding but i do braze a lot and to my big surprise i actually managed to weld sheet metal of similar thickness without much trouble. Sure, any real welder would laugh at them, but sheesh, that exhaust can't get any worse looking.
What has me troubled is distortion. Lets assume i weld up 2 big holes and 3 nasty spots that are almost blown with patches and oxyfuel. Will that thing still fit the bike ?
Should i have that mig welded instead ? Less heat in a smaller area......

Hello again, good to see you back!

A big problem with welding exhausts is that as soon as you start, you will find that there is more thin/rotten metal next to the bit you are welding. I completely re-manufactured a set of headers for a Yamaha RD200 (go on, laugh ) from 3D mitred sections of 35mm OD x 1.5mm 316 stainless pipe, MIG welded with Inconel 82 (I had the Inconel already). No spares were available from Yamaha because of the age of the bike. I would suggest a similar solution to your pipes:-

Make a new section from the 2-1 joints, through to the balance pipe junctions and the connection to the silencers. make it from 316 stainless and use 309 stainless (or better) as a filler. I would advise you to learn TIG or use MIG instead of oxy-fuel or stick (I've got all of them now and would use TIG/WIG, but the original job with MIG/MAG worked very well). Use split pipes and clamps to join the new piece to the old pieces.

Good luck!

PS:- my sister lives close to Evenkamp, near Löningen.

Zonko said:

....snip...
What has me troubled is distortion. Lets assume i weld up 2 big holes and 3 nasty spots that are almost blown with patches and oxyfuel. Will that thing still fit the bike ?---NO
Should i have that mig welded instead ? Less heat in a smaller area......

Click to expand...

From your pic, it appears there already may be numerous weld beads on the tubes and collectors

--have the affected tube ends and collectors gently sand blasted or bead blasted...or even prior
to that try punching thru thin areas with sharp awl..or other wise determine if there's any decent, close
to original thickness on the parts.
--best bet for more than 100 km. use--duplicate from new steel or SS.
Mig can create less total heat input than TIG--but requires adroit weldor that can actually make seal tight welds
in thin....99% of guys will blow/melt holes thru or have leaking.
If one really tunes up a TIG setup, going into pulsing variations, can get less total heat input and relatively
much easier to create seal tight joints, bead profiling, all the good stuff.
Forget the brazing. O/A welding can do good.
-to ensure fit, will need to create function fixturing to use for repair or complete new build

I won't laugh Mark, i'm one of those 'loves all bikes' people, none of that brand or type fanboy stuff. Still have my first proper ride, a 1950s designed 1980s built communist German (thats communist economy for you, the chassis did not change much in 30 years) 50cc scooter i bought 5 years ago or so, haha.
This: http://www.abload.de/img/schwalbe2lu3t.jpg
Next bike could be almost anything.

Which leads back to the sevenfifty. It has to last another 2-3 years and - say - 30k
At that point it will be worn out like hell with 110,000 km on the odometer and i will want something a little less tired.

It also only cost 1000 to begin with.

In other words: No...i am not making a new header from any material. Id rather have misshapen blobs of steel weld and brass spelter stick outta every inch of that pipe than spend any time and money on buying a tig, learning to tig and making an exhaust thats obsolete in 2 years.
Anyone who knows me is well familiar with my Germanic love for perfectionism but this is one of those cases that ask for a "soviet union" grade fix, to quote my ukrainian collegue. I want those holes gone, thats all.
What if i bolt all the Exhaust port flanges to a flat bar of steel before welding ? Distortion of the mounting points should not be an issue, just drill em one size larger.
Then just shut the corrosion down with whatever combination of weld and braze i see fit.

Id really love to do a proper job - but not this time guys
I just need to be sure that it will still fit the bike because a used POS system is 400 bucks and a half ass aftermarket fix 500 and a proper system 700 but illegal. None of that is acceptable because in winter you can buy whole jap bikes in ok condition for 5-800....

For a real bodge-job, how about cleaning up what's rusted by sand or bead blasting, then mapping the holes or really thin sections. Check for material that's collapse into the tubing ID and remove it. Make ~1mm thick conforming panels from a decent sheet steel that will lay over the damaged tubing, then braze those in place to whatever clean metal is present.

You may have to "build it up" section by section, starting at the best areas, then moving into the rotted tubes, using the new plates as foundation so you have a decent adhesion to at least some sections. With enough care (test fitting, figuring out the braze order, etc.), you might have something heavy and ugly, but functional. Then just ride at night in areas where the polizei aren't...

A good spraying with a high-temp paint may even make it presentable.

SteveBausch said:

Mac 4-Into-1 Canister System 81-261

Click to expand...

Illegal in Germany.....believe me, i would love the roar of a nice sporty 4 in 1. It could be legalized since this is a pre CAT bike, but it negates any cost and time savings by far.
Illegal ain't an option, this is my daily commuter/general use vehicle/ i don't even HAVE a car.

Any horrible bodge that preserves the original geometry is legal, however.
Duplicating the original geometry from SS would also be legal, but as i said, not worth it either.

Replicating with patches is what i wanted to do from the get go, yes miland, you understood me
I just wonder how much distortion i will get. Just lack of experience. Never welded a pipe system of any kind before. Have brazed a good lot of copper tubing but well, its copper, no distortion.

Weight and looks are no concern, most of that ugly manifold disappears behind covers and under the vehicle and at 230 Kg she is rather heavy to begin with. Best bike tuning in recent years was me loosing 25 kilos....

Edit: I think i will make a video about the fix....just because i can. Either to proove that it can be done or to show how it failed....

Actually - this thread deserves a follow-up. Because people never post a follow up, and i actually did make a Video. This work was done in October '17, been a while....LOL. I'm still too busy and on edge all the time - no time to indulge in PM.

I am no gas welding genious at all. But i had tried a few test coupons over the years and found that my brazing was good enough that my welding, while definitely rough and not to code, produced useable welds.

So a friend brought a rotten exhaust without any tubing attached to it. A simpler piece of kit than my kraken of an exhaust. Welded that for him and it turned out just fine.


So I kinda lost it that evening. I took no anti warping precautions whatsoever. Just welded the shit outta the thing to my hearts content *facepalming*. Used good new gaskets and managed to get it re-mounted without trouble. Despite that fact that i even had to remake the interconnect.

Here is how that looks:

*Disclaimer* This is NOT professional work and it will fail in other places because most of the manifold is almost gone. But it got sme paint and should now last another year or two. And thats enough.

Too much rust and no idea what I'm doing..... - YouTube

So, that's that.

One thing i got wrong was burrs. I left some rolled edge under a patch somewhere. Now the exhaust makes a whistling sound at 3k rpms....and i BET it's that disturbance causing it. Forgot my die grinder that day and couldn't grind that out before welding -.-
Doesn't seem to harm performance, though.