Breaking downpipes

Hi - can anyone shed any light on my breaking downpipes? Off and on we've owned commandos for many years - MkII 850cc. They've had balance pipes and the connection to the cylinder head has been by means of a flaired (swaged?) end clamped in position by collets behind the big nut. No problems! They last for years and give no trouble at all.

We decided we needed some new ones and were told that the 750 type with no balance pipe and a big lump on the end to locate behind the big nut was the way to go. After 3 months or so the big lump on the end broke off and we had to continue our journey up the motorway with the downpipe tied on with bits of wire. 

It was replaced free of charge by Norvil (bless 'em) but the replacement also broke in the same place after a similar length of time.

I'll try someone else, I thought, and bought a pair from Andover. A different design - this time the main tube went right through the lump on the end and was welded at the outer end. It's lasted longer but now this too has failed. The weld has cracked through and the tube, having worked its way out, is now hanging lose.

It's getting beyond a joke. If we were able to make downpipes in the 70's that lasted for ever, why are we reverting to something that lasts 3 months or so. I suppose its cheaper, but I would be prepared to pay the extra for something I could rely upon.

Any ideas....?

John

Thanks for the response. Yep, engine's fine - as smooth as you like and shimmed to perfection. Rubber mounts holding the silencer on. I mentioned my problem this morning to a local workshop that just happened to have a beautiful red interstate on the ramps and they nodded wisely and said it was a common problem.

My guess is that if the bike doesn't do many miles then it's not seen as a problem but I use mine everyday and on long journeys if the opportunity arises, so it's a bit of a nightmare. If I could find a seller of the traditional design with swaged ends and balance pipes I would go back to this and problems would be over.

thanks for trying - John

I've not had this happen on my Commandos, but that actually surprises me! I think that the design is flawed. The only secure mount is where the pipe fits the head - everything else moves around. The rubber silencer mounts fail with monotonous regularity, increasing the amount of movement of the silencers (particularly if the heavier black-caps are fitted.)

So the pipes want to move, and this strains the fitting in the cylinder head. Little wonder that it either comes loose or breaks something.

One day, I'll make some brackets to connect the pipes to the engine/gearbox mounts, somewhere underneath the 'box. The pipes will then not be able to move at all at the head, but will be fixed with the engine/gearbox assembly. No doubt this will have some effect on the silencer mounts, but it would be an interesting project!

Hi - thanks for the advice. I've ordered a  set of MkIII pipes from RGM which apparently have nothing but a flaired end and a set of collets for fixing them. If there isn't a problem with welding a big lump on the end, then I wonder why Norton made the change?? Perhaps an ex-employee can tell us....?

cheers - John

There was a spate of breaking downpipes amongst some of the lads up here, some put it down to the wrong silencer brackets (The Mk3 ones are different and I suppose then so are the ones which supported the annular discharge silencers) Worth having a look round at the International, I'm sure there will be plenty to compare with!!

Downpipes, 750 type:

I can only speak for Andover Norton's offerings, so can not comment on what most others sell.

The "big lump" (flange) at the front is brazed in. Unfortunately, using a firm that has been making exhaust systems for decades did not save us from them coming up with some virtually unbrazed. In our first batch from this supplier these just had a layer around the joint, but nothing flowed into the gap between pipe and flange as it should have.

We have since replaced the pipes that fell apart f.o.c.- it wasn't all of them, but a small part of that batch.

This is now about two years ago, if not more, and all further batches since have been fine. I even dared ride my Commando to work this morning fitted with those pipes!

I know of one (pirate) supplier who used very light gauge tubing, and of these many just fell apart in mid-section, not at the flange. Not something you want to experience, as there is danger of the broken pipe digging into the road and acting like a lever.

Whilst Andover Norton operates a quality control system for incoming goods, this flange brazing problem was, unfortunately, a mistake that could not be detected short of x-raying the pipes, and at the time we weren't even aware there could BE a problem!

Previously wrote:

Funnily this is the one part on mine that I've not been troubled with. When you bolt them up leave everything loose, just snug up the exh port nuts so the pipes are held in position, leave the silencer clamp loose and the rubber mount nuts then tighten from the head backwards. You may already be doing this of course! This way it all seems to find a happy unstressed position, well at idle or engine off!! Not had a breakage in 32k miles and the pipes are from Norvil (about 1999) though who knows who makes them for them. ( never had one break in 30yrs)

 

Exactly what I would have said Keith, but also he needs to match his silencers to the correct silencer brackets. He doesnt say what silencers he is using ?? If the bike started life with 'black caps' and he's now got 'pea shooters' he would have to change all the brackets.

Possibly try fitting the exhaust pipes then slipping the silencers on and see where they want to sit in relation to the brackets fitted. Dont force them into position !