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Siamese exhaust


Just about to order a  stainless system for my slimline 99  (inside frame) ,from Armours, has anyone had any experience  of these.?. Do  you finish up with a nice line or a droopy silencer?.


Robert..I have had siamesed on my 88SS for the past 10+ years.  Finally swapped back to twin pipes. It now runs noticeably smoother, especially at motorway speeds.  I holed one piston a few years back and I suspect something out of balance with the carburation at high speed..about 70 on A3.

The inside junction between the side pipe and main tube had a restriction which might have caused it.

It was an expensive experiment I won't repeat.  Maybe the Norton original was better made.

Also...if stainless, this doesn't apply...but with chrome plating the part between the pipes as they come together doesn't take plating very well.

Last time I checked with Armours, the web site ordering system didn't say clearly which cylinder head they fit (later wider splayed like 650SS  or earlier like the 99), but a telephone call sorted that out.  They do (or did) both. Do they do stainless? They used not to.

Since Norton only used the siamesed for a short time, I wonder if they weren't as good as they claimed at first? They save the weight and cost of one silencer.

Mine did fit well. No droop.



Morning David. I fitted a pair of chrome siamese  (from RGM)  about 25 years ago , Not a good fit then  and the chrome has gone  . I prefer the uncluterred primary area . They are a bit more rorty than twin pipes but I think provide a better mid range ( which is as far as I intend to go these days.)  The RGM set at least gave a little kick up to the silencer line . I am more interested in what the current offerring looks like (on a bike) and more to the point do Armours stand behind their product?  At  around £212  delivered I want to know what service they can provide if they don't fit.


I can't help in that case. Mine are for the splayed head.

I see from my notes I fitted mine in April 2001.  It says mid range seemed better but slower top end. It reached 85mph on the clock which I seemed to think was a bit slow.   I can't remember riding that fast very often!

Also note says "it seemed more vibratory  especially at low revs".  Since then they have been off and on.


It would be really nice to see  a photo of a bike with the system on. Would instill confidence that its going to look acceptable .  Would be good for sales.  Can't believe that a company that lives by its sales can't see that.


i fit one to a 1960 99. it a lovely fit . you have to send the rose you want to fit I removed a norton 

Siamese system which had been on the bike for years. It needed re chroming still have it. The Armour one fit as good


Slightly off tangent, but on the power gain/loss, does anybody have any objective experience in regard to balanced vs unbalanced pipes on Commando systems- I believe the relevant "correct" silencers have an effect?

Asking for a friend!


This is my 650 fitted with a Siamese system, sorry you can't see the cross over well, I took the picture last Sunday at a Show.

The bike is not fitted with an ss head but the Siamese fitted OK. (Bit tight getting it in but clearance all round.

The bike has been together for about 12 years now, but I bought the system a few years before that, in anticipation before I even had the engine. I bought the system new in a sealed plastic clear bag at a local auto jumble, a lucky find. The chrome is failing around the cross over but otherwise fine. The head is single carb and I think that helps with other running issues.

Never tried a two pipe job on the 650 but I can tell you that my 1960 99 runs much smoother on two pipes, though the performance on both is very good.. (Both single concentric's, 930 on the 650)

I must update with more builds, but if you go to my vmcc site you can see the bikes in question by clicking on 'Neil's Norton Notes.


Dunstall in 'Norton Tuning' writes that the best arrangement "by far" is to have a balance tube across close to the ports. I've not the slightest idea if the Commando ones might fit a featherbed.


Hi Neil,  Your 650 is a bit unusual in having the 1960 on  88/99  head.. This having the Ex ports at a shallower angle  but the same size valves as a 650 head. I think your siamese system is the same as my rusty set as I can see its got a very slight kick up under the kickstart. . I  think it most unlikely that a Dunstall two into one would fit this head David. When they built the 650 standard they used the Downdraught head with a special manifold which for the 650 DL  also had to contend with matching into the opening of the  88/99 deluxe panels  which they did not alter. This manifold must be a very rare item ( not the same as yours  Neil which is std for the 88/99). I am guessing that Neils bike was built up from parts. 


Hi Robert,

Yes indeed, I bought the frame (1960 99) in 2008 to save it from being turned into a real bitza.

I bought the engine, a 64 650ss (Re sleaved) in Spring 2010, just a few months after I bought an 58 88 that was the original plan.The 88 is now in my just finished Dommie Racer (Apart from the Tacho not working right.

The head came with the engine, but something was a miss when running. I found some witness marks on the push rods and in the head. These were dressed and it all ewnt back together.. Still a bit noisy at tick over and low revs but now with just over 2000 miles on the clock the engine noise is quite acceptable. The mudguards are stainless and I have fitted Boyer electronic ignition and a power box with battery.

The SLS front brake has a home made air scoop and works well. Rear shocks are NJB.

As said, I struck it lucky with the 2-1.Brand new and not bad for 30 quid...Worth the gamble.


Hi Robert,

Never had the barrels off so no way of knowing. (Only the head)

Then again the 99 barrels can be machined to fit a 650. Don't know about these...

But, the 650 is sounding better all the time. (I put it down to the none std head)

If it does pack up I will investigate.


When I first partially rebuilt my 1959 99 in 1965, in my correspondence with the factory (Mr. Hudson I believe) said that the 99s, in particular, benefited from small-bore Siamese pipes with an SS silencer - so I fitted a set of inside-the-frame pipes, but a standard silencer.  I still don't know the difference between the silencers.  I have never tried the bike with twins, but it came as a poorly maintained bike with rotten silencer (?from Pride & Clarke), so the only was was up!

Due to one silencer being particularly rotted away, it had contributed to partially melting the piston on that side side!  After it was up and running I discovered another fault that had also contributed to the melting - but that's a story for another day and a possible "Quiz" question!


…look forward to the rest of the story Lionel.


Best regards



Have you ordered the new pipes, Robert?

An observation...nickle plated pipes can avoid the permanent blackening in the event of overheating (e.g. from retarded exhaust).  It happened recently to a new nickle plated downpipe of mine...went black on the first outing, but polished out. (Careless with timing...).

I believe the darkening we are familiar with is due to the nickle oxidising, because the hard, thin layer of chrome is porous to oxygen.  The chrome then protects the oxide from being polished out (unless you polish so much that you remove the chrome). Obviously you can't do it very often....

So there doesn't seem to be much point in chrome on pipes anyway.  Armours will supply nickle plated at the same cost (it should be cheaper and less environmentally problematical).

Does anyone care to comment?  It's proved very difficult to find out exactly how the darkening happens. Even Wikipedia just says "because it's hot"...but we know that already.


Hi David,   No ,I chickenned out at the last moment ! ,  Something to do with the slightly sneaky way the enquiry went to  an order  while I was still  in the  making my mind up  mode.  May enquire at my local platers about a re-chrome. 


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