Bean Can Blues

No it's not the new title track of the latest Norton Rumble and Roll album, but it could lead to …

I had the blues, so bad one time
It put my face in a permanent frown

Taj Mahal - Cakewalk Into Town https://www.youtube.com/watch?v=oBGbrFlWwsE

Recently picked up a fairly original MK3 with bean can silencers that had been in storage for 30 years. The bike runs fine and is very quiet relative to my MKIIa with pea shooters. I am writing now in hopes of getting an explanation regarding coloration of the bean can silencers. Normal experience with blue pipes suggest that pipes blue where the temperature is the highest (near the head) and as pipe temp drops further downstream the std chrome finish prevails. And this bike certainly has that. However, one of the bean cans on this bike is blue from the point it reaches full dia all the way to the bean can lid, and the pipe ahead of it is simply chrome like one would expect.




So a simple minded guy like me is left with the image of a blue pipe at header (expected), then chrome along downstream pipe length (expected), then entire full dia section of silencer is blue (unexpected - air leak and post-combustion after-burn?). No life experience of mine provides a basis to explain this phenomenon. Aside from saying the silencer got damn hot I am at a loss to understand why and what transpired to provide this result. However, I'm sure folks here are sufficiently insightful to set me straight on this one.

Another question on bean cans. Once upon a time someone relayed a rumor to me that the bean can silencers were designed by Prof Gordon Blair of Queen's University. Would anybody have any knowledge if Blair ever passed his hand over anything Norton and/or did actually have anything to do with the bean can silencers?

Thanks in advance for any knowledge that can be offered. Maybe in the end if I better understand the Bean Can Blues....

Now, I'm feelin' so much better
I could cakewalk into town

on a Norton of course!

The original bean-cans on my MK3 bought new in '75 cracked spirally around the taper that's from the inlet to the straight main body of the muffler. As I recall it was bluing a bit too. They appeared to be made from very light-weight material and not too consistently either. I think bluing on an original bean-can is the result of very thin material and iffy chrome quality. I don't think they needed to get nearly as hot to discolor as the header pipe.

I believe Dr. Blair developed the 2-1-2 Dunstall exhaust system amongst other things.

http://www.woodgate.org/dunstall/history.html

In the search for a more efficient exhaust system without additional noise, Dunstall worked closely with Dr Gordon Blair of Queens University Belfast, Ireland. The exhaust pipe design (two-into-one-back-into-two) was entirely Dr Blair's while one of Dr Blair's students, Sam Coates, and Paul Dunstall helped to work out the silencer design and dimensions.

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According to the patent information, the invention of the annular discharge black cap/bean can silencer design was attributed to John Favill and Dr. Geoffrey Roe.
https://www.google.com/patents/US388833 ... CB4Q6wEwAA


http://www.ozlaverda.com/suck-it-and-se ... -t194.html

The Norton 750 Commando, Roe describes it as "a rather nasty motorcycle", mainly because of stability problems. Roe and Thorpe approached NVT with the idea of improving the Norton, but the company wasn't receptive. NVT used Roe and Thorpe's annular discharge silencer design though, apparently because silencing was more important than stability!

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WZ507 said:

Aside from saying the silencer got damn hot I am at a loss to understand why and what transpired to provide this result. However, I'm sure folks here are sufficiently insightful to set me straight on this one.

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Is there a part number, or any other markings stamped on it?

L.A.B. said:

WZ507 said:

Aside from saying the silencer got damn hot I am at a loss to understand why and what transpired to provide this result. However, I'm sure folks here are sufficiently insightful to set me straight on this one.

Click to expand...

Is there a part number, or any other markings stamped on it?

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The bean cans have the part# 06-4170 stamped on their end cap along with several lines of text. Grabbing this "can end", which is presumably rigidly attached to the entire baffle system, and attempting to shake it showed that the blue colored silencer is somewhat loose in the outer housing (can definitely move it around and there is significant play inside the housing), whereas performing the same test on the opposite silencer shows no movement between the baffle system and the outer silencer housing. So, perhaps some extra air is being introduced into the silencer via this loose baffle assembly?

WZ507 said:

The bean cans have the part# 06-4170 stamped on their end cap along with several lines of text.

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06-4170 is the right part number, as the original T160 Trident bean can (71-4402) is outwardly identical but different internally.

WZ507 said:

Grabbing this "can end", which is presumably rigidly attached to the entire baffle system, and attempting to shake it showed that the blue colored silencer is somewhat loose in the outer housing (can definitely move it around and there is significant play inside the housing), whereas performing the same test on the opposite silencer shows no movement between the baffle system and the outer silencer housing.

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It's obviously rotted internally and is breaking-up, and has probably overheated because the exhaust gas is not following the proper route through the baffling.

Basically, "it's knackered"!

What ever innate construction factors made it prone to bluing, one thing that can heat whole exhaust length quickly to reddened bluing temps is a rather over advanced spark, > varrooom red to the bend, > varooom red almost to muffler and a few more varoooms > red to the exit till eventually noticed, often a bit too late. Drunken slurred slangy song would go good getting a "Blue Job" rub off.