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cracking exhausts

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Hi - the bike's a MkII commando from 1974 and has exhausts connected by a balance pipe. Standard in all other respects except for the large full length ex-police fairing which keeps me nice and warm and dry. Unfortunately, I wonder whether the exhaust are suffering from being too hot and this is causing the following problem.

My right hand exhaust has cracked as shown in the attached photo. This exhaust hasn't done many miles from new (2,000?). I've been biking many years and this has become a recurring problem.

Any ideas??

Best wishes - John (Cowie)

 

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A common issue mentioned a lot, I have never had it with my 74 MK2a but I went to MK3 balanced pipes with the spherical seats fitted inside the head so they can be moved around more for proper alignment back in the 90's. Also seen a tip where one of the 2 clamps on the short balance pipe is left loose to reduce stress on the pipes, will be trying that when I fit a new set of MK3 balanced pipes with better chrome than my current ones. Heat is not the issue.

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John,

I rode a Mk3 Interpol for a number of years with the standard balanced, mild steel exhausts without issue, albeit my fairing left the exhausts exposed to the air flow.  From your photo, with the foam wrapping around the down tubes, there does not appear to be much room for air to get through, though this might just be an optical illusion?

Given the location of the crack after the balance pipe joint, it would suggest that the stresses causing cracking have been generated between that joint and the silencer, rather than by the balance pipe?  How are your silencers attached - on the standard rubber mounts? Is there any contact or close proximity between the down pipe or silencer and a hard point that could be inducing resonance, taking into account potential isolastic flex?

Andy

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Thanks for your interest. 

The reason I went for the balance pipe arrangement was to include the spherical seats fitted inside the head that John alluded to - doesn't seem to have helped much though.

I will check the isolastics and see if there's a lot of slack and, yes, I am using the standard rubbers on the silencers. 

I know we  blame  a lot of present day problems on poor quality (cheap) replacement parts, but is it possible that this is what is happening here? We bought our first MkIIa way back in 1980 and I remember bouncing around corners with wildly floppy isolastics. We drove it all the way to see the Pyramids in Egypt and returned using the same old exhausts!

.....so what's changed? Were the originals made from different materials? Were they designed differently? If it is the common problem that John suggests, we could do with finding a solution or long distance touring will become quite impractical.

Sorry to be depressing - keep the comment coming. 

Best wishes - John

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.. but I'm sure I used unbalanced pipes on my 850 with the spherical seats.

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I currently use unbalanced pipes and have kept the spherical seats, that allows you to tuck the timing side pipe well in away from the kickstart lever.

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Unbalanced pipes with spherical seats...... this does sound promising, especially if it enables me to stop clouting it with the kick start!

Where do you get these from? The only ones I've seen have a large collar welded on the engine end which you clamp up tight using the exhaust ring nuts.

thanks again, John

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The large collar unbalanced version is the one I currently use, it still fits over the spherical seat and some realignment can be obtained unlike with the flat surface inside the head. I do have a set of balanced MK3 pipes with the flared ends to fit, they do not have the large weld next to the rose nut like in your pic, bought them from RGM who identify them as UK made and sold by Wassells.

https://www.rgmnorton.co.uk/buy/commando-850-balanced-exhaust-pipes-mk3_1297.htm

The bends and flared ends are exactly the same as the very rusty but uncracked set I took off some years ago but fitted back in the 90's.

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Thanks John - I'll give RGM a ring and will probably give those a go. 

I'll make enquiries about what sort of guarantee they can offer.

Thanks for the help - best wishes

John

 

 

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