Exhaust Rose Nut Question

I'm finding my way around Commando ownership, and of course coming across issues that need sorting as they present themselves.

After putting the carbs back on the bike after an overhaul and during initial set up, the r/h exhaust nut popped off. I understand it's common for them to loosen. So, sorting that has now become a priority above setting up the carbs.

I was wanting to utilize as many of the original parts as realistically possible, but may need to look at an improvement over the stock alloy item (unless this can be made to work). From searches here on the forum, it seems bronze or copper plated rose nut are the way to go? Which look the closest to original?

My bike is a '74 850 mk2, but from viewing the A-N drawings, I don't seem to have the retaining collet 06.3990

Indeed, I can't even see a seal washer 06.3995 , but assume it's buried inside the head. The only thing there that I can see is the lock ring tab washer, so I'm unsure which header pipes I have (mk2/2a/3?).

When the nut broke free, it was covered in what I assume is HT RTV

.

Here's the right hand nut, which seems to have the thread intact, as I can tighten it on (but haven't fully pending new nuts or make these work with some high temp copper RTV - bank holiday here at the moment)







Any guidance, gratefully received.

Bonzo said:

...the r/h exhaust nut popped off.

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If it did simply "pop" out then the head thread is probably stripped? Sometime previously judging by the look of the pieces in your first picture.

Bonzo said:

My bike is a '74 850 mk2, but from viewing the A-N drawings, I don't seem to have the retaining collet 06.3990

Indeed, I can't even see a seal washer 06.3995 , but assume it's buried inside the head. The only thing there that I can see is the lock ring tab washer, so I'm unsure which header pipes I have (mk2/2a/3?).

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You wouldn't normally see the collets or the sealing washer until the pipe is removed from the head at which time the pipes should look like this...

Thanks for sharing your wisdom L.A.B.

Yes, it did 'pop' off, rather than slowly loosen and unwind. I was hopeful the thread was ok as it is taking nice & tight. At any rate, it seems pipes off & out for further investigation is the order of the day.

Oh goody. I didn't want to ride it in this lovely weather anyway

I wonder if this CHASER TOOL from RGM might help?

I use bronze 750 nuts on all my bikes (850’s too) and do not use cross-over style header pipes. Tighten them as tight as you can on a hot engine, and they don’t come loose. Of course, the exhaust threads have to be in good shape to start.

AN sell a plated bronze nut that is at least a similar colour to the 850 nuts, although they are the 750 form factor.

My $0.02 - FWIW

Bonzo said:

I was hopeful the thread was ok as it is taking nice & tight.

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It may just about hold when the head is cold but the alloy head expands at a faster rate than the steel exhaust rose so it could pop off again when the cylinder head heats up. The good thing about the bronze roses is they expand at a similar rate to the head.

Bonzo said:

I wonder if this CHASER TOOL from RGM might help?

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If the head thread has stripped or has worn away from the rose loosening then I doubt a chaser tool would fix it and probably needs an 'insert' thread repair.

I have MK2 850 and have been through this same issue. The balance pipe headers do require the collets and the shorter threaded rose nuts are then necessary.
When mine starting coming loose while riding, no amount of tightening seemed to keep them held. Went with an exhaust spring and safety wiring between the roses, attached to a fin on each rose such that a bit of spring tension was applying in the tighten direction on both roses. This seemed to hold OK for awhile. Ultimately I have the head fitted with threaded sleeves and new roses. Been fine now.

BTW, be very careful with the balance pipe connections. These setups seem to give cracks at the header branch and thought to be from creating excess stress between the headers along the balance pipe connections. Mine had cracked before I got bike so now I'm running non balanced pipes.

Also, when selecting new roses, I opted for the older type with the three wider fins, b/c trying to place the "C" spanner on a rose with only one wide fin can be near impossible depending on where the wide fin ends up in its rotation. So having three wides gives you more spanner placement options. Do not try to use the thin fins for torquing on and do not ask how I learned this....

Tornado said:

Also, when selecting new roses, I opted for the older type with the three wider fins, b/c trying to place the "C" spanner on a rose with only one wide fin can be near impossible depending on where the wide fin ends up in its rotation. So having three wides gives you more spanner placement options. Do not try to use the thin fins for torquing on and do not ask how I learned this....

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Are these the nuts you went for Tornado?

Si

Bonzo said:

Are these the nuts you went for Tornado?

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Similar. B/C I went to non balance tube headers, I needed the longer thread nuts, so mine are from earlier 750 parts list and the fins are rounded, not wedge shaped and angular at in your link.

I believe Jim Comstock has a video out on his channel showing his testing of various rose nut materials for expansion compared to the head alloy. Think he settled on stainless or bronze as closest to alu vs the chromed steel type.
RGM does a nice stainless rose nut I think.

CNW also has one without any fins, just two notches for a C spanner to latch into. Quite a clean look.

IMHO stainless and aluminum don't get along, as a simple explanation. Aluminum tends to stick to stainless gaulding as it is tightened or removed. Use an anti-seize when assembling the two. Stainless is beautiful but not always the replacement for chrome

I find a BMW R series exhaust nut spanner much better than my Norton C spanner. See https://www.motobins.co.uk/ item 99100.

Ibex said:

I find a BMW R series exhaust nut spanner much better than my Norton C spanner. See https://www.motobins.co.uk/ item 99100.

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I had one of those back when I had a couple of R65's. Of course I gave it away with the bike.

A couple of quid cheaper on the bay too.

eBay item number: 133155595884

I also gave up on the Norton C spanner after finding my BMW flat twin exhaust spanner fitted the original 850 lockrings as if it had been made for them and has gaps to accommodate the wide fin although it is slightly tricky to insert and remove at various positions due to the balance pipe.

The Moto-Bins etc. tool, below, however, doesn't fit (some/all?) other lockrings.

Which model Commando is yours L.A.B.?

Bonzo said:

Which model Commando is yours L.A.B.?

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850 Mk3. The original 850 exhaust roses/lockrings were the same part (with the single* wide fin) although 850 exhaust pipes/systems are not.

*Edit: The latest 06.3988 lockrings available from AN and elsewhere have grown another couple of wide fins.

After I asked I spotted the r/h rear brake.

Yes Comnoz tested various nuts and the RGM stainless ones came out top in terms of expansion coefficient match to the alloy head and quality / fit of the threads etc. They also sell an excellent spanner for them. I use them and highly recommend them.

Fast Eddie said:

Yes Comnoz tested various nuts and the RGM stainless ones came out top in terms of expansion coefficient match to the alloy head and quality / fit of the threads etc. They also sell an excellent spanner for them. I use them and highly recommend them.

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I have a coupe of those RGM nuts (063988) in my basket, although the spanners for the 850's are currently out of stock.

Good price too