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stripped exhaust thread 850


          fallen foul of motorway speeds coming back from camping holiday, looked down and Aargh! the exhaust nut round its ankles and on inspection, mashed threads. I used copious amounts of GunGum paste to get me  home, but clearly , something has to be done   .It has had a weakness for some time but coped with local blasts and wire locking, but the sustained speeds of the  A421 was too much for it. What to do? My go to Guru can"t cope with helicoiling ,  Norman White was suggested, but that would be mucho expensivo methinks.   Norvil was mentioned ,an oversize nut ? sort of thing , but looking in an old paper catalogue copy  I see  069383 Insert Exhaust Port Repair each   Not a Helicoil   . So is that A421 proof? can an idiot like me fix it at home with no special tools or great skill?


No its not DIY so it has to be sent out, Norman or


John is right, a big job for the inexperienced engineer.

However, I did manage to repair a damaged exhaust thread with an appropriate thread file. Threads turned to mush are beyond such a repair.

I always use the tab locking washers but only after the joint is fully warmed and tightened for the final time. (That will be after the repair job)


... used to do an on-bike repair with oversized bronze nuts. I rode my first Commando down to his place from Brum, he did the job and I rode it back the same day. Don't know if he (or his successors) still do the job - that was 40 years ago.


Once you have had your exhaust port thread repaired, fit the large washers with the tangs that bend over into the fins of both the cylinder head and the exhaust ring nut.  If you do it properly the threads will be A421 proof.


There are a few options for you, Seager engineering also Steve Maney (if he still does this), i think he does. If you can find someone who has the helicoil equipment you may be able to get it done without removing the head

I have repaired a couple of exhaust port threads using bronze screwed inserts.

I don't recommend using the tab washers, instead two loops of stainless welding wire with a spring inbetween, arranged to tighten the exhaust roses. This effectively 'live loads' them and prevents any loosening.
















Or tighten when hot before any long journey and take the C Spanner with you.

Yes I know it's a bit of a lump.

As an emergency, I once double gloved them hand tight several times.  Got me home without damage. Perhaps a shorter fulcrum tool for such occasions could be made?


    Thanks all for your replies, Norman White repairs the thread by building up the damaged thread with more aluminium , not as good as a brass insert but good enough and a darn sight  cheaper.   So I think thats the way to go . Truely I have tried the bent tabs and the lock wire thing and have found when push comes to shove they"re not up to it.     On long trips I always carry the C spanner, but this time I was caught  out.       The Spring Loaded loops around the exhaust tabs sounds worth a try, but how would it not foul /saw away the fins etc?


Since Mick Hemmings repaired mine decades ago this setup has never come loose and threads are still as good as new.

1. I use Bronze nuts as the expansion rate is nearer aluminium than steel. Use a thin smear of copper grease on the nut threads before inserting.

2. Tighten the nuts and then go for a ride with the C spanner, when the engine is up to temp I stop , retighten the nuts and rev the engine at the same time. The vibration seems to allow more torque to be applied.

The steel tabs are a mistake waiting to happen, they allow the nuts to come loose but never enough for you to notice until the threads are gone. NOC Commando Note confirm this isssue.



Had the very same problem recently on my wife's 850 MK2A.

So, I phoned Seager Engineering who were all 'Singing and Dancing' about how good they are but their estimated delivery time was begrudgingly given as 8 months. (They do this job in batches of 10 or so)!

As we had a trip abroad in 4 weeks' time that was as Good as A Chocolate Tea Pot!

Last night I spoke to someone at the Velocette Owners Club, and he gave up on Seager Engineering and left his head there.

I found a firm fairly local Saunders Engineering at Netley Marsh and explained that we needed the head back sharpish and 3 weeks later all done.

This gave me time to refit and test and all is good after our trip abroad.


I couldn't get Seager to answer emails but emailed SRM, quick reply, posted off and two weeks later returned with brass inserts on both ports and very good price 



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