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Next question.

I noticed that the rear exhaust pipe was leaking slightly where it exits the head, easy fix I thought just tighten the exhaust collar.

On closer inspection I could see that the collar was not screwed squarely into the head, it also looks to have a much coarser thread than the one in the front cylinder which is sat in nice and squarely.

The castellation's in the rear collar are also a different than the one on the front cylinder.

My assumption is that an incorrect size / thread collar has been used on the rear cylinder (500 collar perhaps), suspect that the threads will be shot so it will be a head off / repair the thread.

I have a local machine shop who I know will make a very good job of the repair - they have been going for years and I have had loads of precision stuff done by them over the years.

Guess there is a slim chance that the threads might still be useable as the rear collar is not screwed in anything like as far as the front one.

I'll pull it apart this evening.

That's bad luck! Yes those threads are a bit delicate and they easily get messed up by well meaning owners. Good that you have a decent engineer on call and the gland nuts are easy to find new or used. 500 nuts are larger diameter.

Taken the exhaust pipe off, once I removed the silencer and balance pipe the downpipe was pretty loose in head and you could really see just how out of square the collar had been screwed in - only hanging on by few threads, surprised the thing was not backfiring.

I have found that if I carefully offer the collar up to the head, I can get it to screw in squarley, but it gets pretty tight.

It will almost go in far enough to trap the pipe against the gasket, probably a couple of mm short.

Thinking of possible solutions, I could make up some collets that are a couple of mm longer - that would work.

Other possible approach might be to take an old collar with a good thread and grind some flutes in it with a dremmel or similar to make it into a thread chaser - thinking being that the collar material (brass) will be harder than the threads so might tidy them up.

Could use 2 exhaust gaskets but that stinks of a bodge, and I dont like bodges.

What do you think ?

Well that didn't work.

Believe that the thread is M40 X 1.5 - can anyone confirm thread size?

If someone can confirm thread size I should be able to borrow a tap from work - if I can then I can see if that will fix the problem.

If it does not, nothing lost as the head would have to come off anyway to fix the problem.

The 350 nuts are 42 x 1.5, the 500 nuts are 45 x 1.5 as mentioned here...

viewtopic.php?f=4&t=766

Also, Paul Compton mentions the 42mm thread at 14m 20s in this video which you may find interesting...

https://www.youtube.com/watch?v=ZfFbA9Y3jHw

Paul is still the MRC Technical Guru although he seems to be more involved with sports cars these days. His contact details are in 'A Tutto Gas'.

Jenny.

Hi Andy
Are you sure that you have all of the parts? The order of assy into the head is gasket then exhaust, then split collets (2 parts like for a valve spring cap) and finally the screw in nut.

Cleaning the threads definitely helps as they tend to gather carbon; a right angle scriber works quite well.

The collets are defined as half rings in the parts book; part no. 12.04.09. Without them you will have the slack you describe.

Jen

Thankyou for the thread info.

Now I know what to ask for at work

Martha

Yes all parts are present and correct - the problem is that the collar just wont screw in very far because the threads are damaged.

Now that Jen has supplied the thread size and if you can get hold of a tap from work yo should be in business. Somebody here actually bought such a tap for his project but he's just a show-off as they don't come cheap! Also as Martha says you can chase the thread out with a scriber, I have one that I ground the end to a sort of chisel profile to do this! It's been used more than once, and another time I used a smaller tap of approx 10mm with the right pitch to do the same job. Mind you, I have also resorted to a 2 gasket bodge before now-it does work!

Don't mess around, get some bronze inserts fitted!





Trevor

Nice job - who did it ?

Need to see how I feel about my bike before doing anything like that to it.

Only been a few miles on it since buying it - mainly due to fettling it and the weather.

It was done by MDA Motorcycle Engineering in Adelaide, Australia. Martyn is a Norton guru, so he knows what he is doing!

A bit far for me here in the UK

I did persevere with the home made chaser made from an old exhaust nut.

If I screwed it in until it got really tight then used the collar spanner to move it in another half turn, then take it out, clean the threads, apply some lube and then repeat I found that I could get the collar to go all the way in without needing gorilla like strength / effort to do it.

The threads looked half decent by the time I had finished.

Its all back together now with a new gasket and of course the collets. The nut has snugged up really tight, so I wil keep an eye on it.

When I refitted everything to keep things even I measured the distance from the outer edge of the rear tyre to the outside of the exhaust downpipes, and also measured the distance from ground to the outside of the exhaust pipe - got them to within a few mm before tightening everything up.

When I went to refit the silencers I had to make adjustments to the holes in the brackets and spacers between the brackets and pipes to get everything to align without pulling the pipes out of line. Dont think these are OE silencers, unless they were fitted with Busso silencers.

The most important:
During the next rides allways tighten up the nuts with hot motor!!! You will see that you can turn them in euch time. When you cannot tighten them up anymore they are fixed.

http://www.seager-engineering.com/

https://www.facebook.com/seagerengineering

This place puts a lot of photos of exhaust thread repairs on Facebook. Their work looks top-notch!

Trevor

Be sure to get a turn around time if sending parts away.