Mechanical Exhaust-Thread Fix

[kraakevik]

I finally broke down and bought a 1 15/16 - 14 NS bottom tap which cuts threads to within 1/16" of the port or so. Does anyone have experience with Helicoil-type solutions to exhaust-thread repair? I've got an internally threaded aluminum insert which I'd rather not have to weld to my (naturally) just-rebuilt RH10.

Ideally I'd like to find a kit that lets my machinist drill out and thread the port to accept an insert that I can now tap for the exhaust nut. After 46 years of Norton exhaust-thread foibles (I bought my first 650SS in Africa when I was twelve) I'm finally ready to take the law in my own hands.

Many thanks for your help, gents



Tim Kraakevik
kraakevik@voyager.net
'72 Combat, spare 850 engine
630 474-9164

Also, does anyone have experience opening 750 cases to accept 850 barrels? I'm organizing my life to ensure that the Norton is never more than a weekend away from being back on the road

 

[ML]

Tim,

Just about every RH10 I've seen has had a machined brass insert fitted into the exhaust ports. They seem to take the exhaust header nut tension OK and never become a problem. I agree aluminium would be feeble, steel corrodes and does not expand equally with the alloy head as does the brass. Time to fit up the lathe and make some swarf!

Mick

 

[rvich]

Comnoz posted a repair thread back in August. It is not a kit (yet anyway) but you might find it useful.


documentary-exhaust-thread-repair-t4517.html#p39887

 

[Flo]

My last cylinder head had helicoils in it & it was perfect. The exhaust nuts were taken out & put back in a fair number of times, & it used to be thrashed a lot. The only reason I took the head off was because it started leaking oil somewhere else. If I had to do my present head, I would helicoil again. I wouldn't trust brass or bronze inserts or welding.

 

[pelican]

I noticed on mine there is an insert welded in, but only on the right side... The threads are ok until you get about half way in then they are starting to strip out. Can you put an insert in more than once? At any rate I'll deal with it when the valves need changed. Is there a problem with one side more than the other, or do you think the PO let the nut loosen up sufficiently to where the header vibration was grinding the threads down?

 

[RennieK]

Is there a problem with one side more than the other, or do you think the PO let the nut loosen up sufficiently to where the header vibration was grinding the threads down?

I think that's what happens, guys let them rattle loose too long. I never liked the big tab lock rings because of that. They would be loose and rattling but you knew the tabs would keep the nut from coming right off so you would leave them loose longer than you should. I drilled and wired mine and always kept them tight as soon as I noticed them even a little loose and I never had problems with threads.

 

[RennieK]

Also, does anyone have experience opening 750 cases to accept 850 barrels? I'm organizing my life to ensure that the Norton is never more than a weekend away from being back on the road

I haven't done this myself but have been considering it. I've done a bit of a photo comparison of some 750 and 850 cases and just started a new thread for this topic as it will show up in a search easier in the future. Here's the link:

750-850-cases-comparison-850-barrels-750-cases-t5674.html

 

[kraakevik]

Thanks to all who replied.

If anyone has a source for inserts, threaded internally, externally or both, please let me know. I'm not looking to break anyone's rice bowl--just interested in devising a mechanical fix for my own use--thanks again


Tim Kraakevik
kraakevik@voyager.net
'72 Combat, spare 850 engine

 

[comnoz]

Rennie is right . The original threads will last a long time if they are never run loose. Even a few miles will destroy the threads. Lock tabs don't help. The best thing I have used to keep them tight is a little silicone RTV sealant on the threads. It won't help if the threads are worn to the point of slipping but if the threads are good the sealant will keep them from rattling loose and will still allow you remove them when it's time.
Unfortunately welded in aluminum inserts don't usually last as long as original. The hardest weldable alloy is 6061 t6. It is similar to the original metal in the head in the T6 condition. The problem is it begins to anneal and loose it's strength at about 500 degrees F. and by 700 degrees it is fully annealed to T0 condition. Keeping the insert that cool when welding it in is nearly impossible so after it's welded in it has little strength left. They can usually be stripped just by tightening the nut a little too tight.
Helicoills work OK. The only problem I have seen with them is sometimes they come out with the exhaust nut when you take them apart.
Hard bronze inserts are the way to go. I have been installing them for over 20 years and have yet to see one fail. I keep inserts here but they require a mill and some special tools and taps to install them so I don't sell them without installation. Jim Comstock