Machining new exhaust threads

[amartina75]

I've been working on this for awhile now. I have gone back to school for CNC machining. I thought I would take advantage of having the use of the shop and fix some of mine and my brothers cylinder heads with bad exhaust threads. I have already made the jig that holds the heads in the mill. I am basically ready to mill out the old threads but I wanted to figure out the insert first.

I need to decide what material to make the insert out of, 6061 or something else? what size to make it, how thick of wall does it need to be?
How much of interference fit should it be? how much will the cylinder head expand when heated? should I weld the inserts in or do something else?

Any suggestions would be appreciated
Thanks,
Aaron

[amartina75]

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Pictures of the jig I'll post a better on of it finished tomorrow

[Jordan]

Some data here:
https://www.flickr.com/photos/galpalu/3 ... 514178329/

See photos to left and right of this one, Flickr photostream.

[amartina75]

thanks Jordan

I might try to contact that person to ask some questions. 200 C almost 400 F, seems pretty hot to me, but I don't know.
I will be doing it a bit differently then he did. My jig will hold the head in the CNC mill. The old threads will be removed by helical milling the hole. Then the insert will be fit with a slightly oversized ID (welded in?) then back in the mill, mill the hole to final size and helically thread mill the threads.
I need to figure out what alloy to use first because I need to buy some, of course I will need just over 2" od so I will have to buy 2 1/2"

[Rick]

I researched this a few years ago and thought this guy had the best solution- he works on Nortons.
http://www.nortonmachineshop.com/
Here's a video of his process:
https://www.youtube.com/watch?v=ZPQc-mHtkiA
If you do a web search for 'norton exhaust thread repair', you can spend hours reading arguments for other solutions, but I came away convinced that shrinking/welding/staking/threading an aluminum insert was not the best scheme.
If you decide to build the tooling to do this, I'm sure there would be lots of customers.
Rick

[Jordan]

A big advantage of a threaded insert is that it's more discreet as it doesn't need to be welded, which is difficult to hide.

[amartina75]

Thanks Rick,
When I first started looking for information I did see a lot about Norton heads, more then Ducati for sure. I also saw The Comstock website. He definitely has it down to a science and he should if he is doing it professionally.
It looks to me like the Norton heads have more meat to work with there and doing a double threaded insert would be easier on a Norton then a Ducati.
One of my heads in particular was bored off center from the factory so there will only be about 2.5mm left after removing all the old threads. Assuming a 1mm thread height and that doesn't leave much left.

I went to the scrap yard today and bought a 7 foot piece of 2.5x1.5 inch hollow bar aluminum 6061-t511
1$ a pound can't be beat. I hope it's a good alloy choice

Here are a few pics of the finished jig and one showing how off center my threads are

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[amartina75]

I'll have to look at the measurement and see if I could do a threaded insert. I do have a short piece of aluminum bronze, enough to make two inserts. I was going to make an oversized exhaust nut with it. One advantage with the CNC mill and lathe is I can make any thread I want.

[Rick]

I like Timesert plugs better than Helicoils- the Timesert website brags about how thin walled their inserts are because of their 'synchronized internal external threads'- an idea worth copying if you make an internal/external insert- there's a picture on the site- figuring out how to synchronize the threads should be worth extra credit in class.
http://www.timesert.com/
The aluminum bronze also has a thermal expansion rate that's much closer to the exhaust nut- I think they're plated brass, so the nut and insert should be less likely to move away from each other and loosen.
Your fixture looks plenty rigid, should work fine.
Rick

[Nick]

On the off chance that everyone hasn't already read this 2011 post from Bob on the subject... (Personally, given the complexity of the machining operations, my thought was, like Bob's, to make a different ex. nut ring with slightly better threads that could grip the worn threads in the head. Or, plate the threads of the old ex nut so as to add some 'meat' to the threads, then, after installation, lockwire the nut to prevent it from turning and loosening.)

Bob's thoughts on the subject:
_ I'm led to very much suspect that your stock ex.ring-nut's threads are also rounded-down, rather badly ! _ Cuz I've seen cyl.head ex.threads in far much worse condition than that which you're showing (above), and yet a NEW ex.ring-nut still grabs the cyl.head ex.threads quite well enough !
Those threads you've pictured are certainly better than many others I've saved, with the use of a new MotoGuzzi ex.ring-nut !
Providing that your posted-picture's view shows the worse side of your cyl.head's ex.threads, then YOUR threads are way too good to have wasted (as you're intending) !
__ The Guzzi ring-nut's threads are taller & sharper-pointed than a Duc.single's, and thus bite deeper into the cyl.head's ex.threads !
Many times after fixing-up really POOR-threads on a Duke-head after fitting a Guzzi ex.ring-nut into them, afterwords I found that a NON-worn stock Ducati ring-nut would then work quite well enough !
With considerably far worse threads than you have, I've first taken & cleaned-out all dirt from within the threads, THEN next very CAREFULLY inserted a Guzzi ring-nut into the ex.port-threads (a little further each try) until threaded-inward all the way in, which after final removal, seems to refurbish the original head-threads so that a new (or GOOD-used!) stock Ducati ring-nut fits & works very near perfectly well !
YOUR cyl.head ex.threads look to me to be like they would do very well after such a thread clean-up job !
You should at least clean-out those ex.threads and try a new stock ex.ring-nut, before bothering with the expensive repair-work you're considering !
Also, I'm quite positive that a new Guzzi ex.ring-nut would very strongly work quite well in those threads you have left !
____ And by the way, is that cyl.head you have special in any way ? _ If not, you could no-doubt get another Duke-head (with better ex.threads) for less than you're willing to pay for that special ex.port machine-work you've been considering !


Dukaddy-DUKEs,
-Bob