Video of Exhaust Thread Repair

[Duccout]

Hi Brian,

I believe that the usual method is to cut a larger thread in the head and screw in an insert that has the normal thread on the inside. I have seen this done and it looks superb, but possibly would require a very high degree so engineering skills. The welded insert method was not something that I had seen until Nigel Lacey pointed it out in a thread on this forum, and it certainly seemed a simple solution, but having watched these videos it doesn't look as easy as I thought!

Gary at Middleton Engineering has since carried out the same repair on a Ducati 900 head for me, so he is getting some practice!

Colin

[DBDBrian]

Hi Colin
Yes it would certainly be a great deal more work to make a threaded insert, plus the need to remove more material from the head to accommodate it, as it would need to be fairly thick, unless the inner and outer threads are timed, which is very difficult to achieve. Much easier to make an oversize sleeve nut, especially with the C-spanner slots rather than with the fins.
As said just my thoughts, as I haven't actually done one.
When repairing the BSA hubs, I always make an oversize bearing retainer, as this can be made to whatever size the repaired internal thread finishes at.
At the end of the day, I suppose it's all down to what we have at our disposal.

[Jon Pegler]

Mototrans used an exhaust nut with c-spanner slots as standard, rather than the finned item used on the Italian bikes.
It used the same 48 x 1.5 thread as the Italian machines, but I think that the Spanish machining may have occasionally gone awry.
I've come across a number of Spanish cylinder heads with an oversize exhaust thread of 50 x 1.5. One of my Forzas has just such a thread.
The c-spanner slotted exhaust nuts are not too difficult to machine up. I have made quite a few, usually in stainless, to save them rusting and avoid having to get them plated and ending up with an oversize nut.

Jon

[Duccout]

I think that if I had known how much work and expense was involved in the fitting of the thread insert, then I would have seen if Gary could have cut a bigger thread and made a ring nut to suit, because it would possibly have been cheaper, and less risky, but AFAIK the insert method is what Nigel Lacey uses and he must have tried different fixes.

[Ducadini]

The repair in our head was done about the same.
First we fitted the head on a steady so that the flat, where the exhaustpipe presses against, could be very lightly machined (just to establish a correct angle).
Then the port was reamed (as in the video), a insert was turned but just to hold the nut, NOT a flat for the pipe. The heat should be taken away by the pipe, not the head, so as little contact between pipe and head is necessary.
We feared that the thin (1-2 mm) aluminium (as in the video) would burn away and create other problems (like the irregular shape of the exhaust port after cleaning the insert in the video).
Then the head was heated and insert fitted, and filed flat/flush on the outside.
To hold it we drilled three screws M4 to prevent the insert from being pulled out and/or rotate.
Since the insert heats up more rapidly than the head, it tightens. Same when the bike cools down : the head cools faster than the insert and clamps it.
We only didn't tighten the exhaustnut as hard as before, it was wired anyway.

On one of the singles, our friend (Lathe-man) made two oversize taps in high grade steel to clean up the butchered thread in the head. Then he made a slightly oversize nut. Not original, but efficient (and even looking "race-cool").
Luckily we didn't have to pay the manhours made on this repair, just cost us a good meal at the local Italian risto.

ciao
ducadini

[Jordan]

insert was turned but just to hold the nut, NOT a flat for the pipe.

How was a seal between the head and exhaust pipe achieved?

[Ducadini]

Jordan, just on the cleaned up original spot of the head, we didn't machined it deeper, just touched while setting the two angles.
And then just fit a standard seal.

[Jordan]

Jordan, just on the cleaned up original spot of the head, we didn't machined it deeper, just touched while setting the two angles.
And then just fit a standard seal.

Thanks, I see now you mentioned it in your earlier post, the difference from MidEngineering's method is that there is no inner face made on the insert.
When the damaged thread is removed by boring, the original face can be cleaned up afterwards at the same machine setting.
That will ensure it being square with the insert's thread - a good thing for improving the exhaust gas seal.
It may improve cooling, but also has advantages of avoiding a potential failure point (the inner face on the Al insert is a thin flange), and it's simpler.