Exhaust bolt hole not only stripped, but almost completely destroyed by PO

[bwringer]

There are a few more things you can try before giving up and pulling the cylinder head.

What size are the large threads here? I'm talking about the threads that are only present for about 2/3 the circumference:


If I'm understanding things correctly, these would be the exterior of a failed Helicoil.

Is the diameter a bolt size or an external Helicoil size? Is the thread pitch 1.25mm or 1.5mm?

If you can get a bolt to thread in, then you could make a plug. Drill and tap the center of the bolt, thread it into the cylinder head, then cut off the head of the bolt. Then you have solid steel M8 threads.

If the OD isn't a bolt size, then you could possibly do the same with a bolt inside a helicoil.

For example, if that mess was an M10 helicoil, then install a helicoil (it'll be janky and loose, but a bolt inside will help keep it in place), thread in an M10 bolt, then cut off the head of the bolt and drill and tap it. Or, thread in a double-ended stud that's M10 on one end, M8 on the other.

Or, you could use a metric step stud that's M10 on one end, M8 on the other:

https://www.belmetric.com/metric-step-studs-6810mm-c-9_73/?zenid=9v433cpio60i79g6o97hnlmvl3

If the threads are loose, you might also be able to make a sort of expanding anchor out of a bolt or a slug of aluminum.

[roeme]

That would make a spectacular paperweight

(...)buy a good condition head, quick valve job, new seals, bang it on, (...)

(...)but an ebay head can be had for less than $100 and(...)could be less than the welding, drilling, tapping, etc.

I'm not ruling out replacing the cylinder head. I'm currently leaning towards welding the old one up since I may be able to find a welder who is willing to do it for a reasonable price. And apart from welding, I'm able to do the rest of the work by myself.

Not so, or not yet, for replacing the head. What I'm missing for this route is experience, a head, a clean, heated working space and quite possibly some tools.

Well, I could have a shop swap the head for me, that would leave me with the task of getting a head (while the shop owner knows the bikes well, he doesn't acquire second-hand parts, but will use what I bring him). I don't know the head compatibility between the various years, I have only found heads for later years so far.

This will easily exceed the price of the welding option. I'd expect the shop to charge at least $750, if not $1000 for swapping the head. While I can manage that...it's more of a fall-back option.

Just to ask, what are the timeserts made of that you have in mind?
(...) with ally welding the metal has to be completely and perfectly clean.
All this is finally down to the welder, perhaps take the job to him and ask what he is able to do and work around that?

I have no particular timesert selected yet. Regarding the clean-up I will ask the welder, as soon as I found one. But I'm sure one can't properly weld the way it currently is.

What size are the large threads here? I'm talking about the threads that are only present for about 2/3 the circumference:

(...)If you can get a bolt to thread in, then you could make a plug. Drill and tap the center of the bolt, thread it into the cylinder head, then cut off the head of the bolt. Then you have solid steel M8 threads.(...) If the OD isn't a bolt size, then you could possibly do the same with a bolt inside a helicoil. (...)

I really like your idea! This would give me enough time to prepare for a proper, permanent fix, while still being able to ride the bike.

I have to measure them again for the diameter and thread pitch. I think it was 9-10mm when I checked it out of curiosity.

To recap, my options at this point, pending further research:

1. Repair with a weld & insert
2. Have a shop swap the head
3. Try to re-use "external ⅔ (more like ⅝) remaining" threads (temporary fix)

Does anybody know which years' heads would fit on my '78 engine? I would expect until '81, '82 look a bit different...

Thanks y'all so far your input.

[Phred]

As Tatu says, stainless and ally do not go together well. There is a reaction that happens between these two with heat. I thought of doing the same thing a few years ago then read a thread here that explains what happens when these two are combined. It has been a long time now but I ended up using steel inserts instead of stainless.