New head for 1979 GS550 with bad spark plug threads?

[OneMoreGS]

I have a 1979 GS550 that I've been working on and bringing back from about 8 years of not running and neglected. It's got about 22k on it. I've got it mostly running on 3 of the 4 cylinders. I did compression tests on all warm and cold, left to right is 120-120-70-120. It somewhat appears to run, but #3 is bad. I adjusted the shims, no change other than they are in spec now and sound better. I've ultrasonic the carbs, and adjusted the float heights. I did notice that the threads on #3 spark plug are not original and have been poorly fixed. Dumped some oil into the cylinder to check compression and test the #3 spark plug threads. Just as I thought, a mist of oil coming out around the plug or compression brass piece I had in at the time. This would explain the chuf-chuf I can hear and sometime feel around that plug (have to tighten the plug pretty hard to no feel it) Ideas?

Other than a new head, I was thinking I could braze aluminum around the threads and then touch it up for flatness so the plug will seal. This seems reasonable being machine shop time is expensive. A used head on ebay is around $120 but is always a gamble on weather its in spec or damaged.

For the sake of it, the most extreme option would be to do a head swap with a 1980 gs550 or 1983 gs650 I found near by online.

 

[salty_monk]

Helicoil... You can buy kits. Supposedly with some grease & rags down the hole you can do it in Situ although I'm sure it's easier with the head off. I had one done on a car years ago when I was a teenager & it worked out fine.

 

[BeardmanSam]

Yeah, helicoil is probably the way to go.

Word of warning with them though, be very careful about over-torquing the plugs after you've installed the coil. The coils are pretty easy to krangle up if you go ham with the spark plug socket.

 

[OneMoreGS]

That was the other option I forgot to add. My concern is that there is already something in there, in the photo you can see knurled stuff on the outside. Looking closer down the hole its looks like the original threads. There is a bit of a dip from the surface to that ring that is knurled. Its not flat and is uneven as seen by the naked eye. From what I can see, it looks like the first few mm is this ring, then normal threads. If this was done already at some point in time and I drill this out for a helicoil or time-sert, will that even hold? Im wondering how much material I have left to work with assuming some prior repair was done.

 

[Grimly]

That was the other option I forgot to add. My concern is that there is already something in there, in the photo you can see knurled stuff on the outside. Looking closer down the hole its looks like the original threads. There is a bit of a dip from the surface to that ring that is knurled. Its not flat and is uneven as seen by the naked eye. From what I can see, it looks like the first few mm is this ring, then normal threads. If this was done already at some point in time and I drill this out for a helicoil or time-sert, will that even hold? Im wondering how much material I have left to work with assuming some prior repair was done.

I was about to suggest using a small magnet to see if the insert is steel, but some Helicoils and other inserts are non-magnetic stainless, so that's a wash.

 

[OneMoreGS]

Below is with the head off, you can see that the prior owners have replaced the cylinder 3 with an insert, thus the bad compression. Visually the head looks in great shape and so do the pistons. Cylinder #3 is bottom, #4 you can see near the top and is much better. I'm guessing this doesn't leave many options for me?