When I retorqued head bolts, I realized that the right hand side, upper right bolt was relatively loose and pulled out of the engine block when I applied ~18 ft-lbs of torque (it needs 25 ft-lbs). To repair this, the stud needs to be removed. Best way to do this is “double-nutting” it and turning it out of the case.
The bore must be drilled to 13/32 inch or 10.3 mm, and a 12mm thread needs to be cut into the case so that a 12 mm thread insert can be fitted. I decided to use a Heli-Coil. There are other options as well (e.g. Time-Sert), both have their pros and cons.
Again, big shout-out to “Snowbum” who covers all of this in detail on his site.
It is important to pay attention to the oil supply opening in the upper studs, which under no circumstances may be blocked by the thread insert or any metal shavings. Oil is supplied to the rocker arm through this bore. Any obstruction here will starve the cylinder head of oil with possibly disastrous consequences. The small oil bore at the top of the stud hole is clearly visible.
Some folks suggest to block this hole during the drilling and tapping, however I left it alone and greased the drill and tap to catch the metal shavings. More on that below.
I would highly recommend to use a “drill rig” to get the thread perfectly perpendicular to the engine block. There are several options out there, HPD in Colorado sells a device (expensive!), Northwoods Airheads in WI rents it and that is the way I went.
Some of the BMW gurus drill this free hand, but I would advise against this if you don’t have tons of experience doing this.
This is the device mounted to the engine block. There are separate bushings for the drill and the tap. It took a bit of wiggling to get the device seated, it is very well made and a tight fit. Keep in mind, that earlier airhead engines have the 97 mm spigot opening, while models after 76/6 (I think) have the 99 mm spigot. Make sure to get the rig with the correct ring for your model.
It is very important to not get any metal shavings into the engine when drilling and tapping. I stuffed rags in the opening and greased the drill, drilled a few mm, cleaned the drill, greased again, drilled….etc.
I also used a very strong shop vacuum with a special nozzle to clean out the bore every time I cleaned the drill.
Some of the studs have blind holes, some holes go through. This particular stud goes all the way through to the engine case. The upper left stud has a blind hole. Not sure about all the others.
Tapping has to be done very carefully. I also greased the tap to catch the shavings. For about every turn of the tap into the engine case I reversed it for 1/2 turn to break the burrs. After 2 or 3 turns I removed it, completely cleaned out the grease and shavings, re-greased it and went in again. Key is to be careful, patient and to clean, clean, clean.
This is how the freshly tapped hole should look like. After drilling and tapping I used rubbing alcohol, since I had it at hand and Q-Tips to get all remaining grease out of the bore, before I put the Heli-Coil in.
This is the Heli Coil that goes in with the top part in the picture first. A special tool hooks into the tang and the coil can be turned in.
The tang needs to be broken off before the stud goes in.
It is important that a longer Heli-Coil (with 12 windings) is used. You may still get it from the BMW dealers. Mine came with the rental of the tool. The standard length, which I believe is 9 windings is not strong enough for this application.
I installed the Heli-Coil with red (permanent) Loctite. This is important since you don’t want it to turn in the engine case when you screw in the stud. I applied generous amounts of Loctite and then used the special tool to screw in the Heli-Coil. I made sure to turn it in far enough so that the oil bore was not obstructed.
A scary part was the removal of the tang of the Heli-Coil. This is done by hitting the tool with a hammer, breaking it off and most importantly not letting it fall into the engine case.
Immediately after installing the Heli-Coil, I cleaned out the Loctite that started oozing out. I used Q-tips and again, more cleaning is better. I let it sit for 24 hours.
Here is the Heli-Coil inserted into the threaded hole. The oil bore is unobstructed.
To be honest, I think I turned it in a bit too far, I probably could have left it out one more turn. I don’t think it matters much though.
The stud is supposed to go in relatively easily. I screwed it in by hand, I would not recommend to use a wrench with a double-nut, if it doesn’t go in, take a break and check what may be wrong.
I smeared a few drops of oil inside the cylinder bore and on the outside of the push rod seals. I used dielectric grease on the inside of the push rod seals. I used a thin layer of gasket sealant on both sides of the new bottom gasket.
I put everything back together, set the valves and went on a ride. After 100 miles there were no leaks and everything was working fine!
It is necessary to re-torque the cylinders and adjust the valves again at about 100 mls and then again at 600 mls, due to settling of the parts.
To eliminate as many unknowns as possible before attempting to torque the stud, I “scientifically” tested my torque wrench!
I used 25 ft-lbs for the studs as suggested in most of the technical literature.
Update: After 100 mls, the studs needed a few pounds of re-torquing and valve clearance was too tight due to settling of the parts. I checked again after 600 mls and everything was fine.