Corey Thomas - Jaguar XJ6 Series III

- Repairing a broken cylinder head stud in a 1987 Jaguar XJ6 Series III

When removing the head, my worst fears were realized of a cylinder head stud broke in the engine block.

What happened? A cylinder head stud got so corroded that it broke deep down in the block just above the thread

Why did it happen? Probably from a previous owner not flushing the coolant enough or using the wrong coolant - coolant provides anti-corrosion protection inside the block but loses this ability with age which is why it must be changed

How did I fix it? I used a Helicoil thread repair kit. I bought a few tools and modified some to extend their reach down into this cavity. The challenge was through a narrow opening at the top of the block and side access by removing a free plug:

Get access

• Removed the freeze plug to get side access to the area
• Be prepared for a lot of coolant spillage including the corroded gunk - I left a container underneath in anticipation
  

Went shopping (again)

• Bought a Helicoil kit to repair a 7/16"-20 thread
• Bought an 8pt (these are usually 6pt or 12pt) 5/16" socket for a 3/8" drive
• Bought a 29/64" drill bit

Drilling

• Modified the drill bit from a round shape to a square - sized to fit a 1/4" socket
• I mated the 29/64" drill bit to a 1/4" hex socket facing the wrong way
• I also mated a 1/4" extension to another 1/4" hex socket
• To join these two, I used a 1/4" hex screwdriver bit which I cut the ends off - tape held these parts together from sliding apart
• On the other end of the extension was a 1/4" drive to 1/4" hex adapter for the drill
• I used tape wrapped around the tap as a depth guide and a magnet to clean out the area

Taping and Helicoil Insertion

• Used a medium length 3/8" extension - the longer they are, the thicker they are which may not fit
• I inserted the extension in the top of the block and mated it to the 5/16" 8pt socket that I inserted into the side access hole - from the freeze plug
• I lifted that assembly then inserted the tap in the side access hole and mated that with the assembly
• From there, it was simply a matter of using a socket on the top
• The same was done for the coil insertion
• Again, I used tape wrapped around the tap as a depth guide and a magnet to clean out the area
  

Throughout the drilling and tapping, I would spray SeaFoam for lubrication. I would also occasionally back-out the tap a bit for clean-up.

The test with a stud was great but I did notice that it is not perfectly square - so what!

I'll take a break from the Jag for a bit and do some (marriage saving) home maintenance.

Regarding the Helicoil taps - they won't publish their size forcing you to buy a kit, labelling it as a special tap. I did buy a kit but I'm convinced that the Helicoil 7/16-20 STI tap is a regular 1/2-20 tap. The coil insertion tool's functionality could also have been replicated with on of my throw-away studs. The difference would have been $45 for the Helicoil kit versus $7 for a 6-pack of the coils only.

More pictures here: Fixing the broken stud