Repairing exhaust manifold threads

Summary

One common fault with the T16 engine is "ticking" exhaust manifolds caused by loose bolts and failed gaskets. Unfortunately the threads are very fragile being tapped directly into the alloy head. Often when the bolts are removed or tightened the threads can dissolve which makes the blowing around the manifold gasket even worse.

One way to ensure the threads are put under less strain is to replace the bolts with studs and nuts, this means the studs can be screwed in without any load and the nuts take the strain - unfortunately this solution is not much help if the threads are already damaged.


A common fix is to drill out the threads and insert helicoils - these are coiled thread inserts much like a spring which can be inserted into the oversize hole and are profiled to give a new thread for the stud/bolt.


One problem with helicoils is they can give way under high torque loads. An alternative product called time-cert can give better results being a solid steel insert.

Please Note: This guide was based on an original guide and technique developed by Rovertech's 216si - all credit and thanks to Simon!

Requirements

To do this job you will need:

  • Compact electric drill
  • 14mm HSS Drill
  • 16mm taper tap
  • 16mm bottoming tap
  • 5x M10 time-certs
  • Large bladed screwdriver
  • Clear access to the manifold
  • a steady hand!!

Procedure

Remove the manifold and gain access to the head - note the stripped threads.

Set the drill with a depth stop to ensure that the drill does not go deeper than the original hole - In this case 22mm was acceptable.

Ensure drill is lined up perfectly square to the face of the head, keep all swarf away from the exhaust ports or any other vulnerable openings.

Use the taper tap to start the thread - ensure the tap is kept sqaure to the face of the head.

If a suitable tap holder cannot be found, it is possible to improvise with a T bar with 3/16" whitworth socket.

Once the taper tap has reached the end of the hole, use the bottoming tap to run down and cut the remaining thread.

Clear the thread of any swarf

Screw in the time-cert firmly, make sure that the fixing isn't proud of the face of the head.

Metal may be ground from the end of the fixing or filed from the front to keep it from protruding.

The finished fixing can now be used with studs or bolts with confidence!

Repeat the procedure for the remaining four bolt holes. To gain enough room for the fixing closest to the stat end of the engine the alternator must be removed - this can be tricky to do in-situ but possible, given enough clearance to keep the drill square.