By now, most M20 engines have undergone multiple timing belt services. As part of this process, the upper timing cover is typically removed. Two bolts secure the cover to the aluminum cylinder head, and one of them in particular, the smaller M6 bolt is notorious for stripping its threads.

The distributor cap depends on the upper timing cover to maintain proper alignment with the distributor rotor, which indexes off the camshaft. If that critical M6 bolt strips, it can cause the rotor and cap to become misaligned. At best, this results in premature wear of the contact points. At worst, it can cause the distributor rotor to shatter.

While you can certainly disassemble the area and install a Timesert or Helicoil to repair the threads, a simpler and often more reliable solution is to replace the bolt with a 75mm stud (M6-1.0 pitch) paired with an M6 nut.

A stud has a significant advantage: it’s much less likely to pull out if there’s any thread engagement remaining. Unlike a bolt, which can continue to spin in compromised threads, a stud can usually be threaded a couple of turns deeper than the original bolt, reaching fresh, undamaged threads.

Bolt in question indicated by the red arrow:

Always confirm that the locating dowels are present.

Finalized repair:

This entry was posted on May 4, 2025 at 12:08 am and is filed under Do it Yourself. You can follow any responses to this entry through the RSS 2.0 feed.