Thursday, April 13, 2017

Project Bike: Dad's XT500 - Grub screw mod

As I mentioned before, the engine was making a funny ticking noise after Dad got it rebuilt and up and running. A little internet research found a possible solution - The grub screw mod. This is where the top of the rocker box is drilled and tapped to use grub screws to put pressure down on the rockers. The problem stems from the bores where the rocker arms pivot getting worn and allow the rockers to move. This is likely why we saw the exhaust rocker so warn when we tore the engine down. Here is a video where you can hear a big improvement in the engine noise after the mod is done.

EDIT- Here is some more info on the grub screw mod:

 Pull off the head
 Remove the end cap
 Use a M6x1 bolt to thread into the end of the rocker pin and pull it out
 Here the pin and rocker are both pulled out. The pin was set back on top of where it fits. 
 This is where the rocker pin wears. The 'top' side is loaded and will wear. The grub screws lock the pin from rotating while driving it down to the bottom side of the bore which isn't worn. 
 Here are where the grub screws should be located
Drill and tap here. Use red lock tite and a locking nut on the screw. Make sure you wait for the loctite to cure or you'll have oil leaking around the screws.