The 138190034 Rocker Adjusting Nut for Non-Hemi Clone and Predator Heads holds the rocker arm on top of the fastening bolt. 

The Adjusting Nut allows you to adjust the lash clearance on your rocker arm. When checking lash, goes by the cam manufacturers' recommendation. If running a stock cam, set your lash to less than .003"; we suggest setting it at 0 since, in most cases, it will loosen as the engine heats up. A lash that is too wide will make the engine difficult to start. This is due to the compression release on the cam not fully reading the lifter to open the exhaust valve to make it easy to start. Do not overtighten the rocker adjuster nut, or your rocker arm may resist pivoting, resulting in excess friction, heat and lead to rocker arm or valve failure. 

The Rocker Adjuster Nuts are the OEM Replacement for 196cc 6.5hp GX200 Clones, including the Tillotson 196R/RS, Ducar 196, and Box Stock 196. These also fit the Predator 212 Non-Hemi Cylinder Heads.