500 Hour Service Question - JD4600

[rmonio]

I'm getting close to my 500 hour service interval on my 4600. There are two items that require me to take it into the JD Dealer for service - these are:
- Fuel Injector Inspection
- Engine Valve Clearance Adjustment

I can understand the Fuel Injector inspection - but what is the Engine Valve Clearance Adjustment?

Thanks!

-Bob