I did the 100 mike check on both of my 420s today and now I know why the dealer wants 300+ to do this service. You have to do about 2 hours of disassemble and reassemble just to do a 5 minute check of the valve adjustments. Neither one of mine needed any adjustment after I checked them. I think I will be a little faster next time now that I know how everthing comes apart and goes back togeather. Oil and Filter change was easy.