If they are not staying adjusted properly for more than 2500-3K miles, someone is not doing something correct when adjusting the valves....
They do not 'fall out of adjustment' overnight....and should not after only 2 or 3 thousand miles of operation....
Respectfully disagree Steve. While I have been lucky to have periods of 6 or 9 months where I can go without adjusting the valves, there are other times where only weeks are involved. The same goes for several of my customers. There are two main factors for this.
One is camshaft wear. As the lobes wear down, the clearances change. I have adjusted valves for customers and have told them that the cam was going flat, based on the amount of 'slop' that had accumulated since the last adjustment. Then, sometimes, a lobe will surprise me and slow down its rate of deterioration as it gets closer to round. Granted, not near enough valve lift anymore, but goes out of adjustment less often.
Second reason: rebuilt head. Machine shops in these parts are big on pushing new valve seats during the rebuild process. The seats migrate over time. Sometimes they move once and stop. When that happens, that's great! Often times tho, they stay in one place for a while, then move incrementally over time. No particular rhyme or reason, tho I suspect longer trips at higher temperatures play a part. Not that I would know about that!;p
Case in point. I have a customer who drives his 69FJ40 from here in L.A. to Idaho every year or so. I installed a rebuilt head on his rig after he burned a valve from leaving them out of adjustment too long on one of his road trips. I installed a vacuum guage in his truck at that time and told him that if he was on another long road trip and saw the needle flickering, that he should get the valves adjusted at the nearest town and not wait for me.
He didn't listen. Needle started flickering about 300 miles from L.A. [so he says] In any event, 300 miles later, I was pulling the head again to replace another burned valve. I am certain that migrating valve seats caused this.
Bottom line: I don't see a problem with checking them with every oil change. In deference to Stumbaugh, I will admit that my maternal grandparents were German too!;p
