Chuck:
I would definitely follow the specs, and yes, you are at risk of damage if you run your truck for many hours. We have had loose settings hammer valve tips and push rods. Virtually all engines specify a closer setting on the intakes than exhaust because the exhaust valve gains stem length from the heat it experiences on the valve head transferring up the stem. The intake valves have cooler air passing by them, keeping them cooler so they do not gain length like the exhaust valves, so they are designed to have closer settings. To explain it another way, the exhaust valves need bigger clearances because as the stem length grows from the heat, they close up the clearance while the engine is hot, so they need more clearance to start with. This is also why you should be sure top check whether the valves are to be set hot or cold when servicing an engine.
Cam ramps are also designed to spin the lifters as they initiate the lift. If the setting is too loose or too tight, the spinning is compromised, causing possible lifter wear in one spot. We have had engines that had this problem, which eventually results in a groove being worn in the lifter base because it is not spinning to distribute the wear evenly. If you have an engine that frustrates the heck out of you because at one time it sounds like it has a valve lash too loose on one cylinder, pecking more than usual, then at other times sounding like it should, suspect a lifter with a groove in the bottom. When the groove is aligned with the cam lobe, the clearance increases. When it dies not, the clearance closes up a bit resulting in the sound change. As you can deduct from this, if you happen to set the valve with the groove aligned with the lobe, your setting will be too tight when it is not aligned, or vice-versa.
Then, there is the problem of rockers that have a lot of hours on them or have been hammered. The tips where they contact the valve head will get an impression worn in them. The feeler gauge will not work properly when this condition is present. It will bridge the depression, giving a larger clearance than you think you are getting. If you have one or more rockers with this problem, it could be the cause of a valve(s) that peck even though you are sure you got the setting right. The rockers can be removed and the face dressed to remove the depression.
I do not in any way with my cautions about too loose settings invalidate or ignore the risks of settings that are too tight. This is damaging as well.
Best to follow the specs carefully, and check them regularly.
Regards Marti
