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Didnt trucks still run flat-tappet cams? or is it a hydrualic roller? If its a flat tappet I wouldnt even consider attempting to set the lash while the motor is first running (since you must break in the cam)... A motor running at 2500RPM would be pretty hard to set the lash on. To adjust the valve lash on a motor while its not running pull out all the spark plugs or get a long breaker bar and turn the motor over slowly by hand at the crank, watch the valve springs...
Adjust the Intake valve lash when the EXHAUST valve begins to open and adjust the Exhaust valve when the INTAKE valve has just closes. Tighten it down slowly while wiggling the pushrod up and down. Once there is JUST all the slack taken out of the pushrod, tighten it a 1/2 turn more. Remember, just tighten it slightly then take the wrench off because its weight may screw up the adjustment, then wiggle the pushrod, repeat. You may not even want to go the extra 1/2 turn because more than likely your first time doing this you'll tighten them a little too tight
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