i have a 1996 f250 with a 460 with about 196000 on the clock. a few months ago i broke 3 spark plugs off in the head, which due to clearance issues was easier to take the heads off in order to get the plugs out. the heads were gone through at that point and recieved a valve job (seats, seals stems...) the truck developed a miss at idle, and i traced it back to the rearmost cylinder on the passenger side. long story short i have what i believe to be a wiped cam lobe on cyl 4, as it is not compressing the exhaust spring fully. the first compression test on cyl 4 revealed 150 psi, however, another compression test on the same cylinder yielded different results: 75 psi on the inital cycle, the 2nd, 3rd, 4th cycle will not build any compression, and when it hits the 5th cycle, it will build 150 psi. a leakdown test however, only showed about a 16% loss in pressure overall. im about ready to pull my hair out over this, and i was wondering if anyone has had similar issues. i would greatly appreciate any feedback.

 

Common problem on the 460 after a valve job.
You have to put different push rods in the engine.

A 1.52mm (0.060-inch) shorter push rod (6565) or a 1.52mm (0.060-inch) longer push rod is available for service to provide a means of compensating for dimensional changes in the valve mechanism. Refer to the Master Parts List for the pertinent color code.

Valve stem-to-valve rocker arm clearance should be within specifications. With the hydraulic valve tappet (6500) completely collapsed, repeated valve reconditioning operations (valve and/or valve seat refacing) will decrease the clearance to the point that if not compensated for, the hydraulic valve tappet will cease to function and the intake valve (6507) or exhaust valve (6505) will be held open.

The use of a positive stop rocker arm seat bolt eliminates the need for valve clearance adjustment. However, to obtain accurate valve clearance measurements, it is important that all components be serviceable and installed to specification.

Specification:
Collapsed Tappet Gap (Clearance)
Allowable = .075-.175
Desired = .100-.150

To determine whether a shorter or a longer push rod is necessary, perform the following check:

1. Install an auxiliary starter switch. Crank the engine (6007) with the ignition switch in OFF until the No. 1 piston (6108) is on TDC after the compression stroke.

2. With the crankshaft (6303) in the positions designated in the following Steps 3, 4 and 5, position the hydraulic tappet compressor Tappet Bleed Down Wrench T71P-6513-B on the rocker arm (6564). Slowly apply pressure to bleed down the hydraulic valve tappet until the plunger is completely bottomed. Hold the valve tappet in this position and check the available clearance between the rocker arm and the valve stem tip with a feeler gauge. If the clearance is less than specifications, install a shorter push rod. If the clearance is greater than specifications, install a longer push rod.



3. With the No. 1 piston at TDC at the end of the compression stroke (Position No. 1), check clearances on the following valves:

No. 1 Intake No. 1 Exhaust

No. 3 Intake No. 4 Exhaust

No. 7 Intake No. 5 Exhaust

No. 8 Intake No. 8 Exhaust

4. After checking the clearance on these intake valves and exhaust valves, rotate the crankshaft 360 degrees to Position No. 2, then check the following valves:

No. 2 Intake No. 2 Exhaust

No. 4 Intake No. 3 Exhaust

No. 5 Intake No. 6 Exhaust

No. 6 Intake No. 7 Exhaust



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thank you for the info. i just pulled out the lifter on the exhaust side of things and it was really chewed up. not sure how the cam looks, i havent gotten that far. is it pretty typical of them to eat up a lifter without changing the pushrods?

 

Sounds like the lifter was collapsed, bent (or short) push rod or the valve stem was ground to far to compensate for the valve grinding.