351C-4V Exhaust Valve Sticking Causes and Testing?

Just got the my 351C-4V rebuilt. Heads rebuilt new valves, push rods, lifters and bronze guides. In minutes after the first start the engine started running very rough and stalling, was shut down. Problem cleared after sitting for a week only to recur on the first test drive. No compression on #3 and 4 cylinders, and leaking to the exhaust. On disassembly found both valves bent and not seating. Had checked oil flow to cylinder head after the valve damage but before pulling the one head, had strong oil spirts at all rockers except #3 and 4 exhaust when cranking with the starter. No apparent obstruction found in lifters, push rods or rockers. Oil spirts on the pushrod up stroke in all other cylinders, barely dribbles out of the two bad ones. Seems an obvious cause of the sticking but could it be an effect of the reduced valve and lifter stroke from the valves not seating? Valve stem to guide clearances within spec but at the low end. New valves, new lifters, valve guides cleaned up and slightly enlarged going back with right bank. New lifters on both banks.

Any suggestions on:

  • Other things to check
  • Testing oil flow etc. as the engine goes back together?

Thoughts on testing the other bank (5-8), head not removed but lifters and pushrods out

Original 1970 Ford 351C-4V closed chamber heads full bronze guides
New SS valves - Exhaust valve Speedmaster PCE273-1037
New Hardened exhaust valve seats
Springs - Melling VS-280 (just slightly softer than OEM but used with more valve lift)
CAM - Melling MTF-2 (only slightly more lift than original)
Lifters Hydraulic Flat - COMP Cam 862-16
Valve locks - single groove, 0.060 offset
Valve Exhaust stem seals - by machine shop, premium grade
Valve train set up by engine shop and cam degree’d
Holley Sniper EFI previously installed (running 1+ years)
MSD EFI-6 ignition previously installed (running 1+ years)

Other issues, in the distraction of early fuel leaks and rapid onset of rough running, the proper break-in of the new cam was not done (i.e. quickly going to 2000 rpm for 20 minutes). The #4 exhaust cam lobe is flat by about 10 mils and the cam is being replaced. I’ve got some suspicion that the sticking valve may have contributed to this as all the other cam lobes are good. All pushrods are still straight and after inspection are being reused.

Also any hints on how to install the timing cover to oil pan seal without dropping the pan?

Typically two things can cause a sticking valve.

  1. Bad (old) gas.
  2. Faulty machine work - failed to ream the valve guides to the proper size.

I have had several machine shops do the #2. They set the clearance like you would on a small block Chebby - it’s too tight.

Thanks Royce,
The gas was about 4 month old. I’d expect bad gas to take a while to build deposits and be more likely on the intake valves.
The clearance was tight but represented by the shop as within spec. Perhaps they misinformed me but they have an excellent rep and our working relationship has been good.
The puzzling thing is the lack of oil squirting from the rocker on the two bad cylinders unless it is consequence of the shorter valve stroke (bent valves not seating) rather than a cause or contributor to the failure. The shop is saying it’s lubrication failure but the cause is not evident.
When I reassemble I want to try to prove out the oiling. Seems like setting the lifters in place and priming the oil system should show flow at all of the lifters. I’m not fully clear on how the lifters are feeding oil up the pushrod. I’m guessing they channel oil from the galley all the time to lubricate the pushrod ends but not fully clear why it squirts out briefly at only one part of the stroke. Perhaps when the pushrod hole aligns best with the hole in the rocker but it seems to be only when the pushrod is coming up. I can further test by cranking the engine briefly after assembly and watching. The lifters were disassembled and examined, the pushrods and rockers inspected and no dirt or obstruction noted.
I’d appreciate any further thoughts

The oil seal is supposed to eliminate any oil getting to the valve stem as none is needed. The valve guide and the valve stem are made from different materials to prevent galling. I think if you had a lubrication failure you would see galling of the pushrod end, rocker arm to pushrod interface, and the rocker arm to ball interface. It would not cause the valve to stick. The gas being 4 months old is right on the brink but probably OK.

I was skeptical of valve stem lubrication both because of the tight clearance not allowing much oil in and the high valve temperature that would cook the oil. The factory spec on the 0.3411" - 0.3418" stem diameter is 0.0015" - 0.0032"
Still wondering why they didn’t squirt.
Thanks again

Mostly likely worn ream. Machine shops seem to not like to spend the money to replace their reams. Once the ream starts to wear it no longer holds the correct size (smaller). I have adopted the practice to always check the guide clearance. Get the proper pin gauges, min, max & nom and check their work and make sure it’s sized properly. I’ve caught two very good machine shops that said they’re good and checked under sized with gauge pins. Hopefully, they have a gauge pin set and are checking size. But, most likely not. Use white lithium grease on valve stems during assembly. Amazon.com