[Prowbar]

Been a little while. Got a new job as a machinist so that takes up quite a bit of time.

Been working on the cylinder heads and the crankshaft.

Took the cylinder heads apart and cleaned them. Valves and valve jobs looked good, none of them leaked a drop of brake cleaner when I sprayed it in the ports. There was quite a lot of carbon build up on the intake valves. I used EGR cleaner to clean the valves, combustion chambers, and ports.



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I needed to regrind the exhaust valve as they had the weird concave wear, which is often attributed to weak valve springs which was something I already suspected.

Decided to regrind the intake valves too, because I had it all apart. I then checked the valve stem to seat runout in the lathe using a dial indicator. All except 2 were within .0015". I touched them up again and they were were good to go, too.



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I reground the exhaust valve seats and lapped the seats until I had a good contact pattern. I then made a valve seat runout gauge. The book mentions that they should be within .002". Quite a few were outside the spec, around .0025". I touched them up until there were all .0015" or less.



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Knowing all the valves and seats were good, I could move on to the springs.
The Impco article that I posted years ago about 401 V6's on propane foreshadowed that the stock valve springs are too weak. They upgraded to a Ford valve spring using a spacer.

Went ahead and ordered Melling VS840 springs. These are stock replacement springs for a Ford 460 V8. They are too almost identical in dimensions but a little shorter. I went ahead and tested the stock and Ford valve springs using my redneck setup in the drill press to find out what the actual pressures are.



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The GMC manual lists 76-84 pounds of seat pressure. When I tested mine I got 64 pounds of seat pressure. The Ford spring without spacer got around 40 pounds of seat pressure. The GMC springs didn't make the free length spec either. Looks like they have been getting weaker over the years. Better to replace them now.

I wanted around 100 pounds of seat pressure and less than 280 pounds over the nose. Mike Jones recommended me to not exceed 280 pounds open pressure, otherwise break in springs are needed to break in the camshaft.
A little extra seat pressure should aid in heat transfer and prevent the valve from bouncing on the seat.

Using these springs I designed a spacer and made it out of chromoly steel that I had laying around. Old hydraulic cylinder rod makes for great bar stock. It locates on the valve spring rotator and in combination with the Ford spring gives me 100 pounds of seat pressure and 220 pounds of open pressure. I'm about 1/4" away from coil bind at full lift, so plenty safe.
The spacer in the picture sits cockeyed, was due to a locating bore that was too tight, fixed that later.



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I milled a small groove in the nose of the spacer to aid in oil drain.



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Installed new valve stem seals and reassembled the heads. I already cleaned up the rocker arms and polished the rocker arm shafts.