I'm building a 1985 351W motor, and am nearing the end of the head work.
I have my own valve grinding system, and have completed the seat grind, and am using new valves. What I need to know is the range of allowable installed valve stem heights for a hydraulic lifter equipped motor. No specs are printed in the manuals. I know there is a limit on the height before new seats need to be put in. Does anyone have any data on this?

Thanks

 

I don't have any real world experience with this -but this is what I think. Valve stem height it not a true measure of seat depth, unless you use brand new valves to take the measurement. The height would vary depending on how much was taken off the valves and which angles were used to resurface the valves and seats. The limit on valves is determined by the margins left and the seats by the depth the valves are set in. Besides structural integrity, the first thing that goes is flow. The effect on flow is more discussed than structural integrity. What the limits are- I couldn't tell you.

 

I don't ever recall seeing a spec for that particular measurement. Like Doug1222556 said, it would depend on how far the seat was ground. I would THINK . . . . . please note the emphasis . . . . that unless you had to grind one excessively, you should be OK. Lay a straight edge across the tops of the valve stems. If they're all close to the same height, you should be OK. Valve spring installed height is probably more important.

 

And if I were to guess- the valve margins should stay above the seats.

 

Thanks guys,

I appreciate your thoughts on this. I finally did however find a local machine shop that had a book ..... one from a head service ......and in it was the published figure. Turns out that the maximum the stems can extend above the spring seat is 2.187" for 351W engines from 78 -85. Because the rockers are not adjustable, this is a critical figure, in that too much will keep the valves open. Too little can be a problem in that the valves will be noisy from no lash take up at low rpms, but that is not a typical problem. It is possible to grind the stems to bring the height back into spec, but you can lose the hardened ends that way if too much is taken off. That can lead to premature stem wear.

One other trick that the machinist mentioned is to use hardened shim washers under the rocker arm stands, and that may be in fact what I'll do. It all depends on how the rockers line up at zero lash, and whether too much angle might get involved. Ford has shorter pushrods available too, but are pricey. Crane sells some as well, and has ones you can custom cut to make work.

I'm enough of a cheapskate to like the shim idea first ... If .. it can work
out.

There is one other bit of data I got in this ... and that's the actual hydraulic lifter preload that you can work backwards from to see it things are going to work out. The lifters evidently have about .125 travel available in them. The final position of the pushrod into the lifter needs to be between .020 and .060 to make it all work right. That gives you enough "preload" of the lifter to keep the valve noise gone, but not have the valve pushed open too long.

Thanks again ... and hopefully this information will be of use to others.

 

I wasn't even thinking about that end of the valves, or the reason for the limit. It makes more sense now(kinda). How much material is removed from the head surface and head gasket thickness affect the pushrod length as well. For myself and some others on this site- an oddball pushrod length is almost a given. By the time we surface our heads, change out rocker arms, and valves,seats or whole heads... whatever, we expect to use an adjustable pushrod and then get the correct length pushrods to make the geometry correct. Adjustable rocker arms don't necessarily mean you can stick with the stock length pushrods in these circumstances either. Finally, we're running spring pressures too high to use stock pushrods anyway. It's easy to forget not everyone is building for power. Thanks for the education and the info-it's not possible to know too much about this stuff.