Dumb Valve ?

[88911coupe]

On a top end rebuild can the valves be reused if in spec (or whatever the term is) I was reviewing Wayne's book and did not see the valves listed as absolute replace items. Is this b/c no one would EVER re use valves or is it a judgement call based on condition? Hope this is not too dumb a question.
TIA

[HarryD]

I had the same question for my wrench for my rebuild. He recommneded we remove and inspect/measure the valves to determine if I had enough material on the seats to grind and the stems were not worn. I ended up buying (only ) 12 valves mostly due to worn stems.

[Wayne 962]

There are several pages in the book related to heads and valves, in which I discuss reusing valves. I don't have it in front of me right now, so I can't tell you page numbers. There a nice picture of a valve on a grinder in the machine shop section...

-Wayne

[TimT]

Valves can be reused...to a point.

If after a few "three angle" valve jobs the valve will show a sharp edge, then its time to replace, if the valve is to thin at its perimeter it will propogate cracks, not a good thing..

Also if the seats are to deeply reground, you may have some problems getting the proper spring height....

If the valves arent bent or obviously damaged, and this is a first time engine rebuild, a few turns with some valve grinding compound should make things good to go

Of course others will disagree

[Doug Zielke]

Originally posted by TimT

If the valves arent bent or obviously damaged, and this is a first time engine rebuild, a few turns with some valve grinding compound should make things good to go

Of course others will disagree

I *do* agree with Tim, and so did the machinist who did the head work on my 3.0. He checked each valve for wear, and inspected all the seats. As everything showed minimal wear, he lapped in the valves, replaced the guides, and that was basically it.

[Wayne 962]

The amount on the tip of the valve is called the margin. You can grind off the end of the valve to get the proper spring heights - it's typically not a problem when the seat is compressed too far into the head. The problem with the seat is apparent with the change in combustion chamber size and thus change in compression ratio.

-Wayne

[911pcars]

Just to complete Wayne's description, the margin is on the tulip end of the valve; the thickness of the valve head starting from the end of the valve face bevel to the flat valve area facing the combustion chamber. Enough margin is needed so this area doesn't become a hot spot for pre-ignition and/or a starting point for a burnt valve.

In addition, reducing valve margin by grinding the valve face increases the valve face width which in turn requires adjusting (grinding) the valve seat to maintain the proper valve-to-seat contact position. As the valve head sinks lower relative to the seat, this tends to shroud the valve and reduces air flow as the valve opens. Individual combustion chamber volume (and thus compression ratio) is also slightly affected. Reducing the margin also reduces the number of times the valve can be reground. Some replacement valves (especially '69 S exhaust valves are quite pricey).

If installed spring height is insufficient and removing valve spring shims is not possible, then lowering (spot facing) the spring seat is probably the best alternative, then readjust to specified height with shims under the spring.

Sherwood Lee
http://members.rennlist.org/911pcars