From: Dave Baker
Newsgroups: rec.autos.tech
Subject: Re: A Valve Manufacture Question
Date: 24 Sep 2000 14:21:09 GMT

>What is a typical engine valve made from and how is it made?  Is the
>valve cast iron - cast in one piece and turned, or is it made from two
>cast pieces welded together, or turned from billet steel, etc?  If
>welded, is it treated to keep the weld from being brittle?  How is the
>valve finished - electroplated? With what?  Are intake and exhaust valves
>mfg differently or using different materials?
>
>If it varies from mfg to mfg, I'm most interested in factory valves for a
>Ford 351 Windsor, 1985 or newer.

Firstly, valves are NOT made from iron - they would disintegrate at idle speed
if that were the case - nor are they cast - that is the process used for large,
low strength components like blocks and gearboxes. Valves are forged - from
steel.

Inlet valves are usually one piece steel forgings using EN52B alloy steel or
similar. This is a magnetic steel which can be hardened by heating and
quenching - only the tips and collet groove areas need to be hardened to
prevent distortion from the pounding forces sustained in those areas. Sometimes
the seat area is induction hardened to increase wear resistance there too.

EN52B is ideal for the temperatures sustained on the inlet side but does not
have sufficient high temperature strength to endure the temperatures reached in
the heads of exhaust valves. For this reason exhaust valves are generally two
piece - the stems are EN52B and the heads and about 1 inch of the stem are
21/4N austenitic stainless. The two parts are joined by friction welding in a
machine similar to a lathe - one part is spun and brought into contact with the
other stationary part under hydraulic pressure - the friction generated melts
the ends in contact and at a given point a clutch in the machine slips and lets
both parts spin together and fuse. The weld so generated is very homogenous and
as strong as the parent materials.

The stems are then heat treated, ground to size and may be chrome plated or
tuftrided for increased wear resistance.

21/4N is non magnetic and cannot be heat treated - it does not get harder with
heating and quenching - for that reason it is prone to distortion on the tips
and collet grooves if one piece valves are made from it and so a material
called Stellite can be used on the tip to give this sufficient hardness to
resist rocker loadings.

The Stellite is used in powder form and induction melted into a blob on the
valve tip which is then turned and ground to size to leave a pad about 2mm
thick.

A magnet can be used to make a rough and ready determination of the type of
valve material. Inlets will generally be magnetic all over and only the top
half of the stem of exhaust valves. If you use the magnet to find the point on
the valve stem where the magnetism stops and look carefully at this point you
can sometimes see the colour change where the two different types of steel
join.

Racing valves are generally one piece 21/4N simply because it is easier to
avoid the friction welding process for low volume production. There is no
benefit and many disadvantages in having 21/4N stems - you'll often see badly
mangled collet groove areas on stainless race valves, especially with the
triple collet grooves used by Ford and VW which are not a good design IMO.
Using a two piece valve means this area can be heat treated properly and thats
the way I generally make my own race valves.

It's an urban myth that a friction welded valve is weaker than a one piece
valve - valves never ever break at the weld if they have been made properly. I
doubt anyone in this forum has ever seen a std production car grind to a halt
because of a valve breakage that wasn't itself caused by contact with the
piston due to timing belt or similar problems. Considering the loads they
endure, modern valves are extraordinarily reliable components.