From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: Q about automotive valves and seats
Date: 25 Jan 2000 22:53:10 GMT

>From: kinsler@frognet.net  (Mark Kinsler)
>
>>450°F maximum temp.
>
>That's about the standard operating temperature of a Volkswagen air-cooled
>engine.  Around 425 degrees, said the gauge I installed on a cylinder head
>bolt.
>
>>Is stellite a coating or a plain material ?
>
>Stellite is a variant of high speed steel,

Stellite in its various derivatives is not a variant of high speed steel or any
other steel for that matter. It is a cobalt based alloy, normally with high
(approx 1/3) chromium content. Tungsten (up to 20%) and nickel are common
constituents.


> and it's a solid material, not
>a coating.

It's a coating - normally applied to valves by induction melting stellite
powder to the tips of 21/4N valve stems and to the seats of exhaust valves
designed for high stress applications.

>  Really nice stuff: makes great cutting tools.  Not attracted
>to a magnet, I don't think.  I don't know what the recommended
>application for exhaust valves made of this is, though.

There are no recommended applications for valves made of this because no valves
are made from it in the first place.


>>What is the difference between stainless steel and stellite for valves?
>
>Dunno.  My guess is that they're recommended for different applications--I
>don't really think you can say one material is better than another.

I think you can - stainless steel, usually 21/4N is an excellent material for
both standard application exhaust valves and both inlet and exhaust race engine
valves. Stellite isn't a valve material full stop - just a coating. 21/4N isn't
heat treatable and needs a hard stem tip to resist wear from rocker pads etc.
Stellite provides the necessary hardness as an induction melted deposit approx
1mm thick. For seats, stellite provides better wear characteristics for high
temperature and unleaded fuel use.


>>What is the benefit of a chrome layer on the stem of some valves.
>
>It would have to have something to do with its ability to resist galling
>and maintain a low coefficient of friction under high temperatures with
>lousy lubrication.  I don't think that valve-stem wear is all that big an
>issue, though certainly a hard chrome plate would minimize that as well.

Chrome is used on stems primarily because it minimises stem wear considerably,
especially with cast iron guides. EN52B can run in cast iron guides without
chrome plating but 21/4N can't although it operates well enough in bronze
without a plating. Deposited as hard chrome at approx 70 Rockwell C it
increases stem life by 2 or 3 times. Care has to be taken with chrome stems in
conditions of high temperature with poor lubrication as it can gall and the
cutting edges so formed will rapidly destroy the valve guide. For very severe
use therefore, such as race valves, chrome is normally not used and the valves
are lifed instead. Tuftriding can be used instead in severe applications to
avoid this potential problem.


>I snipped the rest of this because I don't know much about valve seats.


or valves I'm afraid.


Dave Baker at Puma Race Engines (London - England)  - specialist cylinder head
work, flow development and engine blueprinting. Web page at
http://members.aol.com/pumaracing/index.htm

From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: Q about automotive valves and seats
Date: 26 Jan 2000 08:03:21 GMT

>From: mmdcl@aol.comatose  (David)
>
>
> Coincidence. I just came in from the cold after attempting to make a small
>gouge from a valve stem to carve copper.The head had broken off ; inside was
>gray and grainy, similar to cast iron. I annealed it, forged to shape,
>sharpened, and quenched in icy brine.Tested on the copper before tempering;
>it`s soft and useless. Whatever it`s made of, seems to not be heat treatable,
>unless I did something awfully wrong.
>
>   Dave

Test it with a magnet. 21/4N exhaust valve steel is stainless austenitic - non
magnetic and not heat treatable. You may find that the other end of the stem
'is' magnetic. Most std production exhaust valves are two piece friction welded
- 21/4N head and half the stem - the top half of the stem being magnetic EN52B
or similar. Heat treatable at 900C plus. Try the same trick with an inlet valve
and you'll make a perfectly usable cutting tool for light use.


Dave Baker at Puma Race Engines (London - England)  - specialist cylinder head
work, flow development and engine blueprinting. Web page at
http://members.aol.com/pumaracing/index.htm

From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: Q about automotive valves and seats
Date: 26 Jan 2000 21:29:18 GMT

>Seems that all modern cars, even not sport, have at least SS exhaust
>valves. I thought hi rev bikes or aircraft engine had better materials.

21/4N 'is' a better material.


>I am surprised to learn that stellite is used on the tip, not on the
>face despite some vendor claims. That's why some aftermarket SS valves
>need a lash cap. They don't have hardened tips.

It is used on the face - I made that clear above I thought. I quote:

> It's a coating - normally applied to valves by induction melting
> stellite powder to the tips of 21/4N valve stems and to the seats of
> exhaust valves designed for high stress applications.


>And whatabout valve seat material ?

Normally a high chrome steel - any aftermarket seat designed for unleaded use
will be fine in most applications. Bronze is a bit old hat and there are a wide
variety of materials used by OEM manufacturers - mainly steels of different
grades. You'll get all the info you require by asking any supplier of seat
inserts.



Dave Baker at Puma Race Engines (London - England)  - specialist cylinder head
work, flow development and engine blueprinting. Web page at
http://members.aol.com/pumaracing/index.htm