This document describes improvements to valve guides for internal combustion engines made through powder metallurgy processes. Specifically, it describes coating the surfaces of the powder metallurgy-produced valve guides with corrosion-resistant metallic coatings to protect them from oxidation caused by exposure to hot exhaust gases. The coating can be applied through electrodeposition, spraying, or impregnation by dipping the guide in molten metal. This protects the guide from disintegrating due to internal oxidation.

1. * GB780073 (A)
Description: GB780073 (A) ? 1957-07-31
Improvements in or relating to valve-guides for internal combustion engines
Description of GB780073 (A)
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PATENT SPECIFICATION
Inventor: PETER ROBERT MARSHALL Date of filing Complete Specification:
Dec. I, 1955.
Application Date: June 23, 1954.
No. 18365/54.
....... Complete Specification Published: July 31, 1957.
Index at acceptance:-Class 83(2), A(11; 26).
International Classification:-B23p.
COMPLETE:SPECIFICATION
Improvements in or relating to Valve-Guides for Internal Combustion
Engines We, THE BIRMINGHAM SMALL ARMS COMPANY LIMITED, of Armoury
Road, Small Heath, Birmingham, a British Company, do hereby declare
the invention, for which we pray that a patent may be granted to us,
and the method: by which it is to be performed, to be particularly
described in and by the following statement:-
This invention relates to valve guides for internal combustion
engines. At present such guides are usually made from small hollow
cast iron cylinders through which passes the stem of a poppet valve,
the guide being ground on. the outside so that it is,a tight fit in a
machined seating in the cylinder block or the cylinder head.
Investigation, has shown that valve guides manufactured by powder

2. metallurgy processes possess a number of advantages over those made by
the older and conventional method, among which are the following:(1)
The valve guides can be made within the tolerances required without
machining, such as is necessary with cast guides and are ready for
assembly in the engine.
(2) Solid lubricants, such as graphite, can be incorporated in the
metal powders before pressing and sintering.
(3) The valve guide, due to its method of manufacture, may be porous
and these pores are adapted to be filled with lubricating oil.
(4') In use, very little wear of valve guide and valve stem ensue due
to the efficientlubrication mentioned under (2) and (3) above, and due
also,to the continual replenishment of the oil in the pores of the
valve guide by oil spray and splash! arising in the normal course of
operation of the engine.
It has been found, however, that a serious disadvantage and one that
prevents the use of valve guides made by powder metallurgy processes,
is the severe oxidation or corrosion leading to disintegration of the
guide, especially the guides for the exhaust valves of the engine. It
appears that this is caused by the extremely hot gases, laying on the
exposed T F hs portions of the guides, entering into the pores
themselves and causing oxidation or carburisation together with
disintegration to occur throughout the material. The oxidation that
occurs is not always confined to the exposed portion of the guide but
may extend down the guide into portions that 'are protected by the
surrounding cylinder block or cylinder head castings.
An object of this invention is to manufacture a valve guide for
internal combustion engines by powder metallurgy methods such that it
is not, or is less, susceptible to this internal oxidation and
disintegration.
We are aware that it had already been proposed to coat a metallic
strip with a porous metallic lining by a powder metal process, and
subsequently to impregnate the porous lining by immersion in a molten
bath of the impregnating metal.
It has also been proposed in a powder metal process for the production
of porous metal bearings using powdered spongy iron particles, to
subsequently close the pores on the surface of the bearing by
precipitation of a fusible metal such as tin or solder.
According to this invention a valve guide for internal combustion
engines produced by powder metallurgy methods has the surfaces subject
to oxidation or corrosion by exposure to combustion gases protected by
a corrosionresistant metallic coating.
Coatings of suitable metals may be applied by electro-deposition, by
spraying, or by impregnation. It will be appreciated that the
protective treatment need only extend along the valve guide for a

3. distance suitable for thq purposes of the invention, that is to say il
will usually extend over the end and along the sides of that portion
which wil'l be exposed to corrosive conditions when the guide is in
use.
As stated above the metallic coating may be put on by
electro-deposition and' suitable metals include copper, nickel,
chromium, tinnickel alloy, copper-tin alloy, iron, nickel780,073
cobalt alloys and the like. The manner of applying such a coating is
well known to electrodepositors, and those knowledgeable in the art
will take the necessary precautions fox the removal of chemical
solutions that may become trapped in the pores of the valve guide.
Alternatively, as mentioned above the corrosion-resistant coating may
be applied by metal spraying by means of a metal spraying device. In,
this case, the protective coating may be any metal or alloy that can
be sprayed but the use of easily oxidisable metals will be excluded
for obvious reasons. To carry out this part of the invention, a number
of valve guides are placed in a suitable holding fixture, made so that
only the ends to a suitable depth protrude, a rod is inserted in each
bore to prevent the bore from receiving a deposit, and the protective
metal is then sprayed on. It is found that a sprayed metal deposit
adheres well to the slightly porous surface presented by the powder
metal component.
Yet another way of applying a corrosionresistant coating consists of
dipping the end of the valve guide, that is later to be exposed to the
hot gases, in a bath of molten metal or alloy so that the molten metal
is drawn into the pores by capillary attraction. The valve guide will
normally be made from iron powder with graphite as a solid lubricant
and suitable molten metals or alloys are copper, bronze, brass or the
lower melting point alloys of this type known as "silver solder" or "
brazing solder." On the surface of the molten metal may be placed a
layer of molten flux and this will not only serve to protect the
molten metal substantially from oxidation but will also flux the valve
guide as it is dipped into the molten metal.
A preferred method of carrying out the invention consists in heating
the valve guide in a protective atmosphere in contact with the
impregnating metal so that the latter melts and flows into the pores,
for example by standing the valve guides in a jig with their ends that
are to be impregnated downwards and including on the jig pieces of the
impregnating metal or alloy. On heating in a protective atmosphere,
the latter melts and is drawn into the pores of the valve guide.
It may be mentioned that, generally speaking, the total impregnation
or protection of the valve guide will be avoided so that the pores in
the unexposed portion of the guide may be utilized for subsequent
impregnation with lubricant for the lubrication of the valve stem.

4. The manufacture of the valve guide itself may be carried out by any
suitable powder metallurgy process. For example it may be made by
pressing the metal powder from both ends in a cylindrical die equipped
with a core rod. The resulting "green" compact is then sintered by
heating to a suitable temperature in a protective atmosphere of
hydrogen, cracked ammonia or other suitable gas mixture. The manner of
pressing and sintering is well known to those who practice the art. In
order to facilitate the pressing operation the valve guide may be made
in two pieces, the division being transverse of the longitudinal axis.
The valve guide may also be protected, according to this invention, on
its inner surface usually for a distance similar to that already
described. In this event the inner surfaces may be recessed to a depth
corresponding to the thickness of protective coating to be applied, so
that the bore of the guide 80 will, after a metallic coating treatment
as herein described, be substantially uniform or may easily be
rendered so by mechanical methods.
It is to be understood that the term 'metallic' used in the
specification and claims is intended so include alloys.
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