Customer: “Send me some bronze valve guides!” And I don’t want any of those alloys I just want bronze!”
JRC Parts Counter: “Sir, bronze only exists as an alloy.”
A recent call from a dealer took me down the road of valve guide materials. So I thought why not write an article covering materials currently being marketed.
Copper was one of the earliest metals discovered. The Greeks and Romans made it into: tools, weapons, adornments, and there are even historical details showing the application of copper to sterilize wounds and purify drinking water. Today it is most commonly found in electrical materials such as wiring because of its ability to effectively conduct electricity. In nature it is often found in natural occurring alloys. To achieve purity it must be smelted using modern or ancient methods. Pure copper will be found on a British bike only in: copper washers, head gaskets and the wiring harness. All other applications will require alloys which have higher wear resistance.
Copper and its alloys are used extensively on engines for: plain bearings, bushings, and valve guides. Copper has too many alloys to list them all here. We will focus on a few of interest to the British bike restorer. Mostly valve guides and bushings.
Bronze is an alloy that consists primarily of copper with the addition of other ingredients. In most cases the ingredient added is typically tin. Arsenic, phosphorus, aluminum, manganese, nickel, and silicon can also be used to produce different properties in the material. All of these ingredients produce an alloy much harder than copper alone. Bronze is typically characterized by its dull-gold color.
Bronze valve guides were introduced around the time cylinder head material changed from cast iron to aluminium. The job of the guide is: to support the valve, and also conduct heat from the combustion process. This heat is taken out from the exhaust valve and into the cylinder head where it can be radiated away by the cooling system. A balance between stiffness and wear on the valve is essential to achieve a useful service life.
This ability to remove heat is often expressed as the thermal expansion coefficient.
Thermal expansion coefficients expressed as the exponent 10 to the negative 6th power(10-6 in/in degrees F).
Cast Iron is a 6, Aluminium a 13, Manganese Bronze 12, C630 (Ni-Al bronze) 9. We use these differences in heated expansion rates to remove guides and bearings. There are advantages to using materials of similar thermal expansion. A guide with a higher coefficient with be more effective at removing heat. Are there any advantages for bronze valve guides over cast iron? Apart from the expansion qualities of the different metals ?
Guides made of Ampco 45 or CDA 954 aluminum bronze will outlast iron guides and plain steel valves 2 to 1- when matched with a hard coated or chrome plated valve stem.
Cast iron guides will damage the hard coating on Black Diamond valves and usually cause a lot of valve stem wear. Cast iron guides when used with a chrome plated valve stem [if you can find them] can provide reasonable life. In racing, if you over-rev and bend a valve, the bent stem strains the guide. Bits are likely to break off a cast iron guide and drop into the cylinder, damaging everything present. A bronze guide is more likely to stay in one piece.
Common Valve Guide Materials comparison:
copper-nickel-silicon alloy developed in WWII for high performance bearing applications, including main bearings in the Merlin engines in Spitfires. Considered to be close to factory for our British bikes. I have it on good authority that Hidural 5 was used for valve guides at Triumph.
95% copper, 2% nickel, .8% silicon, 0.5% zinc, 0.10% iron
C630 (Ampco 45) Aluminum, Nickel, Bronze:
Can be really difficult to ream without the proper tools and set up.
85% copper, 11% aluminium, 6% nickel, 5% iron, 1.5% manganese
Lacking nickel and iron make this alloy easier to ream and hone than C630
60% copper, 34% zinc, 2.5% manganese, 2.2 % lead, 1% silicon
Colsibro® is a precipitation hardening copper nickel silicon (new to British bike market – and rather expensive):
Colsibro material is a copper nickel silicon alloy, and is second to none as valve guide material. Has high wear resistance and heat conductivity, and the lubricity prolongs valve stem life. Some shops are now making repair valve seats from this material.
97.4% Copper, Nickel 2.0%, Silicone 0.6%
Non-Bronze copper zinc alloys:
Brass is mainly an alloy that consists of copper and zinc. Some trace elements me be added as well. These varying mixtures produce a wide range of properties and variation in color. Increased amounts of zinc provide the material with improved strength and ductility. Brass can range in color from red to yellow depending on the amount of zinc added to the alloy. If the zinc content of the brass ranges from 32% to 39%, it will have increased hot-working abilities but the cold-working will be limited.
Brass is commonly used for decorative purposes primarily because of its resemblance to gold. On British Bikes it will be found in limited use. Mostly in the electrical system as contacts or terminals. Styling items such as tank trims and instrument bezels were chrome plated brass. Brass makes very poor valve guides. Years ago a vendor supplied a batch of, easy to machine, “bronze” valve guides. These didn’t last long at all.