People seem to think they need bronze valve guides in their modified engine if the stock guides are cast iron. The tuning shop will certainly be happy to sell you them or fit them for you. They make a lot of money from selling bronze guides. Trouble is in most cases it's wasted money and in fact usually a backward step not an improvement. In many cases the material isn't even bronze.
Bronze is the term for an infinite variety of different alloys consisting primarily of copper and tin whereas brass alloys are primarily copper and zinc.
Bronze is one of the most common bearing materials used in engineering and in the right application and if the bronze is the correct grade for the job it performs very well. In engines you might find it in the small end conrod bushes, valve guides and sometimes in crank or camshaft bearings.
Whether bronze is used as the stock material for valve guides varies widely in the car world. The Japanese never use it, even in 200 bhp per litre motorbike engines which might tell you something. Ford tend not to use it nor do most American engine manufacturers. You'll also never find it in engines designed for really high mileage like truck diesel engines because it wears out too fast unless you use the strongest grades of bronze. It's mainly in continental Europe that it became fashionable for some obscure reason so you'll find it in Citreon, Fiat, VW, BMW etc car engines. Well at least you might think they use bronze because the valve guides in those engines are copper coloured but in fact the material usually turns out to be a cheap and soft copper zinc manganese alloy called "Manganese Bronze" so in fact it's really a type of brass rather than a true bronze. More on that later though.
Does bronze have any advantage at all over cast iron as a valve guide material? You can certainly find people who will ramble on at length about how bronze conducts heat better than cast iron so the valves must run cooler. Trouble is these people are only repeating what other people once told them and I doubt if you'll ever find anyone who has actually run a scientific test proving any advantage. In fact most of the valve heat is dissipated through the valve seat not the guide. Any potential advantage of bronze to conduct heat away better will certainly soon be lost if the wear rate is higher resulting in bigger valve to guide clearances and that will definitely be the case if a cheap grade of bronze or brass is used. If there was ever a class of engines which ought to benefit from better heat conduction away from the combustion chamber it would be the very highly tuned bike engines but as I said above they all quite happily use cast iron guides or at least some sort of ferrous based sintered material rather than anything copper based. Fancy that. I wonder why?
If for some obscure reason your valve stems are not chrome plated or tuftrided then you have to use bronze guides because cast iron won't run against plain steel at high rpm. However that's the only reason I can think of where you would actually need to use bronze and you should never use valves with plain uncoated stems anyway. Chrome or tuftriding coatings are much harder than plain steel and deliver a much longer service life.
Can bronze work well as valve guide material? Well sure if it's a decent grade of bronze but as I said above the stock material in european car engines is usually Manganese Bronze (material code CZ114) which is really just a high tensile grade of brass and it wears out like the clappers. Anyone who has experience of stripping down and measuring engines such as the Peugeot 205, VW Golf will know just how fast the valve guides wear out compared to engines with cast iron guides like the Ford Pinto or CVH. The usual giveaway is puffs of smoke at startup and that can start happening from as little as 50,000 miles if the engine gets a lot of hard use. I've seen the guides on VW race engines last as little as half a season. So why in god's name do the car manufacturers use this stuff if it wears out so fast? Simples - it's cheap and easy to machine compared to proper grades of bronze. Never underestimate the desire of car companies to save a few quid per engine as long as they don't get too many warranty claims.
Proper grades of bronze i.e alloys with tin not zinc last much longer than Manganese Bronze. One of my favourites is Aluminium Bronze (material code CA104) which is incredibly durable but also very hard to machine. It's one of the strongest grades of bronze ever developed. Even with carbide tooling it squeals at you when you're trying to turn or drill it, gets red hot and generally complains like buggery at every stage of the process. However if you manage to actually make a set of guides out of this stuff without wearing all your tooling out they'll last until hell freezes over. It has a tensile strength about 50% higher than Manganese Bronze and on a par with some of the fairly tough steels. In America you tend to find race engine companies offering silicon based bronzes. Colsibro which is a nickel silicon alloy is a popular grade in the UK and although not as tough as CA104 looks as though it ought to do the job although I've never tried it yet.
You can see the properties of many of the commonly used grades of engineering bronze here. You'll also see that Manganese Bronze CZ114 is listed amongst the brasses where it rightly should be.
One that looks well worth trying is the Colspeed 90. Even stronger than CA104 but easier to machine because it has some lead added to the alloy mix. That also ought to improve its wear properties in conditions of heat and poor lubrication which pretty much sums up the service condition of valve guides with stem seals fitted.
The thing is bronze costs many times as much as cast iron and you have to use a good grade of it just to get a service life anywhere near that of cast iron so why bother replacing perfectly good cast iron guides with bronze ones? In my opinion that's easy - DON'T. You won't see any advantage in the vast majority of cases.
As an aside I was chatting to one of the technical bods at Cosworth many years ago and the subject turned to bronze valve guides. He regaled me with a story about a particular engine they developed, I think it was the BDA but my memory fails me, which they brought out with cast iron guides for road use and bronze ones in the race versions for the sake of fashion. The trouble is the bronze guides wore out so fast the cast iron guide heads started to attract a premium on the second hand market for race use because the guides lasted much longer. He admitted with some embarassment that the grade of bronze they'd selected was purely because it was cheap and easy to machine rather than choosing the right material for a long service life. I'll hazard a guess they also used Manganese Bronze.
If you do decide to fit bronze valve guides ask the seller for the material code which will no doubt perturb them considerably. If it's CZ114 then don't even bother. Ideally you want a tin based copper alloy not a zinc based one and with at least 550 N/mm^2 tensile strength.
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First written 26th February 2011.