Bronze Guides in Cast Iron Cylinder Heads.

Back in the early days of my business I had a very unhappy customer and, to make matters worse, the customer was a lawyer, not a good combination. Our lawyer friend brought me an MGB with its engine, in boxes with a whole bunch of new parts and asked me to reassemble it and put it into his car. He had had various parts of the engine rebuilt including the carburetors and the cylinder head, which sported brand new bronze valve guides but had decided that he lacked the time, or skills to complete the job himself. After negotiating a price for the job and using his parts, I reassembled the engine and put it into the car. It ran well and he was very happy when he picked the car up on a Friday night with a view of driving it directly up to his cottage.  I was more than a little surprised upon my arrival at work the following Monday morning to find the car dropped in the driveway of the shop with a note written in a distinctly cool tone on the driver’s seat and a very seized engine. It turns out that there is a bit of a hill on the highway route to his cottage and seconds after cresting this hill there was a loud bang from his newly rebuilt engine, followed immediately by a squealing of tires as that engine seized and the rear wheels locked up. It must indeed have been a little frightening in the heavy cottage traffic and I could appreciate the reason for the man’s ire.
Various lawyers letters flowed back and forth all of which culminated in a small claims court claim being filed. I disassembled the engine and found that #3 exhaust valve had gone through the top of the piston and caused unimaginable indignities to the rest of the engine. I studied that engine in intense detail before I figured out the sequence of events which resulted in its failure.

Many after market suppliers extol the virtues of the bronze or silicone bronze valve guides that they sell and, although these may wear marginally more slowly than cast iron valve guides, it is my belief that bronze of any type is not an ideal material to use for exhaust valve guides in cast iron heads.

This is a simplified cross section of a guide installed into a cylinder head.

normal-head.bmp

Valve guides are normally a press fit into the cylinder head, that is to say that the guide is very slightly larger than the hole in the head into which it is pressed. The interference, i.e. the difference in diameters, is usually one or two thousandths of an inch. This tight fit is sufficient to keep the guide securely located in its correct position in the cylinder head. Once the guides are installed the inner bore is usually reamed to an exact size to produce, in the case of the exhaust valve, a clearance of around 0.0015 to 0.003” between the valve stem and the guide. This tight fit is necessary to correctly position the valve on its seat and because excessive clearance here can result in hot gasses passing through the space and burning oil in the engine making things all black and yucky.

Now, although I have never stuck my finger into the exhaust gas stream to measure it, I have it on good authority that the gasses exiting the combustion chamber and shooting up the exhaust port have a temperature of the order 1300 – 1500 degrees F. and, under full power, can go as high as 1800 degrees.

If the valve guides are cast iron, the same material as the head, the interference fit of the guide in the head remains constant as the engine heats up or cools down. Bronze, according to my Machinist’s Handbook has a linear expansion per unit length (coefficient of expansion) of 0.00001 per degree F. whereas for cast iron that expansion rate is 0.0000065. In little words this means that, when heated, bronze expands 1.5 times faster as a result of the increased temperature than cast iron and it is these different coefficients of expansion which create problems for our bronze valve guides.

If we presume that a particular exhaust valve guide has an outside diameter of 0.75 inches and it is heated from 20 degrees F to 1500 degrees F its outside diameter will increase by 0.011”. Of course the hole in the head will also expand as a result of this temperature increase but, the cast iron has a lower rate of expansion and is cooled by the engine coolant. If the temperature of the cast iron in that area of the head rises to say 500 degrees the hole will only increase in diameter by about 0.002”. The result of all this is that the one to two thousandths of an inch of interference fit of our guides becomes eleven thousandths of and inch of interference when things get hot!!
The question arises then as to where all this expanded bronze material goes? Those familiar with the properties of cast iron will be well aware that it is not very elastic so it is unlikely, even give the huge forces involved, that the hole will temporarily stretch to accommodate the expanded guide therefore the only realistic conclusion is that the head presses in on the guide and effectively compresses it some 0.011”. One can only imagine the forces involved and, one can occasionally see evidence of this where cylinder heads are cracked radially around the valve guide holes when bronze guides have been used!!
Amazingly, for the bronze guides to continue to function normally, the bronze must be sufficiently elastic to actually return to its original dimensions as the engine cools and, of course, bronze is another material not given to stretching, or in this case being compressed very much!!
I believe that under normal engine operating temperatures this is actually what happens but, if the exhaust gas temperature become further elevated, things really go pear shaped, literally.
Most fuel systems are set up to provide a slightly rich mixture at full throttle. Aircraft always use a very rich mixture at takeoff power. The reason for this is that the excess fuel will actually cool the induction charge and produce more power than would be the case with a perfect mixture setting. Of course this over rich mixture also serves keep the cylinder head temperatures down a little and prevent detonation. However, as anyone who flies a piston engined aircraft will tell you cylinder head temperatures rise very quickly if you try to get power from an engine when the mixture is set lean.
When this over temperature condition occurs in our cast iron headed car engine the expansion of the bronze reaches a point where the “elastic limit” of the bronze is exceeded.
What the term “elastic limit” what this actually means is that beyond that point of deformation the material is distorted to such an extent that, when the deforming forces are removed, the deformed material no longer returns to its original shape.
When this occurs in the case of the valve guide, rather than actually expanding, the guide is actually radially crushed by the expansion forces. Of course, with the small clearance between the exhaust valve and its guide any permanent inward deformation (radial crushing) of the valve guide is, when the guide cools down, going to eliminate this clearance and cause the valve to seize in the guide.
I have only seen this situation occur in a handful of cases but, as in the case of our learned friend, when it does the results will be nothing short of catastrophic.
In addition to the decrease of the size of hole through the guide where the valve stem is located the outside diameter is also going to permanently decrease which will result in the guide becoming loose in the head in the areas where it is enclosed by the head material.

Prior to my figuring this problem out I had frequently noticed, when removing worn bronze valve guides from cast iron heads, that there was a distinct step right where the guide protruded into the port and I it took me a few years to put two and two together on that one.

This is what the result looks like.

normal-head-hot.bmp
Of particular interest to performance car owners will be the fact that this problem is more likely to occur in multi carburettored engines because, in this type of engine, it is relatively common for one carburetor to be delivering a leaner mixture at high throttle without the overall driveability being adversely affected and that leaner mixture will result in substantially higher exhaust port temperatures.
So should one use bronze valve guides? Absolutely, but only if the cylinder head is made of aluminium because, it just so happens, that the coefficient of expansion of aluminium is very similar to that of bronze. If the bronze guides are installed correctly in an aluminium head, they should not suffer from this distortion problem.
If you are rebuilding a cast iron head stick with cast iron valve guides because you never know when a blocked jet or a intake manifold gasket leak is going to provide the leaner mixture conditions which will lead to disaster.

About Michael

Who is this guy? Born in New Zealand some time back. Went to Maori Hill Primary School then Kaikorai Valley High before joining the RNZAF and an Airman Cadet in 1968. Graduated 1972 with an NZCE in Aeronautical Engineering. Then embarked on the typical Kiwis "Big Trip Overseas". Got to see quite a few places, and spent a while in the U.K. "home" as it was refered to by many New Zealanders in those days, before travelling on to New York and then to Canada by bus!! This trip is presently on hold (has been for the last 34 years). Met my dear wife Judy not long after arriving in Ontario and we have been happily married since 1976. After travelling around New Zealand and the pacific in 1979 I started Precision Sportscar andfor the next 23 years grew the business and helped raise 2 boys Drew and Robin.
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17 Responses to Bronze Guides in Cast Iron Cylinder Heads.

  1. Barry says:

    Michael,
    It is not uncommon in cast iron BMC heads to have to give extra clearance to #s 2&3 exhaust valve stems, because they are immediately adjacent to each other and do not shed heat as quickly as the others. I have only experienced this on race engines where we typically run at lean best power.
    I use only Manganese Bronze which is very hard and can only be clearanced by honing. Not a practice that most repair shops have access to.

  2. Potato says:

    I find all the engineering and small details like this that go into each tiny part of a car fascinating!

    I wonder if the bronze guides would work better with an “expansion channel” cut in around the outside?

    I know that lean fuel mixes have more emissions problems, despite saving gas — I always figured it was simply due to having unused oxygen in the hot, compressed gas that can then react with nitrogen, but if it’s hotter too, then that would explain even more! I’m a little confused as to why that happens (to my non-car-specialist thinking, more fuel = more burn = hotter). I can see how fuel would cool the cylinder on the injection phase (the fuel would start cold from the tank and sap heat from the iron, and also the expansion from the spray would cause cooling), but fail to see how that would lead to a net cooling after that fuel gets burned…

  3. Randall says:

    FWIW, the same problem is well-known on TR2-4 engines with bronze guides installed. The exhaust guides in particular must be reamed larger after installation than the factory specified clearance. Fortunately, the TR motor is non-interference meaning a stuck valve just causes a loss in power.

    Reaming them larger of course reduces the life, so I’m inclined to agree that bronze guides are a waste of money. The larger clearance also allows more oil to be sucked past the intake guides.

  4. Dave says:

    Michael,
    The valves are also cooled when they contact the seats.
    Once the stems start to stick in the guides that cooling effect is lost thus and raising the temp at the head of the valve. Probably the reason for the “bulge” in your drawing.

  5. Gordon says:

    I think your numbers on the degree of expansion are possibly flawed. There will be a teperature gradient through the wall of the guide, with the inside obviously hotter, getting up towards the valve stem temperture but will be less than the valve due to the lubricating oil film. The outside of the guide, in the cast iron head will be close to the teperature of the cast iron. The net result will be that the guide will not expand as much as your sums predict. The actaul strains and stresses seen would need a complex mathematicl modelling on a powerful computer to prdict and will have been done by many engine design teams. I wonder if there is any published papers that would give us more info? I don’t mean to imply that you are wrong in your conclusion but your analysis may be over simplistic. The cricality to the fits and tolerancies of guides,valves and their actual composition will ll play some part.

  6. Mark Wallis says:

    So, having established that we need more analysis of the bronze/iron interface, what was the result of the lawsuit, since you didn’t supply the component parts?

  7. Michael says:

    After a lot of toing and froing he ended up paying to have the engine rebuild completed again, and to cover all my research time and court costs. Furthermore he turned out to be a good and happy customer thereafter.

  8. Automotive Machinist says:

    You can run manganese bronze guides at .0013 intake and .0025 exh all day long. Been there and do that every day. You can run bronze liners at .0008 int. and .002 exh. but they won’t take the exhaust heat as long as bronze. The problem is not all machine shop have the proper equipment to install and clearance guides. It requires Head Shops that can machine a valve seat to .0005 run-out or less, Hones, bore gauges and micrometers capable of accurately measuring bores and valves to.0001 along with a HIGHLY skilled machinist who gives a rat ass. Good luck finding people like that. I’ve been in this trade for 30 years and trust me there are very few machinist who can hold the tolerances on anything day after day.

  9. Michael,

    I agree with the conclusion that “in cast iron heads, run a cast guide”.
    We have been reworking cast iron heads since 1981 and we have NEVER had a cast iron head with a cast iron guide stick. Period.

    When bronze guides are installed in a cast head, cross your fingers. The usuall clearances of .0015 intake and .002 to .0022 exhaust are typically fine. However, if the motor is overheated or if the motor is restricted to a small carb and cast iron exhaust you are asking for trouble. We have, over the years, had properly clearanced bronze guides stick in these restricted applications.
    Also, never run a valve seal on the exhaust with a bronze guide in cast or aluminum heads.

    Randy B.

  10. nobody says:

    Bronze valve guides trump cast iron. your valve stuck because the machine shop did a piss poor instalation of the valve guides. i have rebuilt two motorcycle engines bot had bronze. bronze lubricates and wears better. motorcycles need the due to red lining at 9000 rpm.

    nobody

  11. Michael says:

    nobody, if you are running a scooter with a cast iron head it must be one old machine!!

  12. Steve says:

    I am running a scooter and it is old, 1938 inline 4 cyl Indian. It has 1 1/8′ carb for all cylinders. Overhead intakes and side exhausts.I have had problems with exhaust valves and now guides, both cyl and guides are cast iron.

    It has been recommended that i use manganese bronze guides withch i would fit in to the existing cast iron guides after hoging them out a bit. The idea here is to avoid a guide liner situation. Then get new valves with chrome stems. I would like to hear what others think.

    Steve

  13. Machinist says:

    I do a lot of road race MGB, and similar syle motors like the 1500 Cortina in lotus’s. Bronze guides are all I use with clearances set at .0014″ INT/.0018″ EXH on a 7mm stem – never have had an issue with sticking. As was pointed out earlier the guides need to be honed precisely to not only the proper size as measured on a quality gauge, but also be straight and free of taper or other out of shape conditions. The tune is also very important, running with a vacuum leak or very lean will cause these kind of problems too.

    Obviously these numbers are dynamic as is the rest of the engine, if you are running a stock class motor at high rpm while needing to use factory iron manifolds the heat will be more intense and clearances will need to be increased to compensate.

  14. Andy says:

    I also think that bronze valve guides are a good idea.they take up any slack that the old guide would of had.I Know that on some cylinder heads it can give you more power.To me it can be a ali or cast head.

  15. Ragworm says:

    I run a cast iron head on my 500cc single cylinder motorcycle, standard fitment is cast iron guides and valves used to be KE965 nimonic I think. The valves and springs operate in open air (1933 technology) with no lubrication. This set up worked fine for years, 4 to 6 thousand miles on a set of unlubricated guides I considered acceptable. I ran out of nimonic valves and the only direct replacement was 214N stainless and I was assured that as long as they were plasma nitrided they would run fine in cast guides. I have managed to arrange a oil mist feed to the guides, a very hit and miss thing but I thought better than nothing.

    Let me say that the first set of stainless valves and cast guides lasted less than 50 miles, that was the average milage with the next 4 sets!!

    I was told by the very eminant valve supplier that the mixture does work and the fault lay in my abilty to fit them correctly, seats not cut square, guides not reamed to correct clearance, guide not tight enough in the head so not transferring heat properly,
    valve geometry not correct, etc, etc, etc.

    What was happening no matter what I tried was that the valve stem was galling, I think that is the correct term, it was picking up from the cast guide and forming a sort of rasp then procceded to file its way through the guide, typical wear in 50 miles was about 0.060″ to 0.080″, completely knackered, thats 60 to 80 thou!!

    I then had my local engine shop make and fit PB guides, a rush job because the smart tight cast iron guides he had fitted for me two days earleir were in the bin, this was Monday and I was off to Spain on the Tuesday for an old bike rally, boat booked and all that sort of thing, anyway PB guides fitted, no time to test and off down to Plymouth for the ferry, I am sure you can guess what happened next, going along quite nicely when the engine cut out, shut off to slow down, go 100 yards or so and ‘Ping’ and away the engine goes, it took me a little while to decide whether it was a sticking valve or a cooked plug but the ‘Ping gave it away. If I keep the engine on a light load it ran fine but any sort of energetic riding seized the valve in the guide, luckily no piston in the way.

    Now I made it to Spain and did about 700 miles that week and if the valve pinched once it pinched 50 times. Anyway I made it home and had the engine man give me a little more clearance in the guide. After all that seizing the valve stem and the guide were beautifully polished, not a mark on them, the same could not be said for the cam and follower however they were blue and very badly smeared from all the loading from the tightening valve.

    I ran this set up for about 18 months and about 2000 miles with no problems at all and I did another Spanish rally. After I got home I had reason to remove the exhaust pipe and when I did, it rattled and out came the end of the guide, the bit that sits in the port and it was in about three pieces.

    Off with the head and remove what was left of the guide, not at first a problem as I have another PB guide in the drawer waiting but the guide bore looked black and bit corroded after many guide replacements the holes tend to go oval so I bored the guide hole and guess what, a lovely crack down its length, bugger!!

    I am sure the expansion within the guide contributed to this. I now have a dilemma, I have a spare head, not cracked, if I fit a PB guide I risk cracking the head, if I fit cast guides, well I cannot do that for all the reasons above.

    It is possible that the guide broke up in the port due to the expansion and swelling as detailed above, sound plausable.

    So any advice would be most welcome, please

  16. Michael Salter says:

    Hi Ragworm,
    I’m surprised that you managed to get that distance on the bronze guides without some sort of catastrophe!! IMHO you are running a serious risk of cracking the head because of the different thermal expansion rates of the bronze and cast iron. The end of the guide broke off, as you have probably surmised because it cracked at the point where it exited the head casting.
    If, after some use, you were to press one of the bronze guides out of the head downward toward the combustion chamber if you would find a distinct decrease in diameter where it entered the head casting in the port.
    This just happened to a friend’s mini racer and there was a 0.012″ step at that point. You just have to imagine how much force is required to “neck” the bronze down that much to realize why the heads crack.
    I really don’t know what to suggest for your problem. I would certainly stay away from SB guides but maybe SB liners in the guides would do the trick.
    http://www.cylinderheadsupply.com/valve-guide-liners-bronze-bullet-smooth-wall.html
    This is a pretty common practice here in NA and that way you get away from the “lump” of SB issue.

  17. Ragworm says:

    Hello Michael,

    Thanks for the reply, I appreciate it. My motorcycle is a 1933 Model 18 Norton 500cc single overhead valve pushrod engine, however the sports model, the International also has a cast iron head and has always to my knowledge run bronze guides with no trouble which confuses the hell out of me. Your suggestion regarding the bronze liner is certainly worthy of further investigation, I will look into it and if I find an answer I will let you know. Thanks again Michael for your time and advice.

    Pete

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