Michael’s Blog

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.

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.

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.

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The original exhaust systems on Healeys were very different from those currently available.

This deviation from “originality” is particularly evident on the BJ8 and is not mentioned in any of the “authoritative” books I have read. This is my attempt to ensure that this detail of originality will not become lost as the years pass.

Some of the differences that I have noted are as follows.

The front down pipes utilized one continuous sweeping curve rather than the series of small bends on the mild steel systems that have been available for the last 20 or so years.

I believe some of the stainless systems may have the single curve but of course stainless in definitely not original.

These are a pair that I had hanging around for years for when I build my BJ8 “museum” car..which I don’t think is ever going to happen.

The front mufflers are constructed entirely differently from anything I have seen available for many years. The parts book shows what I believe were the original type.

The mufflers originally used were what I describe as “pinched end” mufflers. These were “straight through” type mufflers and were stuffed with fiberglass. The metal used was quite heavy and they actually lasted for a very long time in good climates. Each muffler is 27 1/4″ long and oval in shape 31/2″ x 5 1/8″ and there is a folded and crimped seam along the top center. The edges where the pinched sections meet appear to be TIG welded together.

I had a couple of these they are both lefts, not a pair. 

The parts book shows that the rear mufflers were not of the “pinched end” type but, like a few other things in the parts books, I believe that illustration shows what was available from the parts system rather than what was actually fitted to the cars. This did not happen very often as the original parts books are amazingly accurate and the only other questionable item which springs to mind are the felt insulation pads which are illustrated as fitted to the inside of the bonnet of non U.K. cars. I have never seen these or any evidence of them, however I digress.

The parts book shows the rear mufflers (resonators) as being the crimped end type rather than the “pinched end” type. I have seen a number of very original cars where the resonators were the pinched end type and am fairly convinced that these were what was originally installed.

Fortunately the hanger and clamps have been quite faithfully reproduced.

I’m sure there are people out there who have seen more original systems than I have and I would appreciate any comments or corrections.

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After several days of work we have completed the inventory of the used parts that I want to sell. Anyone who is interested in making an offer please send me an e mail: magicareprotect-usedparts@yahoo.ca and I will forward the complete sorted list in Excel format or you can view it unsorted when you scroll down in the post below

PLEASE NOTE THAT THESE PARTS ARE ONLY AVAILABLE AS ONE JOB LOT but I will forward enquiries for individual parts or groups of parts to the purchaser.

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