Technical Information for Gunsmiths and for Classic Car Enthusiasts

MG Restoration

Crack inspection

for the Hobbyist

Les Bengtson

One problem facing all MG owners is vehicle age. The newest “classic” MGs (MGB and Midget) are now over twenty years old. Relatively few of these cars have lead a pampered life as a show car. A few have been used as “pleasure vehicles”, driven only for fun and only when the weather is predicted to be pleasant. However, the MGs were sports cars; most have been driven hard and many were not maintained, as they really needed to be for maximum life expectancy. In addition to this, there were certain compromises required of the company as a part of a larger corporation—one not always sympathetic to the MG. As a result of age, hard usage, and, in a few cases, poor design, there may be a problem with parts cracking. While this problem may exist with any stressed part, there is a particular problem with the MGB cylinder head.

Recently, I needed a cylinder head to ship off for modification. This was to be used on a 77 MGB roadster (known as a rubber bumper, or RB, car to the community). Cylinder head cracking has been reported by all segments of the MG community. Even the earlier MGA, which used the same BMC “B” series engine, suffered from some cracked cylinder heads. But, this problem is particularly acute with the RB engines. Whether it is due to the change in volume and shape of the combustion chambers, the induction hardening of the valve seat area (for unleaded fuel use), the water take off at the rear of the cylinder head, poor quality casting materials, or some other reason, no one can state definitively. All of these reasons have been cited as possible causes, but no one seems to have done any definitive research. Thus, we are left only with the practical problem of how to determine whether a cylinder head is cracked. There is nothing that we know can be done to prevent cylinder head cracking. All we can do is identify cylinder heads, which have cracks and replace them with heads not having cracks. Finding such a cylinder head may be a problem. I went through four cylinder heads before finding one that was not cracked, a failure rate of 75%. Conversations with professional mechanics specializing in the MG indicate that this is similar to their experience. They indicate that 75% to 80% of the RB cylinder heads they have examined were cracked and that a somewhat lower percentage of MGB, non-RB, and MGA cylinder heads were cracked.

What does this mean to the owner? It means that the cylinder head is a known problem area with the B series engine and some form of inspection needs to be performed periodically. The type of inspection will vary with what is to be done and what problems are observed. The most basic inspection is done on a regular basis -- every time the owner checks the oil. The most commonly observed failure is an external crack, running upwards, in the area of the number three spark plug. When checking the oil level, one simply looks for signs of coolant leakage in this area. When the cooling system is pressurized, a small amount of coolant is forced out and will show up as small droplets of coolant or as a thin line of coolant seeping from the head. This form of leakage will also show up when the cooling system is pressurized with a “radiator/cooling system pressure tester”. Possible cracking is indicated by the system not holding pressure. If the cracking is external, you will see the coolant seeping from the area of the crack. (You will also see coolant leaking from cracked hoses, poor connections and bad radiators or heater cores. A “blown” head gasket will also allow pressure, and coolant, leakage.) Thus, until the crack becomes fairly severe, many people never know that they have a cracked cylinder head. Proper examination for cracking requires removal of the cylinder head and a physical inspection.

Most commonly, a cylinder head is removed because of a blown head gasket or when a valve job is to be accomplished. At that point, inspecting the cylinder head for cracks makes excellent sense from both a performance and an economic standpoint. Rebuilding a cracked cylinder head, as has been done, is a waste of money and time. That being the case, the question becomes—How is a cylinder head, or other part, checked for cracks and which methods are suitable for hobbyist use?

There are three methods of inspecting for cracks—visual inspection, dye-penetrant testing and magnetic inspection. The first step in any of these procedures is a through cleaning of the cylinder head or other part.

The part should be thoroughly de-greased and cleaned. Degreasing can be accomplished by using one of the readily available degreasers such as Gunk, Castrol Super Clean or Simple Green. The parts are sprayed with the degreaser, wire brushed, as necessary, rinsed, then dried. When the parts are degreased and the surface dirt removed, they are inspected for rust and carbon build up. Rust and carbon build up may be removed by wire brushing, but some form of abrasive blasting is both faster and leaves a better surface for inspection. After the part is thoroughly cleaned, it is inspected for cracks. A set of “Opti-Visor” magnifiers or a magnifying glass can be used with good effect during the physical inspection. On the cylinder head, the area around the number three spark plug and the valve seats on number two and three cylinders are the most commonly cracked areas. On other parts, any stressed area should be carefully inspected. Any signs of cracking detected by a visual inspection should be grounds for declaring the part non-serviceable. If you can see the crack, it is severe. But, the fact that the cracking has not progressed to the point it is readily visible does not mean that no cracking is present. There are two other tests, which may be performed after the visual inspection. These are the magnetic inspection and the dye/penetrant inspection.

The magnetic inspection, often referred to by the brand name Magna-Flux, is simply placing the cleaned part in a magnetic field and sprinkling iron filings over it. The filings follow the lines of magnetic force. Where the lines of force are disrupted by a crack, the line of the crack can be seen by a change in the pattern of the filings. The biggest advantage of this system is that it can detect cracks, which have not progressed to the surface yet. The drawback of this system is that it is not suitable for hobbyist use. Thus, you have to find a machine shop using this technique and take the parts to them for inspection. Prices for this service will vary, but having thoroughly cleaned the parts before taking them to the machine shop will reduce inspection time and shop charges.

The second form of inspection is the dye/penetrant test. The drawback to this method is that it will only catch cracks that have reached the surface. Its greatest asset is that it can be used by the home hobbyist and is a fairly simple procedure. In fact, it is used in many different industries as an easy replacement for the magnetic checking. The fact that it can be used with non-ferrous metals makes it very desirable for use as a general inspection tool. The particular product I use was available from several “machinery supply” sources and industrial hardware stores. It is called “QA Inspector” and is manufactured by Sprayon Products, which is a division of Sherwin-Williams. Since this process is suitable for hobbyist use, I decided to test it and photograph the results for this article.

First, however, be aware that this is an industrial process. It does contain chemicals, which are bad to breath and can cause damage if they get into the eyes. I use the product outside and wear safety glasses when using it. During the process, the wearing of chemical resistant rubber gloves is well advised to prevent absorption through the skin. The cost of the three-can inspection kit was under $20 and the gloves can be purchased for about $10. Thus, for an investment of about $30 you can crack check ferrous and non-ferrous parts. Used properly, this inspection can save hundreds of dollars, perhaps your life. But, you must read and follow the label directions.

The first step is the same as the physical inspection, making for a logical progression. Degrease, wire brush or abrasive blast (blasting is the preferred option if it is available to you), physical inspection, then, dye/penetrant inspection.

After the physical inspection, the part is then cleaned again using the “QA Inspector Cleaner”. This chemically cleans the parts, allowing the dye to penetrate into cracks. I find that spraying the cleaner onto a paper towel and then wiping down the surface to be checked works well.

When the surface is cleaned, it is sprayed with the second can in the system, the dye or penetrant. This is allowed to soak in for up to 20 minutes, but should not be allowed to dry. I used a ten-minute soak for my tests. After the dye has been allowed to soak, and before it dries, you again use the paper towels with cleaner to thoroughly clean the part, removing all of the visible dye.

When the dye has been removed, the part is sprayed with the developer, which is a white colored, thin substance. The dye will “bloom” through the developer producing a red-pink color anywhere the dye remains. This is mainly in any cracked areas. However, the rough “as cast” areas of the cylinder head will produce a faint blooming. This is quite different from the straight or jagged lines produced by a crack, as the photos will show. The entire testing procedure takes less than one half hour and is considered a definitive check for surface cracking. After testing, the part is cleaned again to remove the developer and sprayed with oil to prevent rusting if it is to be taken to the machine shop. If it is not, the part should be sprayed with oil or painted immediately. At this point, the part is “chemically clean” and will rust quickly.

In summary, there are three main methods of crack inspection. Physical—cleaning the part and inspecting visually, the dye/penetrant test—definitive for surface cracks and capable of being used with any metal, and the magnetic testing—will show up sub-surface cracks, but can only be used with ferrous metals. The first two methods are suitable for hobbyist use and are relatively inexpensive. The magnetic testing requires equipment beyond the reach of most hobbyists, but is a good follow up procedure when having a cylinder head modified. The physical inspection, followed by the dye/penetrant inspection, can save the hobbyist significant amounts of money, when properly used. This procedure has been used within industry for many years and there is no reason that the hobbyist cannot take advantage of this technology.

This monograph may be reproduced only for non-commercial use without other permission of the author. Reproduction for commercial use only by written permission.

Copyright © 2003 by Les Bengtson