Used XPAG Cylinder Heads; Report Prepared by; MG Services 349 Glenroy Ave. Cincinnati, Ohio 45238-5762 These two photos are from a cylinder head that was brought into the shop for a valve job. All of the valves were not present and the valve spring parts were off the head and thrown in a box. Of course they were covered with all sorts of gunk. If you have been looking for a replacement head this is what you would typically find. The overall condition of the head is what you should be concerned about at this time. 1 1
Therefore, that water sat in the head and rusted for all of the years that the engine had not been run. What you cannot see in these photos is that water was not in all of the combustion chambers, just a few. Rust is not the only issue that we are looking for. In this same head we see a pair of valve seats that have been cut into the meat of the head quite deep and pitted badly over the years of use. In this case it is not a problem because we are installing larger seats and valves. We are looking for any part of the casting that may have been chipped or otherwise damaged. While we are inspecting the condition of this cylinder head, you can plainly see that, at some time, some of the combustion chambers were filled with water, which was captured in the head when the valves closed. With the valves closed the water could not be expelled out through the exhaust. At this point, is would be wise to understand the capabilities of your machine shop to clean engine parts. How much rust can they get out? What about built up carbon, or grease? You may have to take the head to a de-rusting or cleaning specialist. There are two types of heads that came with the 1250cc XPAG engines. They are virtually the same with the exception of the water jacket holes. The 1500 cc XPEG heads are all round holed heads, see below left, but the diameter of the combustion chamber is slightly larger. 2 2
The top left photo on the previous page shows that the water jacket holes that go through the head are round. This is known as a round holed head. The top right photo on the previous page shows the details of a head with elongated water jacket holes. This is known as a banana head. The corresponding engine blocks have the same type of water jacket holes. If you are buying a spare head, try to find the one that matches your engine block. If you do not know which type of head- block that you have, check the engine numbers; Engine numbers up to 22734 should be banana holes. Engine numbers from 22735 should be round holes. If you find a head that has a slight amount of rust that has appeared after sitting unused for an extended period of time it should not be a concern with the performance of the head. The photo at left shows the area below an exhaust valve highly rusted. How did that happen? Let us take a closer look. If you look at the area of the elongated water jacket holes you will see the remnants of the circular pattern of the round holed head gasket. At some point in the past there was a huge water leak into the combustion chamber possibly from the use of the incorrect head gasket. This engine was probably retired from service at that time and the water sat in the combustion chamber and caused it too rust. This is a condition that I commonly see. A round holed gasket was used on a banana head. It is best if the correct head gasket is used. But I do see engines where the head or block has been replaced with a non-matching unit. If you are in a bind it is acceptable to use a banana gasket on a mis-matched set of components. If you find yourself in need of a cylinder head to replace the one you were using, because it was damaged. Maybe you are just looking for a spare just to have on hand, just in case. First, these cylinder heads have been around for quite a number of years, and you must check the overall condition of the head. Is it massively rusted, chipped, broken, or otherwise compromised? If you have been working with a particular machine shop, you should know the capabilities of its cleaning techniques. Additionally, you will need to determine what work had been previously done to the head. The original thickness of the cylinder head casting was 3.022 thick. 3 3
After years of service, your head may have been warped or twisted from the heat or stress of the engine. If it had a valve job done to it, probably the head would have been decked to straighten or flatten it out so that it would mate up with the deck of the engine block. If this machining had not been done and the head or block was warped, then the head gasket may not been able to seal the area between the head and the block. Any time the thickness of the casting has been altered, the geometry of the valve train movements will also change. As the head gets thinner from the machine shop the valve train can start to bind up on itself. Let us first understand how the change of thickness due to this machining of the casting, affects the valve s operation. By using by the adjusters in the rocker arm you can change the lash on the valves. As the casting becomes thinner, in effect, the push rods become longer. Again, the threads will limit the travel of the adjusters. The above two photos show the adjusters and lock nuts in the rocker arms. When they are adjusted to achieve the proper valve last, care must be taken to check to see if there is any interference with the pushrods. I show this because the head thickness is the determining factor in this condition. This situation can be overcome by placing shims under the rocker shaft pedestals. Again, there are limits to this fix. There is a photo on the next page showing this condition. If you look closely, at the photo at left you can see, that the cup at the top of the pushrod is hitting the underside of the rocker arm because the adjuster is threaded all the way up into the rocker. This is totally unacceptable and will cause a valve train failure (very expensive). This particular head was still usable because, as we stated earlier, we inserted a shim under the rocker pedestal, which allowed the adjuster to be readjusted down from the rocker arm to gain more clearance. 4 4
Now that we know that the thickness of the cylinder head is very important, and that the thickness of the head was originally 3.022". How do we determine if it a too thin? The first thing to do is turn the head over to look at the underside of the head (the combustion chamber side). If you look at the photos above, look at the area around the pushrod tube holes. In he photo at left, you can see a distinct difference between the rough casting of the head (dark area) which is where the push rod go through the head, and the lighter color of the machined area of the head. The thickness of this head will be acceptable. The right photo shows no distinction between these two areas not only has the machining cut into the push rod holes it has also started to cut into the spark plug hole inside of the combustion chamber. This is the round hole cylinder head shown on the previous page. Here is the method we used to measuring the thickness. This head is measured to be a thickness of 2.888 and is usable as a replacement. This is the banana head shown previously. Its thickness is only 2.862 and it can possibly be used if it passes further inspections. If you had a choice between these two the first one is the obvious choice. 5 5
Report Prepared by; Not only does the thinner head effect the valve geometry, if can also allow the valve itself to have an interference fit with the top of the piston. Again, this can be a very bad situation and very expensive one to repair. You can check this by installing a head gasket between the cylinder head and engine block. Place a small amount of plumber's putty or any other thin (non viscous) putty placed on top of the piston before the head is temporarily bolted and tourqued in place on the engine block. MG Services 349 Glenroy Ave. Cincinnati, Ohio 45238-5762 After tightening the head nuts rotate the engine two complete turns so both valves will open and close. Remove the head and measure the thickness of the putty that was under the head of the valve. If you have a very small amount of clearance, remember that if you over rev. your engine you could possibly cause your valves to float. And they can still come into contact with your pistons. VERY BAD!! Unfortunately, we are not done yet. These heads do tend to crack with some regularity so they will need to be magnafluxed to check for cracks. If it has cracks then the head is unusable, so stay away from it. There must be a visual inspection to see if there are any other irregular issues with the head. Check around the valve seats to see if there has been any damage from failed valves. In conclusion; If you have visually checked the head and measured it to determine if it is usable or not, then you can proceed with its rebuilding. At MG Services, we highly recommend the use of harden seats be cut into exhaust valve seats and stellite valves. We also recommend using the larger TF valves so that the engine can breath better. About twenty years ago bronze guides were introduced for these cars. When we first used them, we had problems with the valves sticking in their guides when the engine reached operating temperature. The car would begin to miss and loose power. If you stopped for an extended lunch so that the engine could cool down, the valve would free itself and the car would run fine again. The valve may stick at a later time or not, you just never knew. It was discovered that the guides needed a slightly larger clearance for the valve stem and that solved the problem. 6 6
In conclusion Cont,; If you are not comfortable identifying or measuring these two different types of cylinder heads take it to someone who you trust is knowledgeable in this area to do your work. Purchasing a cylinder head that will fail on you is not at all what you want to happen, both financially or emotionally. It is much more economical to pay someone to guar against you spending a lot more money purchasing a head rebuilding it and installing it onto your engine and possibly having a major failure miles from home. Paying attention may prevent you from spending any money! Disclaimer: The advice and guidelines given in these articles are given in good faith. MG Services will take no responsibility for any injuries or loss sustained while carrying out any of the described tasks and procedures or any consequences arising. If you are a member of an MG club that would like to have these technical reports sent to your newsletter editor. Please contact Jim at; MG Services 513-532-1795, WWW.MG Repairs.com The information contained in this report is protected by US copyright laws as assigned to MG Services and cannot be copied or reproduced in any fashion without prior written consent from MG Services. 7 7