Whenever you are confronted by the shaped cast metal that is a cylinder head you want to ‘port’, you are also confronted by problems in tooling. What do you really use to get the results you want and what is it called?
For the new, non-professional porter, this is one of the first hurdles and one of the areas that it is easy to spend money on what looks good verses what will work well for you.
Asking professional porters what they use was of little benefit to me when I first started trying to modify cylinder heads. You see, asking a bloke who may or may not want to tell you his ‘secrets’ is a fraught business at the best of times. But the other aspect to this question-asking, is that whoever you ask the questions of may have been developing his skills for many years and on thousands of cylinder heads. As such whoever you ask for advice has probably worked out exactly how to work with the tooling and they may not be willing to tell you that! Of course even if they did, telling you how to and then actually doing it are two different things…well at least they are for me; I like to watch how people do the work.
So what I will do is discuss some of the abrasives options used for porting an aluminium cylinder head. We will start off with cushion abrasives and not carbide burrs etc as abrasives are the most forgiving things to use when starting to port cylinder heads.
Naturally enough this will not be a comprehensive list and I will limit the discussion to what is usually available in tool shops or industrial supplies companies such as Blackwoods etc.
Cushion abrasives are vital tools to anyone who does any porting work. However cushion abrasives are not something that you may be familiar with by this particular name. The ‘Abrasives’ part is straight forward enough; the ‘cushion’ part of the label refers to the deformation that occurs in the abrasive material when pressure is applied by the operator. As such carbide burrs and mounted points (high speed stone used mostly on cast iron/steel) are not cushion abrasives for the simple fact that they will not deform when pressure is applied.
What does fit into the category of cushion abrasives, are fan wheels, cartridge rolls, spiral bands, cross buffs etc. All of these will deform to different amounts given the same level of pressure applied to them. This may not sound like a particularly important ability or something worth considering, but having the correct tool can make the world of difference when porting a cylinder head, particularly to the less frequent head porter like myself.
Instead of recounting all of the physics involved, which may or may not interest you, we will go through some of the commonly available cushion abrasives and how they can be used in modifying a cylinder head.
Spiral bands are ‘tube’ type construction with the abrasive on the outside. They may remind of a cardboard tube and they require a special mandrel (mount), which may be of a particular size to suit the brand of abrasives that you end up buying. The mandrels are usually a steel shaft with a rubber wheel on the end. The nut on the threaded section of the shaft expands the rubber and locks the abrasive band in place. The rubber provides the cushioning and thus the amount of ‘cushion’ can be altered a little by the amount of pressure the rubber exerts on the band.
This is more theoretical than practical as you rarely put enough pressure on the band when porting aluminium heads to get any benefit from the cushioning. There is another aspect to the spiral bands that is useful: they can be moved so that the mandrel is holding only the back 60% of the spiral band. The level of deformation and cushion available increases greatly as the spiral band is unsupported for 40% of its length, which is the prime working area. This is handy for getting into some areas such as combustion chambers in iron heads, but the spira band in possibly too aggressive for such work on an alloy head. Spiral bands have the most aggressive action and material removal rate of all of the cushion abrasives; they are also relatively expensive unless you take care to use them properly by using lubrication to control the heat.
Cartridge rolls are one of the most economical and frequently used cushion abrasives. As their name suggests, they are a ‘roll’ type construction, much like if you rolled up a newspaper from one side. Adhesive is used within the roll to keep it in shape, with the majority of the glue applied at the rolls base. As with the spiral bands above, you must control the heat that is generated by the abrasive rolls, particularly the heat at the base of the roll. When the rolls become too hot the adhesive will fail and the roll will suddenly unroll, making a startlingly loud whirring noise and vigorously shaking the tool you are using. If this happens in the port when you are working, remove the tool as quickly as possible and turn it off. Also, you should hope that you have not damaged any of your work or the port!
Because of the heat related failure you should not use the rear or base of the cartridge roll to remove material…or if you absolutely have to, then you should use it sparingly and with lots of lubricant. Where a cartridge roll is good is when you have lots of work to do and you can approach the working area on a slight angle. This allows you to use the front section of the roll and also affords the best vision of the work area. The cartridge roll will gradually wear down exposing new abrasive material. This also means that the cartridge roll will be constantly changing shape. If you are attempting to make a sequence of ports all the same, it may be worthwhile starting with a new roll in the same area of each port, so that you are attempting to repeat the same work/shape port with the same shape tool. The change in shape of the cartridge roll also allows us to use cartridge rolls in more difficult to access areas, but it also means that the cartridge roll will be getting shorter.
When this happens the supporting mandrel will be exposed and it may mark your work, and the cartridge roll may have reached its useful life. I say “may” because there is a solution: you can use two or three mandrels of different lengths by shortening the shaft of the mandrel. Thus when the end of the shaft is exposed, you can take it out of the die grinder, switch to a shorter shafted mandrel and keep working with the same cartridge roll. These mandrels also come in overall different lengths, which is one great advantage the cartridge roll has over a spiral band, as when using such a mandrel we have the capability to reach deep into a port to access otherwise inaccessible areas. You should take great care with the long shafted mandrels as you cannot turn them as fast, nor can you put as much pressure on them when removing material. Doing either puts you at much greater personal risk of the shaft bending and either making a mess of the port you are working on or even worse: coming out of the die grinder and flying around the workshop (see safety tips).
Sometimes called ‘flap’ wheels, these can often been seen by the box full in hardware shops in a wide variety of diameters. Whatever you end up purchasing, make sure that it is has a speed rating that matches the die grinder/drill that you are intend to use as some Chinese fan wheels have quite a low speed rating.
Fan wheels are one of the ‘softest’ cushion abrasive tools, as each abrasive element attached to the hub can move along its full length (see Quality section). This flexibility means that the fan wheel also has a greater contact patch relative to diameter when compared to other cushion abrasives and thus puts a greater load on the machinery driving it. This flexibility and soft contact also means that the flap wheel is relatively gentle in its action, both in how it removes material and also in the transition point from material being removed to not being removed. Both of these two factors make the fan wheel an excellent tool for ‘blending in’ (smoothing the transition) from one shape to another. The other aspect is that both ends of the fan wheel can be used, allowing the non-sighted area of a port to be blended in as well.
As with all of the cushion abrasives, excess heat from using a fan wheel is to be avoided as it will greatly increase wear and as a result the fan wheel will change shape much quicker than may be desirable.
A fan wheel is not a perfect tool; due to the pressure exerted on them in other applications, fan wheels are nearly always short-shafted and thus they cannot be used deep in the port. Now this is short-shafted supply is usually the case in Australia, the caveat being that there are specialist cylinder head porting abrasives available from companies such as Standard Abrasives in the USA. Standard Abrasives sell a expansive ranges of porting abrasives which includes small fan wheels which can be mounted onto an extension shaft to get deep into the port. I have not personally used such tooling, but it seems to be well regarded by others. The other aspect to fan wheels is that they do not do that good a job if you want to work on a flat surface (e.g. combustion chamber roof) as they do not provide an even pressure over their whole working surface and thus tend to create a ‘lumpy’ surface. Secondly Fan wheels are not ideal when you require great precision in a specific area (e.g. post valve seat area in combustion chamber) as it is sometimes difficult to see exactly where the working area is. There are better tools for these jobs.
Rexcut looks very much like a mounted point (shaft mounted grinding stone) and in many respects it is the same in its use. However there is a very important difference; it is intended for grinding/finishing work on non-ferrous metals such as aluminium, whereas the mounted points will load up and become clogged/useless very quickly when used with Aluminium.
The Rexcut is made from latex, cotton and abrasive grit cured onto a shaft at high temperature/pressure. Due to this construction they are not able to run at as high a speed at some other tooling and it is always best to ask the optimal seep for each tool.
These Rexcut are at the hardest end of the cushion abrasive scale, however this is one of their advantages. They come in a wide range of shapes/sizes and are good for working in areas such as the combustion chamber, or other areas where precision is required. Their important features are their lower removal rate (compared to a carbide burr) and thus more forgiving nature, their good ability to maintain their shape (gives consistency across the cylinder head) and their availability in different grits/finishes, which allows better polishing potential if you are so motivated.
Abrasive Caps were intended to address some of the issues that Rexcut addresses; in particular the ability to maintain the original profile. As noted above, this consistency in profile is important for work in places such as the combustion chamber and other areas where a particular profile is useful. The caps are moulded shapes and somewhat resemble a thimble that is placed on a finger. The abrasive grit glued to the outside and the caps themselves are pushed firmly onto a shaft mounted, split rubber mandrel. This expands at speed keeping the cap in place and allowing it to be used in fairly light applications. Caps are generally intended for detail work and as such are ideal for some cylinder head jobs. However they are relatively expensive and in my limited experience they do not last as long as I desired, but they may suit you needs, so have a look at them.
This is not a true category, but one that I made up. These are abrasives that are good for working on relatively flat areas of the cylinder head. You may not be able to think of many uses for these immediately and they may not suit what you want to do, but for me they were perfect for one particular job. When I had finished modifying the intake ports on the test heads I then started having a look at the combustion chambers. Typically, combustion chamber work does not yield the benefits for time spent, that the intake ports do. However I noticed the intake/exhaust seats in some of the heads were protruding about 0.3mm above the surface of the cylinder head casting. Naturally this is far from ideal and had to be removed. The solution was to use a shaft mounted flat flap wheel. This is very similar to the flap wheels used on angle grinders. This was used to remove the protruding steel lip of the valve seat. After the edge of the flap wheel was reshaped to a more curved shape, it was then used to provide a flat surface from the valve seat to the chamber wall, which was then radiused with the reshaped edge of the flap wheel. Whether this has any beneficial effect will be determined when the head is back on the flow bench. The same procedure was repeated on another cylinder head ported later and again it removed the valve seat lip, a variable that occurred in two out of the four combustion chambers.
The other form of flat abrasive is similar in application to the flap wheel, but comes as a flat abrasive disc that clips or sticks to a mandrel, and it thus has a similar appearance to an intake/exhaust valve. These work in exactly the same way as the flap wheel previously mentioned with a couple of exceptions. First - as they are a single layer abrasive they tend to wear out much faster, but they are cheaper and you will always be working with the same shape if you replace it. Second - depending on the system they can be more flexible, which may not suit all the work you want to do. Finally – you cannot shape the edge to work into a radius.
If you try these abrasives you, will naturally need a high speed power source. Check to see what you can get in the cheapest brands of tools, as I have seen some very cheap 500W routers that can be stripped down and used as a small die grinder; just make sure it is variable speed. Otherwise a die grinder with an extension is the way to go if you want to make the investment.
So now, what I suggest is that you get a couple of clean, scrap or worthless cylinder heads. There are nearly always some in a car club member’s garage, or at a cylinder head reconditioner’s. Get the heads and have a go! You may find out that it’s not for you, or you may find out that you enjoy the challenge. However you will gain a new level of respect for those professional porters who can make a sow’s ear of a cylinder head look like it has come fresh from a CNC machine.