How to Modify Heads

A number of people have been asking about an article on what to do when modifying heads. Well this is a difficult subject to cover completely as their needs to be engineering drawings and the like. So, all I can do is show pictures and describe the do's and don'ts. However what I can cover is what most other authors leave out, the what steps and how to go about the project of head modifying. I will also cover the rocker geometry as this is part of the head and most people leave this out completely. Also included are CR calculations and all the different aspects of the machining operations needed to successfully complete your task.
The DIY method is cheaper but not by that much, but you should end up with a product that is superior to many heads that you can buy over the counter because this head will be fully optimised to work with the engine that it will be bolted too which is something that no STAGE 'X' off the shelf head ever is.

Firstly, the engine what type are we looking at? The example I am writing about here is a fresh 1293cc engine with 21253 pistons, featuring a 266 Kent cam and a single 1.75in SU. With the head that I am going to describe it will produce about 84-86 bhp @ flywheel and around 90 lbft of torque.
I will be using 35.7mm inlets and 29.4mm exhausts (leaded M.G Metro Valvesizes). It will have hardened valve seat inserts for use with unleaded fuel, basically a good solid long lifed torquey road engine.

As a basis use a 1275 late metro casting '84 on to '89, all of these have the big 35.7mm inlet valve. These heads are also found on 1.3 maestros and 1.3 montegos. These larger valved heads already have a slightly bigger inlet port and throat to suit the bigger valve, so are different to the later unleaded and earlier small (33.3mm) inlet valve heads.

Selection of casting

The whole process starts with a good casting. So if you have a pile of these heads then you need to look for a few key points. If you have only the one then please buy a few more, if you have a cracked head then keep this as it will be useful at a later stage.
Start by looking at the bottom (deck face) look for a thick casting at least 1/8" of material thickness between the gasket face and the water jacket. The thinnest areas on a visual inspection or those along the front edge where the small head to block waterways are.
A thick deck means that the casting will take a substantial skim without the problem of weakening caused by an excessively thin deck.
Check for valve recession, heads which have recessed exhaust valves will not present a problem if exhaust valve seat inserts are fitted. Heads which have valve recession more than 0.060" should be rejected.
Next check the rocker/exhaust side of the head look for digs and scores, too deep and they will not clean up when the head is skimmed which will cause leakages in service. Reject a head that has damage deeper than 0.010".
Check for cracks between the valves, use a screwdriver to scrape any carbon away and use a torch to light up the area, LOOK VERY CLOSELY. If you are buying a head for modding don't buy one, buy at least 2, 3 is better. As when stripping you may find cracks so the casting is useless if cracked. You can then just start the next head. Also when buying see if you can get hold of a known cracked head, this will be helpful later.
Next check for stripped/pulled threads. These will not be a problem to a machine shop as they can be heli-coiled to effect a permanent repair. If you have not the equipment to deal with broken studs, avoid the head. Although for some reason, I usually find that heads that have broken studs and stripped/missing threads are also the best castings to start from, (Sods law!!!!)


Clear yourself a work area on a bench put some newspaper down to work on. The first thing to do is remove the old spark plugs if any are fitted. Then remove the temperature sender unit. With the head sitting deck down (head gasket face on the bench) use a mallet an a socket to loosen the valve spring retainer keepers. The idea is to use sharp blow to depress the spring and loosen the keepers. Don't hit the retainer too hard as the keepers may drop out and the spring/retainer will fly off dangerously.
Turn the head onto its side (Spark plug face down). Using a valve spring compressor start at valve number 1 (the thermostat end of the head) compress the valve spring and remove the keepers. Retain the keepers and retainer together in a small bag. Bin the spring and valve stem seal if fitted. Remove the valve.
The valve should be a TAM 1061 Exhaust valve. If the stem and head are in good condition, and I mean GOOD, then they can be reused when suitably modified. If good keep the valve in a suitable holder. I use an 18 " length of wood with 8, 7.1mm dia holes, to form a valve rack.
Valve number 2 should be a TAM 1059, again these can be re used if in mint condition.
Remove the rest of the valves as above.
Clean up the combustion chambers and ports with a wire brush bit in an electric drill. Pay particular attention to between the valve seats. Once clean with all traces of carbon removed then visually inspect the head for cracks. Pay particular attention to between the valve seats. To give you some idea, I use a 500 Watt floodlight mounted 5 ft above my head bench to illuminate the head when visually inspecting for defects. This kind of illumination really shows up the smallest flaws.
If any cracks are found, reject the head.
Finish removing the studs from the head. If you have stubborn studs, removing them with a little heat from a plumbers blowlamp will greatly reduce the chance of the stud shearing in the head. Remember to heat the casting and not the stud but don't overdo it!
By now you should have a head that only have the valve guides left in. Clean off all gasket residue. If you are in any doubt as to the integrity of the casting take it to a machine shop that can do a pressure test on the casting. This involves filling the internal waterways with soapy water. Then the head is sealed an pressurised with compressed air to around 60psi. Any cracks into the ports or combustion chambers will show up as drips or bubbles of soapy water. This usually cost around £35-40 to have this process carried out.

Modifications and machining

Now that you have a clean sound head we can start on the modification work. This particular head will have 11 stud fixing, so we need to mark out the position of the holes. To do this you need a genuine gasket. I use an old TAM 1521 headgasket. Lay this on to the deck face such that it lines up with all the stud holes in the head EXACTLY. Mark out the position of the 2 extra stud holes at each end of the head. Locate the centres of the holes and mark using a centre punch. At this time as the headgasket is perfectly positioned on the deck face, scribe around the combustion chambers so as to produce a gasket line mark onto the head.
Time for special tools, we need to machine the floor of the combustion cambers adjacent to the valve seat and chamber wall (I call this the production of valve seat platforms). This achieves a number of goals. Firstly, this will effectively narrow the outer diamter (OD) of the valve seats. This is required as the valve sizes are to be used are the same as the original. So if we were to re-machine the valve seat after fitting new guides it would be too large. Secondly we can cut into the floor of each combustion chamber in such a way that will generate a set depth in each chamber so that when we come to grinding there is a guide. This helps greatly when grinding the chambers to a depth. If they are all the same then balancing is that much easier.
Thirdly by setting all the valve seats the same, the valve stem tips will also come into line which will help greatly when setting the rocker geometry.
Finally the chamber wall can be cut out to within a few thousands of an inch from the gasket line at the same time. So to sum it up we use a single point cutting tool mounted in a pillar drill with a valve guide pilot to machine the chambers to produce an even depth on all chambers and to take the chamber wall out to the gasket line save for a few thou. Use a 0.040" radius on the cutting tool so not to produce too sharp a corner in the chamber.
Some chambers may require more material to be machined out. The idea is to produce valve seat platforms that are all within 0.005" of each other from the deck face. A typical head that has been machined in this way would have valve seat platforms at around 0.325 - 0.350" from the Deck face.
Another by useful feature of this machining process is that if you did not have the head pressure tested it will show up cracks across the valve seats if they were not spotted earlier. Finally drill the 2 extra stud holes as marked earlier. Use a 10mm drill mounted in a proper pillar drill that has its table square to the chuck.
The head is now ready for the exhaust valve seat inserts to be fitted. I send my heads to have this work done for me as I do not posses the required equipment. Basically the exhaust seat platform is machined to take an insert of the appropriate size to the valve being used. I specify 31mm inserts for my own heads when using 29.4mm exhaust valves. The insert hole is machine 0.004" smaller than that of the insert (this depends on the type of insert being used). The head is then heated in the machine shop hot wash tank to around 80 degrees celcius. The inserts are then cooled in liquid nitrogen, -260 degrees, yep thats damn cold!!! They are then simply drifted in place. The inserts will not come out if they are the correct type, they expand faster than the cast iron of the head, so the hotter the engine gets the tighter the seat becomes. Unless the engine is overheated. The extra stress will cause the casting to crack and the insert will fall out. Typical cost is around 95 for 4 inserts.
With the head back its now time to push the guides out. The use of a 10 tonne press is required here. Bin the old guides. You now have a casting ready for porting.


Before you can dive in, you need equipment. Not just any old thing, the right tools for the job are definately required. Firstly you need a bench, something sturdy that will take the weight of you leaning on it. Old office desks make perfect head benches. Now if you are planning on doing a lot of head grinding then please think of health and safety, have some form of vacuum to suck up the grit as you produce it. If you don't then you've been warned, it will do you more harm than good in the long run. Also wear a decent particulate mask. You only get one pair of lungs so look after them!
Next you will need some lighting, as I mentioned before, I use a 500Watt halogen floodlight over the bench. This is good for the surface work, but you will also need some form of additional lighting for the port/throat work. I use a 12 Volt 50watt inspection lamp, placed to illuminate down the port of the throat that I am working on.
Cutters, you are going to need some decent ones. I use a tungsten carbide rotary burr (13mm ball) for roughing, cost around 12 - 20 each. Now really you will only need one to cut a head, but buy 3 or so if you have not used them before as you will ruin them trying to get the hang of the grinder. In my case I use an electric Bosch GGS 27L (27,000rpm) which cost 275.00. There are better units around now with speed controllers, if you can get one as the are far more useful than a single speed machine. Now that may be a bit steep if you are only going to do one head, but maybe if you are in a decent miniclub they can buy the tool and lone it for a small fee to different people (May I suggest a reasonable deposit though). You can use a dremel but it will be slow and useless with the carbide burrs. It will also kill the thing in no time as they are not designed for hacking up heads. Forget air die grinders unless you have at least a 14CFM compressor with a 200 litre tank. They are also extremely noisey.
You will also need a selection of stones, again not the ones from your local DIY place, proper 35,000 rpm jobbies. You need a dressing tool for shaping and cleaning the stones. A 1" ball, a 1/2" ball and a 1" cylinder stone is all that is needed for mini heads. These stones will cost 3 - 4 each.
When using this equipment it is very noisey, so you will need ear defenders, availiable from any decent hardware store. You MUST also wear goggles with this equipment, not glasses but proper airgun pellet stopping googles, basically the old impact grade 1.

Before you dive in, think about what you want from your head. Get books, read. I have been thinking alot about how to describe grinding heads and the easiest way would be for me to show people as you cannot accurately enough describe to someone how to profile a head. What I can do though, is say If you read vizards book Tuning the A-series engine. It does give a good idea of what to do and what not to do. t also gives pictures and shapes to follow.
What I can do is describe what points should be followed in this case and which methodical order to follow. I will also say that please do not gt carried away with the grinding as you can ruin a head by having delusions of grandure and grinding too big!!!!
I would urge you now to reach for that knackered cracked casting to practice on. Practice with the carbide burr. Hold onto he machine tightly and cut some metal. Don't worry about the casting it is useless afterall, practice, practice, practice. Try to avoid getting the burr caugh up you, will know when this happens it hurts and you will bend and ruin the burr! You will soon find out that lightly cutting with the carbide burr will rapidly cause it to go dull (blunt),. To get the best from these burrs slow the cutter to around 18,000 or so RPM and push hard into the casting you will find that by adjusting the speed you will notice that at the correct speed the burr will sail though the metal. Run the stones flatout and practice with these. Stones are used for finishing (All my heads are ground finish) and/or light shaping.

Getting down to some serious cutting....

When starting with a raw casting I always start with the valve guide bosses, select a new burr and just cut out all 8 bosses, don't blend the burr will dull. Just use the brute cutting ability of the burr, remove 95 % of each boss. Now switch over to a previosly used burr perhaps one that has just lost its best cutting performance. Start with the inlet valve thoats, blend the boss into the roof of the port as shown in the vizards bible, then continue the smooth shape up to the inside diameter (ID) of the valve seat. Cut round the valve seat such that you cut away a quarter of the original valveseat. This will leave a ridge on the throat which you can blend in to match the port. The main roof of the port should now be a smooth althought slightly rippled shape from the port round the guide hole right up till the valve seat. On my heads I do not use a 3 angle cut, I have not seen the need on road heads as you can get enough power to break things without using every trick in the book!!!!(I can hear Sgault Tutting as I type!) I use a 2 angle.
Next move onto the wall which divided the two inlet valves. The standard shape is convex, this is totally wrong, it actually needs to be concave. You have to be careful here as it is easy to get carried away and grind through into the water jacket. Use your practice head and deliberately grind though the jacket at this point, it will give you an idea of how far to grind. The bear in mind when shaping the inlet ports/throats that the gasses are moving from the port through into the combustion chamber. If you remeber this when grinding you will notice that when you grind the exhaust you will produce a completely different shape to promote flow in the reverse direction (exhausts - well I do anyways)
Back to where we were, produce this concave shape on all inlet throats and blend the sides into the floor. Next, move onto the radius that blends the floor into the valve seat opposite the valve guide. This is commonly called the short side radius. This needs to be radiused so as to produce a nice smooth turn from the port to the valve seat. Inicidentally if you are insisting on 3 angle valave job, you must not grind the port throat right out to the inside dia of the seat, you must leave the throat approx 2mm smaller than the valve seat ID for the production of the 3rd cut. The inlet port size On this head does not need enlarging to any great degree. Basically all that is required is to grind the narrowest part of the port until the existing shape is merely squared off. Ie only remove enough material to acheive the slight change in shape. The port can then be blended using a stone. Do not polish, leave with the just ground finish as a guide the port should measure after grinding 26.5mm across and 30.5mm in height. Blend the area that you have prevouisly worked from the throat into the port from the manifold face. Its up to you if you want to remove the location rings from the head, (you may need them, ie and injection engine) it will not make any difference on this spec. I simply grind out enough material to remove them.
At this stage you should have a head that has the inlets completely ported.
Moving onto the exhaust, remember that the gasses are moving in the opposite direction, shape the valve throat to form a smooth almost bowl like shape, that leads into the port, with the exhaust try to minimise material removal as the large amounts of heat present in the exhaust tracts will be more reliably removed through a thicker casting. But do make sure that everything is well radiused, though this time not just for flow purposes so as to not generate hot spots which overtime can cause casting failure.
The exhaust ports always seem to cause lots of arguments as to the ultimate shape, well heres my view. I use a standard rectangular shape. This will flow 95% as good as a carlos fandango theoretially perfect ultra high tech shape. This head is not being used in a high tech race engine it will simply not need any extra flow from sexy port shapes.
Using a standard manifold gasket scribe around the exhaust ports to produce a line to work to. Grind the port at the manifold face to the shape of the scribed line. If the port has moved such that the outer wall is already over the line, just grind this wall square. Only remove significant material from the areas of flow, ie the outer sides of the end exhaust ports and the ceiling of the port, as you look in the port with the casting positioned with the thermostat face up most. Aim for a port that is 29mm high and 26mm wide, with a 1/4" radius in the corners. Continue the shape right down the port till it meets up with the valve throat porting.
The centre port should be ground to a size of 30.5mm high and 26.5mm wide. Continue this size into the port.

You should now have a head that is fully ported apart from the chambers. Now its time to visit your machine shop. This time it is for the skimming of the thermostat or top face of the head. This is to ensure it is flat and that there is no areas of corrotion to cause leaks. Do not skim so much off that the thermostat will now not seat correctly in its locating recess.
At this time you can fit the new guides, or have them fitted. Fit them to a height of 14mm above the spring seats. Use 12G 1963 taper nosed iron valve guides availiable from Minispares, in my opinion these are the best guides to use for any road engine. Because the guides are a interferance fit in the head the guides need to reamed after fitting. (The correct interferance fit for the guides is 0.0015"). This is because the interferance fit will cause the internal bore of the guide to be squeezed smaller than before it was installed. So to this end the guide needs to be reamed to restore the true size and hence running clearance. The correct size for these is 9/32" or 7.143mm. A simple hand reamer is all that is required for this. Failure to ream the guide bore may result in the valve stem seizing in the guide, causing all sorts of nightmares!!!. Once reamed the valve seats need to be cut. Using an actual valve that you plan to install and use as a guide cut the seats to a diameter of the valve seat on the valve minus 0.005". The rule here is the outer diameter (OD) of the valve seat must be smaller than the valve head seat diameter that will run in it. This is to allow the valve head to rise up out of the seat as thermal expansion happens in normal engine operation.

Now, its chamber time. The easiest way here is for the novice to copy a known shape. Ie vizards bible or some such equivalent. You will need to mark out the head chambers. You need an old valve modified such that the valve face has a small centre punch mark exactly in its centre. You then insert this into the valve guide. You can now use a pair of dividers to scribe the radial dimensions from the drawings you are using onto the deck face. Practice grinding your chambers on the cracked head as before. I use a 1 inch cylindrical stone with a 1mm radius ground on the bottom. As you grind you will notice that it is very difficult not to damage the valveseats. To get round this you use an old valve ground such that when seated it fits exactly flush with the chamber wall and at the same time it completely covers the freshly cut valve seats. Because the valves are quite hard in comparison with the cast iron they will offer very good protection from the grinding stone. It will also allow you to grind the chamber floor completey flat right around the valve head without fear or wrecking the seats and possibly the whole job.
During the grinding you will notice that the grinding stone will dull, this is because it becomes clogged and needs dressing. Your dressing tool will be needed to clean the stone and restore its performance probably 2-3 times during chamber work. Remember only grind the minimum amount from the floor, use the valve seat platforms as a guide for depth. Try to gind the chambers equally the trick here is to grind the chambers equally to achieve the same volumes!!

DO NOT grind past your scribed gasket lines this will ruin your head and you will have to start again.

Once you have finished grinding the shape, and you are happy, you can leave the chamber with the stone ground finish. It will quite honestly make no difference if you polish them for this application.

Right Valve seats, and valves. The best thing for you to do here width wise is use 0.050" intake and 0.070" wide exhausts. You can at this point cut the seat in the head say 0.040" bigger than the OD of the valve heads. You then use a 30 degree cut to reduce the seat width at the chamber side of the seat to a) give the correct seat diameter and b) to gain a little extra flow over the seat. The inside diamter of he freshly cut seat can be blended using a ball stone into the port throat such that the valve seat forms an even shape with a constant width right around it diameter. Take care not to damage the valve guides.
You have a choice with the valves. Basically if you can use the originals, use them for this application. Check for stem wear. If the hard chrome facing has started to come away or is pitted replace them. If the stems are fine and the valves have not been overheated and oxidised they can be refaced. This involves mounting the valves into a valve grinding machine, the machine then simply grinds material from the valve seat at which ever angle you wish to use (45 degree in this case). This effectively makes a used valve into good as new condition. Again once the seat has been established its width needs to be adjusted to match that of the particular seat into which it ill be inserted. Use a 30 degree back cut on the valve to reduce the seat width to the required amount. This operation will also give a useful boost to flow.
Use your local engineering firm for this work, if you have not got access to this equipement. This will also have the added bonus of being able to accurately match the seat angle on the valve to that on the head.
You can now finally lap in the valves. Use only fine paste 'watered' down using parrafin. Aim for a light grey even seat all the way round the seat circumference. If the lapping takes more than a minute or two there is something wrong with the concentricity of the seat to valve, or valve seat angles. See the machine shop for assistance.

The chambers volumes can now be measured. To do this you require a burrette (20 from lab suppliers), some vaseline, a spark plug of the type to be used in this case a NGK BP7ES. You need some perspex sheet (3-6mm acrylic clear) with a 6 mm hole in the centre. Set the head up on head stands and insert the valves for the chamber to be measured. Ensure the seat are clean and perfectly free from all traces of valve grinding paste. Secure the valves into the head using the retainers and some dummy springs such as rocker shaft spacer springs. Install and tighten the sparkplug into the chamber. Seal the perspex sheet over the chamber to be measured using a light smear of vaseline around the periphery of the chamber. Use parrafin in the burrette. Zero the reading after filling. Then fill the chamber until the liquid comes to the bottom of the filling hole in the perspex sheet. Note the reading. Usually this will equate to 24.5 - 25.5 cc for the fresh road heads that I cut to this spec. I will use 25 cc for the calculations.

Firstly we need to work out the Compression ratio that the new chamber size will give on our engine (1293 with 21253 pistons)

Let us use the equation CR=



Swept volume of 1 cylinder (SV)

= 1293/4
= 323.25cc

Ring land volume (RLV)

= 0.90cc for a +0.020" 21253 piston

Piston dish volume

= 8.3cc for these particular pistons

Deck volume (DV)

This is the volume contained above the piston crown to the top of the block deck.

= Bore area(sqcm) x deck height (cm)

= 39.758952 sqcm x 0.0127cm

= 0.5cc

Gasket volume (GV) for BK450 gasket

= 3.2 cc

Head chamber volume (HV)

25cc (The typical example size I am using here)

So plugging it all in we get CR=


= 361.15/37.9

= 9.53

so CR = 9.53:1

Now if we were to use the head as is we would only have the 9.53:1 CR. Not high enough for good performance and decent economy. So I usually use 10.25:1 on this spec with the 266 cam. So to work out the HV required to get the CR to 10.25:1 we need to re-arrange the formula and substitute the new CR figure.

So HV =

((SV+RLV+PDV+DV+GV) - (CR x (RLV+PDV+DV+GV))) / CR-1

((323.25+0.9+8.3+0.5+3.2)-(10.25 x (0.9+8.3+0.5+3.2)))/9.25

= ((336.15) - (10.25 x (12.9))) / 9.25

= ((336.15) - (132.225)) / 9.25

= 203.925 / 9.25

= 22.04cc

We conclude that the Head chamber needs to be skimmed to leave a 22cc chamber to give us the required 10.25 : 1 CR.

How do we work out how much to machine off the head to give us the 22 cc?

Well start off by setting the head up as for measuring the chambers. But this time the head needs to be perfectly level. Use a spirit level for this. Now with the valves and spark plug installed as for the measuring the total volume, run 22cc of parrafin into the smallest chamber. Then using a depth micrometer measure down to the surface of the liquid. This distance represents the amount to be machined from the deck face. Typically for this application 0.038" is the total amount. You then send the head off to the machine shop for skimming and final cleaning. Once returned deburr the deck face and then you are ready for checking the installed spring heights and rocker geometry.
For this head I use Kent VS2 valve springs. They reccomend a 36mm installed height. To check the installed height of the springs I use a 20mm spacer which I install complete with a rocker shaft spring. With the retainer and keepers fitted it is a simple case of measuring the distance between the spacer and the retainer with vernier calipers. In this case 36 mm would read as 16mm on the vernier. ie 36mm-20mm(spacer width) = 16mm. Because the valve spring platforms vary in height you may need to shim some springs and not others either way try to achieve a tolerance or 36 +- 0.5 mm. I use 1/2" table 4 light washers for shims they are 0.057" (1.45mm) thick.

Install all of the valves and retainers with dummy springs. Set the head up on the head stands so that the rockers can be sat onto the head in their correct running position. What you are going to do now is set the mean rocker geometry for the valve side of the rockers. What this will acheive is the elimination of excessive guide and valve tip wear whilst giving a certain amount of increased lift. These are the basics of a road engine valve train where high mileages are more important than out and out power.
It is actually very simple. Calculate the total theoretical valve lift:

Valve lift =

(cam lift - follower clearance) x theoretical rocker ratio

= 0.263" - 0.012" x 1.25 = 0.314"

Now take this figure and halve it to give 0.157"

Now the idea is to set the rocker shaft centre line so that at half valve lift 0.157" the valve tip is exactly on the centreline of the rocker shaft and that the contact pad of the rocker is exactly in the centre of the valve tip.
If you find it is not, you simply machine the rocker posts or add shims and machine the rocker posts to acheive the desired result.
Some of you will notice that with a cam such as the kent 266 it has a split lift profile so enabling true geometry to be impossible to achieve. Just optimise for the average of the 2 lift figures.
Of course this is not the full story, to properly optimise the geometry more favourable positions can be used together with the correct length push rod, but with the mods here the differnces will be small so I will keep these tweeks to myself. You will however, have given your valve gear the best chance of a long trouble free life without have to do anything to labour intensive and expensive.

Finally run a tap though the all the threads on the head and blow out all orifices with an airline. (Use goggles!)
You can now paint the head, use an engine lacquer and Red oxide as an under coat. Apply 1 Red oxide coat and 3 top coats, allowing at least 12 hours between coats.

Finally clean off the excess paint and you have a decent head that should provide years of service if properly looked after


Last updated 21/05/03


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