I've seen many a question asked on how to modify a cylinder head at home, is it possible to get worthwhile results ? what are the important areas etc etc... so I thought I'd share my recent project with you, with a few words and lots of picks. It may inspire some of you to have a go yourselves, or at the very least, understand some of the work involved in a modified head and what to look for when buying one. It'll take a while to load everthing up while I shuffle pics etc so bear with me. This head is going on a friends 2ltr conversion for road use in his early MK2 GTi on K-jet. The head has standard valve sizes and is being used with a 280+ degree cam (more details as I get to the relivant part of the project) and competition valve springs. I thought I'd kick off with a few basics before we go into the main grinding work... You'll need the following as a minimum really to achieve a similar result... Health and safety plus some tools of the trade. Always wear protection (Ooh err), you "dont" want aluminium swarf in your eyes or up your nose. A pair of cheap gardening gloves are also handy (no pun intended), it'll save washing yr hands every five mins and having to pick ally dist from your nails. You cant really do this sort of work with a black "n" decker or a dremill, you'll need a high speed die grinder. The model I used for most of the work is a Makita 906, about 120 quid new (80 quid when I bought mine 8+ years ago !), or worth a look on good o'l e-bay. You also cant cut aluminium successfully without these... Carbide burrs.. available via the net or local compitant tool supplier (be prepaired for blank looks when you ask for carbide burrs!). Dont let your nice new burrs roll around the bench and fall on the floor, a simple stand will do and makes life easier when working with them. You'll also find one of these handy, a long shank burr (more blank looks from behind the tool counter), if you want to carry out more than the basic mods you'll need one (or a couple of different shapes) to reach down the port. And now the good bits... I'll try and put this in some sort of order, so to start with the minimum of work to achieve a result, we'll start with the inlet throat/seat area. Sticking a cutter or flap wheel down the first couple of inches of the inlet port will achieve nothing (apart from a waste of your electricity), so if you only want to, or feel confident doing the minimum amount of work, this is the area you need to start with... The bit with the rust spots on is the valve seat, the pics not great but you can probably see all the lumps and bumps below the seat that need to go, or be re-shaped. Here's a slightly better angle.. see the ridge below the seat, where the seat insert joins the head, that needs grinding down to something like this... Sorry the pics arnt very sharp, but i'm sure you can see the idea. Basically, your straightening the area down from the seat, down into the throat (although dead straight is'nt always the best shape, but more of that later). A bit of work on the short side turn wont hurt now... Again, not a great pic (note to self..must do better!), but if you look at where the colour changes (and if you have a cylinder head in front of you) you'll see a ridge that carries around into the throat... this nees re-shaping to look more like this... I did'nt use any at this stage (hence the finish), but a better finish can be achieved by spraying a little WD40 in the area that your working on... it reduces clogging of the burr and gives a better finish, though what it look like at this stage is'nt important, it's the shape that counts. The port floor already has a leading edge sweeping into the throat area, its the sharp(ish) edge that you need to cut back and blend-in.. you 'll see this much better on an actual head. The next area of worthwhile gain is the valve guide boss, shown here in yet another fuzzy picture (they do get better, honest). The idea is to thin down the guide boss, and widen the area between the boss and the wall of the throat... And looking at the same area from another (fuzzy) angle... If followed from above, this is the sort of stage you will/should be at... And from a different angle, you can see the shamfer on the guide boss... Something you can obviously see thats missing are the valve guides. They really need removing (pressing out) by your local machine shop before you start for a couple of reasons.. They'll get in the way when your trying to get the tool in the throat (easy !).. They'll no doubt need replacing due to wear.. And before the new ones are fitted they can be re-shaped for a little extra flow (more of that later). You can probably see at this stage that the roof of the port/throat area has been raised around the guide boss, and blended back into the roof of the port. You can also see at this stage that no material has been removed from the port runner itself. If you felt this was as far as you wanted to go, you could indeed tidy/smooth up the work so far with a flap wheel or some emery tape on a split rod, have the new guides fitted and seats cut, blend the lower part of the new 3 angle seat (a must) into the throat, lap your valves in, clean and assemble. You'd then have a basic stage 1 (if you must call it that) type inlet which would work much better than some "ported and polished" effort from a dubious source. But as power hungry enthusiasts we can do much better than that... For the next round of mods you'll need a pair of these... And one of these... The genarally accepted rule of thumb, is to open the throat area below the valve seat insert to around 0.95 of the valve diameter. For those without a Casio to hand, based on the standard 40mm GTi inlet valve, the throat needs opening to 38mm. Between the throat area and the bottom of the seat insert, a venturi type effect is desirable, though not easy to achive. The size here is generally 0.85-0.88 of the valve diameter, so in the case of the 40mm inlet, the diameter directly below the seat insert should be 34-35mm, opening out to the 38mm of the throat. Now get measuring with you calipers and see how small the standard throat area is... yep, theres a fair bit of material to come out.. The bulk of the work still needs to be done in the throat area, but at this stage, we can start looking at some inlet port runner mods. So we'll start here... Yep, it is what it looks like, an old valve. More accepted wisdom sizes the inlet port at around 0.82 of the valve diameter.. save your Casio's battery's.. it works out at approx 33mm based on our 40mm inlet. Now take a close look at the above pic, looks like the edge of the valve has been ground (machined) down.. yep, you've guessed it, to 33mm. This is our guide to sizeing and shapeing the inlet port, like so... Now an important bit.. "DO NOT" grind any material from the floor of the port. You need to open the roof and the sides of the port equally to start with so you can fit the head of the valve in. Bear in mind the floor is at the bottom of the port and the roof at the top (bear with me) as you would look at the head the correct way up... when working on the head, a lot of the time the head will need to be upside down so you can see what your doing.