DIRTY STUFF

IF DONE by a professional, porting cylinder heads is an expensive business due to the many hours of work involved. But with basic cutters and grinding stones, plus either an air or electric die-grinder with a 6mm chuck, all you need is know-how, a good pair of protective goggles, and patience to do it all yourself.

Although tungsten carbide cutters are expensive, they really are quick at chewing through cast iron and aluminium. For combustion chamber work, a combination of a TC cutter and a flathead grinding stone is mostly all you will need. However, make sure the cutter doesn’t bounce; these tools have a soft ceramic core overlaid by tungsten and ground to form sharp teeth, and if they bounce off a port wall at 20,000rpm, the head will shatter into small pieces. Also, don’t buy cheap-shit stones; they will only cut very slowly and wear out quickly. Specialist suppliers like engineering shops should have good-quality ones in stock.

Don’t face the head before you begin work. It’s way too easy to slip with a stone while reshaping the chambers, usually right across the head gasket sealing line. Fit a used head gasket to outline the combustion chamber shape with a marking pen before you start, and leave a couple of millimetres from the chamber shape to the gasket line in case of accidents.

If you are fitting oversize valves, a machine shop can open the valve throat with a 70-degree cutter to the inside diameter of the new valve seat. But don’t get them to cut the seats just yet; again, a slip with a TC cutter across a new valve seat will be a disaster.

And unless you are intending to fit bronze valve guides, don’t cut down the existing guides in both the inlet and exhaust ports – or at least think very carefully about doing that. Performance engines generate a huge amount of waste heat in the ports; if the guides are cut down too far, the valves will struggle to keep their stems cool, and at high rpm, heads will drop off and take out a piston or two.

When enlarging ports, stay on the safe side of 3mm-deep cuts into the walls, spaced apart so that the trenches can be joined up later. That way, you don’t go too deep and the intended size is easily controlled – unless of course you want to chop up an old head to see how much thickness you have in the port walls.

If you are not going up in valve size, a pair of old valves fitted into where they normally live will protect the seats when reshaping a chamber. To avoid valve pocketing when oversize valves are too close to a chamber wall and not enough critical gas flow is happening, it is possible to cut a trench into the side of the chamber in that area, leaving at least 4mm between the top of this indentation and the upper face of the cylinder head. Allow for any intended milling and later refacing, of course.

You have a choice of high-speed die grinders. Purpose-built, stand-mounted high-speed motors that drive a flexible cable to a handpiece work quite well. However, although the small diameter of the handpiece is good for working in ports, they do heat up to unbearable levels, particularly if the wrong ball bearings are fitted. Operators tend to whack in normal shielded bearings when the originals die, but their plastic shields generate too much friction, causing overheating. Bearing companies can supply the same bearings with no-friction steel shields, solving the problem. Air die grinders are the way to go if you can afford a large-enough compressor, and the tool stays cool, although the feed hose is a bit of a bother.

As a notorious cheapskate, I use a carpenter’s laminate trimmer, which has the high rpm I need as well as both 6mm and quarter-inch collets to fit the carbide cutters and stones I require.

Alloy head work is different; you can still use carbide cutters, but they need to be coated with dry soap, as the cutter teeth pick up molten aluminium, and the soap makes cleaning between teeth with a small flat-blade screwdriver a bit easier.

Once all the cutting and grinding is done, it’s time to think about polishing. If it’s your own head, don’t bother; polishing is purely cosmetic and does nothing to increase horsepower. In fact, polished ports actually slow down critical gas flow speeds, as those inlet port walls are coated with unburnt fuel droplets, creating drag.

However, if you do need to polish, a variety of low-speed polishing products are available from engineering supply shops, from multibladed emery mops to rubberised polishing heads that contain emery pieces. You can also make your own port polisher from a simple 6 or 9mm short shaft. Using a hacksaw, cut a slot of 25-30mm down the shaft so you can fit a contained roll of flexible emery cloth, and get into the port walls with that. Doesn’t last long at electric drill revs, but it works really well!

WITH BASIC CUTTERS AND GRINDING STONES, PLUS A DIE-GRINDER WITH A 6MM CHUCK, ALL YOU NEED IS KNOWHOW, A GOOD PAIR OF PROTECTIVE GOGGLES, AND PATIENCE TO PORT CYLINDER HEADS YOURSELF