Decarbonising and Grinding-in Valves

This work does not have to be done often, but it is beneficial to remove the cylinder heads, clean out any carbon deposit, and very lightly grind in the valves after a new engine has run about 2,000 miles. Thereafter much longer distances can be run before the same attention is required. No fixed mileage can be stated as being the maximum that can be run between decarbonising and valve grinding. It varies according to working conditions, and upon the condition of the engine. It is however safe to assume that if the engine runs well, does not pink readily on Premium grade fuel, and shows no obvious signs of lack of power, no attention is necessary.

Removal of Cylinder Heads

Take off both carburetters, and detach the exhaust pipes, and the rocker oil feed pipes that are attached to the heads. In dealing with the oil pipe connections do not use excessive force on the hollow bolts that hold the banjo unions or they may be sheared off. Be careful to preserve the metal gaskets from each side of the banjos. Remove the oil return pipes by disconnecting from the heads and oil sump. Remove both rocker covers, and one of the sparking plugs.

Rotate the crankshaft by the starter until compression is felt in the cylinder that has the sparking plug in it. It will be seen that both valves on this side are closed leaving clearance between the valves and rockers. Unscrew the four cylinder head nuts, each a little at a time in turn until they are all quite free. They are situated between the second and third fins on the cylinders. Draw the head off the studs as far as the nuts will allow, and run the nuts right off. As the head is moved separate the push rods from the rocker tips and press them in against the tappets to ensure that they do not come away with the head and allow the tappets to fall out of the guides. These will be difficult to replace and the cylinders might have to be taken off to get them back.

When the head is clear of the cylinder face stretch a stout rubber band (this can be cut from an old inner tube) over the push rods to hold them in place.

Before removing the other head in a similar manner turn the engine one complete turn to close the valves. Preserve the head gaskets. If removed see that they are both marked to enable them to be refitted on the sides from which they were taken and the same way round.

Decarbonising

With both pistons at the top of the stroke the crowns can be cleaned. Most of the carbon will wipe off, but if hard deposits have to be scraped off use a scraper that will not scratch the piston. A stick of solder hammered flat at the end and trimmed to a straight edge will serve very well. Clean the crowns of all deposits, but do not use glass paper, emery cloth or anything abrasive to get a high finish.

When clean move the pistons down a little way in the bores and wipe off all traces of carbon from the mouths of the cylinders.

Cleaning the Cylinder Heads

It is better to clean the combustion chambers of carbon before taking out the valves. This is done in the same way as the pistons. The carbon deposit can be scraped from the exhaust ports after valve removal. If desired the ports can be polished up finally by rubbing with emery cloth, but this is best done after the valves have been taken out.

Removal of Valves and Springs etc.

Unscrew both rocker pin lock-nuts and remove them. Tap the rocker pins out of the rockers, and remove the rockers, marking them for replacement in the same position that they occupied originally.

Preserve the spring thrust washers from above the rockers, and the hardened ones from below them. One of each is fitted to each rocker pin.

Compress the valve springs and when the collar exposes the cotter prise the two halves from the valve stem and lift them out of the collar. Keep the halves of each cotter together, and also see that they are refitted to the valve from which they were taken. Do not use unpaired half cotters. After removal of the cotter, the valve, and springs will lift out. Remove and preserve the valve spring bottom washers from the valve guides.

Cleaning and Regrinding Valves

It is best to polish the valve heads by holding the valve stems, close up to the head, in a lathe or driller, rotating them rapidly whilst emery cloth is held up against them. Be sure not to reduce the stem diameter and also be careful not to bend them. It is usually possible by this means to remove any slight pits or irregularities from the seatings and thus to reduce the amount of regrinding, that will be needed.

The 'leaded' fuels on sale nowadays often produce a hard scale on the valve heads, particularly on the exhaust. If this does not respond to treatment with emery cloth it may be necessary to remove it by applying a piece of carborundum to the surfaces that are coated, and polish up afterwards with the emery cloth.

The valve seats in the head seldom require attention except after very prolonged running indeed, and may then have to be refaced with a special reseating stone. They are too hard to cut with an ordinary valve seating cutter. The seat angle is 45°.

Valves that are very badly pitted and will not clean up on the seatings without reducing the thickness of metal behind the seating must be replaced with new ones.

FIG. 10. The Valve Lapping Tool No. LET 687 (with valve).

To grind in the valves apply a very little fine grade grinding compound to the seat of the valve. Push the valve into place through the guide and attach a valve grinding tool LET687 (Fig. 10) to the stem. Hold the valve lightly against the seating in the head and give it a semi-rotary motion backwards and forwards, lifting it from contact with the seat after about every half-dozen movements, bringing it back into contact in a different position relative to the head. This will distribute the grinding compound evenly and prevent the formation of concentric rings on the seats. Continuous rotation, in one direction only, will form these rings or ridges on the seats. This must be avoided.

Examine the progress of the grinding from time to time and as soon as an even grey matt surface is obtained all round the seat of the valve and in the head the grinding is completed. When satisfied with the seats wipe off all traces of abrasive from the valve and the cylinder head. If the ports have been cleaned of carbon the valves can now be refitted.

Refitting Valves and Springs

Fit the valve spring bottom washers over the valve guides and the valve springs. Place the collars in the springs, compress them enough to expose the cotter groove, and fit the two halves of the cotter, setting the small ends facing the valve head. Hold them against the valve stem and release the springs until the collar locates over the cotter. See that the action of releasing the springs does not force the cotter out of the groove. A little grease inside it will help to hold it in place.

Refitting the Cylinder Heads

See that they are both clean, particularly the face joint surface and the sparking plug threads. It is a good plan to fit the sparking plugs before fitting the heads to the cylinders as any dirt that has lodged in the threads will be forced out and can be wiped or blown away, and will not get into the combustion chambers.
Fit the joint gasket, the same way round as it was originally. Offer up the head to the cylinder, guiding the push rods through the tubes in the head to engage with the rockers. Just before the faces meet remove the rubber band that was used to hold the push rods and tappets from falling out. Before tightening the cylinder head nuts make sure that the piston is at the top of the compression stroke so that the action of tightening them does not have the effect of opening the valves. Note that both pistons are at top dead centre at the same time, and when one is at the top of the compression stroke with both valves closed, the other is at the top of the exhaust stroke, with both valves open.

Tighten the cylinder head nuts evenly and each a little at a time, avoiding putting all the pressure on the faces at one point. Work round them alternately until they are all tight.

Completing the Work.

Readjust the rocker clearances (see page 7). Refit the carburetters, and exhaust pipes, and connect up the oil pipes.