Cylinder head and pistons - decarbonising

1 With the cylinder head removed as described in Section 7, the carbon deposits should be removed from the combustion surfaces using a blunt scraper. Take great care as the head is of light alloy construction and avoid the use of a rotary (power-driven) wire brush.

2 Where a more thorough job is to be carried out, the cylinder head should be dismantled as described in Section 13 so that the valves may be ground in, and the ports and combustion spaces cleaned and blown out after the manifolds have been removed.

3 Before grinding in a valve, remove the carbon and deposits completely from its head and stem. With an inlet valve this is usually quite easy, simply a case of scraping off the soft carbon with a blunt knife and finishing with a wire brush. With an exhaust valve, the deposits are very much harder and those on the valve head may need a rub on coarse emery cloth to remove them. An old woodworking chisel is a useful tool to remove the worst of the valve head deposits.

4 Make sure that the valve heads are really clean, otherwise the rubber suction cup grinding tool will not stick during the grindingin operations.

5 Before starting to grind in a valve, support the cylinder head so that there is sufficient clearance under it for the valve stem to project fully without being obstructed, otherwise the valve will not seat properly during grinding.

6 Take the first valve and apply a little coarse grinding paste to the bevelled edge of the valve head. Insert the valve into its guide and apply the suction grinding tool to its head.

Rotate the tool between the palms of the hands in a back-and-forth rotary movement until the gritty action of the grinding-in process disappears. Repeat the operation with fine paste and then wipe away all trace of grinding paste and examine the seat and bevelled edge of the valve. A matt silver mating band should be observed on both components, without any sign of black spots. If some spots do remain, repeat the grinding-in process until they have disappeared. A drop or two of paraffin applied to the contact surfaces will speed the grinding process, but do not allow any paste to run down into the valve guide. On completion, wipe away every trace of grinding paste using a paraffin-moistened cloth.

7 Repeat the operations on the remaining valves, taking care not to mix up their originally fitted sequence.

8 An important part of the decarbonising operation is to remove the carbon deposits from the piston crowns. To do this (engine in vehicle), turn the crankshaft so that two pistons are at the top of their stroke and press some grease between the pistons and the cylinder walls. This will prevent carbon particles falling down into the piston ring grooves. Plug the other two bores with rag.

9 Cover the oilways and coolant passages with masking tape and then using a blunt scraper, remove all the carbon from the piston crowns. Take great care not to score the soft alloy of the crown or the surface of the cylinder bore.

10 Rotate the crankshaft to bring the other two pistons to TDC and repeat the operations.

11 Wipe away the circles of grease and carbon from the cylinder bores.

12 Clean the top surfaces of the cylinder block by careful scraping.

    See also:

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