This blog, in addition to the usual maintenance details for classic mini cars, an attempt is made to explain the "whys" and "wherefores" of the various jobs.

Saturday, 17 October 2009

When is a Top-overhaul Needed ?

It is a sound recommendation that an engine that is running well should not be disturbed: on the other hand if decarbonizing and attention to the valves is postponed for too long, the amount of work involved, and the expense incurred, may be greater than would have been the case had the head been removed at an earlier stage. Badly burnt exhaust valves, for example, must be scrapped, whereas slightly pitted valves can be given another lease of life by refacing and grinding-in.

Moreover, an engine that has a relatively high compression ratio will benefit from the removal of even a light coaling of carbon, which retards the flow of heat through the piston crowns to the lubricating oil; the oil acts as a coolant as well as a lubricant. Overheating that results from carbon deposits is probably more directly responsible for causing detona¬tion or "pinking" than the slight increase in compression ratio caused by the deposit itself.

The exhaust valves can also cause pinking. Most of the heat from the head of the valve is transferred through the seating to the cooling water during the instant that the valve'is closed; it is not surprising, therefore, that restoring a good contact between the valve and seating by careful grinding-in will often cure an acute case of pinking.

A further point is that there is a lot to be said for removing the cylinder head and lightly grinding-in the valves after a new or reconditioned engine has covered about 5,000 miles. During this "settling-down" period the strains set up in the engine components have been relieved and the slight distortion that may have taken place may be sufficient to impair the seating of the valves.

After this initial attention, the need for a subsequent top overhaul will be indicated by a progressive deterioration in performance, sometimes accompanied by a tendency for the engine to overheat and for "pinking" to occur at low speeds in top gear, together with symptoms of pre-ignition and a tendency for the engine to run-on when the ignition is switched off.

It is possible to delay the point at which decarbonization is necessary by slightly retarding the ignition, as described in Chapter 7, but this should be regarded as a purely temporary measure as the valves will probably require attention.