'Glaze busting'

You've just reached the midpoint of the season, your engine is a little 'tired' and so you send it back for a general rebuild to tide you through to the end of the year. As a 'spec' engine, identical to others in the formula, the builder will no doubt strip it and check it for wear on the valve seats and tappets before rebuilding with new bearings and gaskets. But what does he do about the piston rings and cylinder bore? The engine may be slightly down on a cylinder leak-down test and so some form of remedial work may be necessary. But how much and what kind?

In the case of a purpose-designed race engine the answer is simple - new liners, new pistons and rings and she'll be like new. However, in the case of our 'spec' unit and where rebuild costs may be included in the lease agreement, such luxury is out of the question, and anyway, the liners are most likely cast into the cylinder block. The chances are that the standard production rings, designed for up to 200,000 miles of sedentary road use, will have hardly worn at all and it could be that the increased leak-down is all in the valve seating anyway. Replacing the rings will be no great hardship since the engine will need some form of initial bedding-in on the dyno, but in order to assist this using new rings do we need to do something to the surface of the cylinder bore as well?

This is a dilemma faced by many an engine builder, particularly those, say, supplying Formula Ford engines where the regulations are quite strict. Should we 'glaze bust' the engine or should we just fit new rings (assuming no obvious bore damage or excessive wear) and hope for the best?

To start with, let's get our terminology correct. Although the title to this article is 'glaze busting', bore glazing is a phenomenon normally observed only on diesel engines that run a light load for long periods of time. The 'glaze' is in fact a lacquer coming from the oil or products of combustion that condense on the cold surface of the cylinder bore and fill in all the ridges and furrows (however invisible to the naked eye) that represent the surface topology. Hard and almost impossible to remove, the ring will pass directly over it, producing excessive exhaust gas blow-by and high oil consumption.

In the case of race engines, however, what I am referring to is 'bore polishing' - when the peaks and valleys are worn away gradually by the passage of the piston ring. In diesel engines this can be caused by the use of high-detergent oils that produce ash which acts as a grinding paste in the ring bore contact zone. In race gasoline engines, however, when these detergent additives are present in lower concentrations, this is attributed to just general wear and tear.

In the case of our Formula Ford engine, when new rings have been fitted there are a number of possible approaches. One option may be to do absolutely nothing. After only less than 1000 miles the amount of wear against the standard Ford ring pack would, it is assumed, be almost negligible, so the plateau finish produced at the Ford factory under closely controlled procedures would be hardly affected. At the other extreme, re-honing is surely out of the question. This would remove too much material, increasing the piston-to-bore clearance, which would increase even further after bedding in.

The middle option, and one used by some engine builders known to me, is to use what is known as a 'Flex hone' or 'bog brush'. A resilient flexible cutting tool with a soft cutting action, the Flex hone consists of a number of abrasive globules bonded to the ends of nylon filaments, mounted spirally around the axis of the tool. Placed in a simple pillar drill or even handheld electric drill, the tool is run in and out of the bore at about 350-500 rpm and 30-200 strokes per minute for no more than 10-20 s. Depending on the abrasives selected, this produces a surface finish that is less harsh than that of the traditional hone and, done correctly, once the engine has been run-in it can restore leak-down test results to their original values.

Fig. 1 - The Flex hone

Written by John Coxon