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Deposits on cylinder liners have been reported in many applications
including tractor diesel engines, four stroke gasoline engines found in
motorcycles, vintage cars, light propeller aircraft and lawnmowers. The
distinction between lacquer and glaze appears to be used primarily for
medium speed engines. In case of other engines the term 'glaze' or 'bore
glaze' appears universally applied to liner deposits with appearances
ranging from invisible to dark black and blueish deposits.
Bore glaze is frequently reported during break in following cylinder
overhaul. Montgommery [16] found that a smoothing surface coating
consisting of Fe3O4 and a carbon was formed during simulated runningin
experiments. These constituents were also found in authentic glaze
formed during run-in. A mechanic with extensive engine service and
teaching experience provided the observations on the topic of glaze
formation presented in Table 2-1. [17]
Personal correspondence with this author has established that glaze in
these engines is typically black or blue in colour and is related to the use
of multigrade oils. The use of a high performance monograde reportedly
eliminated this problem. It was also established that the engines
frequently were running on less than full oil levels.
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TABLE 2-1 ABOUT BORE GLAZE IN SMALL ENGINES [17]
- Glaze describes oil burned and baked onto a surface.
- Glaze is insulative, very hard, polishes to a high shine, wears
piston rings, and can be removed only with an abrasive material or a cylinder hone.
- A glazed bore is invariably the result of surplus lubricating oil being left behind by the piston rings because they have not been "pushed hard enough" against the cylinder bore surface to remove most of the oil.
- The piston rings of any engine under a too light load will not be pressed as firmly against the cylinder bore surface as in an engine under a heavier loading.
- Piston rings scrape harder against the cylinder bore surface
during the combustion and exhaust strokes as the gas pressure
increases between the piston rings and piston ring grooves.
- When excess oil is left behind it is burned onto the bore surface and forms a very hard glazed smooth finish.
- Light or under load running will always result in a glazed bore
- The time taken to glaze a bore will vary according to the
lubricating oil used, the load applied, the length of time run under light load, and local air temperature.
- Any engine with a glazed cylinder bore will always use lots of oil and will cause complete engine failure if not quickly andorrectly repaired.
- A glazed bore will usually be restored to standard by re-honing and fitting new rings.
- A glazed bore is not an engine fault it is an operator or application problem.
- Sometimes the only solution to overcome light loading is to include a dummy load such as an electric bar heater on a generator.
- Most petrol engines prefer 60% to 80% loading if running for long periods of time.
- Diesel engines are "happier" with 75% to 90% loading if running
for long periods of time.
- Engines (using correct grade and quantity of frequently changed
oil) that run at variable speeds and changing loads very rarely get a glazed bore.