Join Date: 10th August 2007
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From the wiki...........(if you've not already seen it)
This is all about piston rings and bores. In an engine the compressed gas must not escape the combustion chamber down into the innards of the engine (causing loss of power) and the oil must not escape at too great a rate into the combustion chamber (causing loss of oil). The seals that stop this are springy rings that go around the piston, that are forced by the pressure of the gasses in the combustion chamber to squeeze against the inside of the bore. They mostly don't touch the bore - there is a microscopic film of oil between the ring and the bore, held in some tiny little grooves that cover all of the bore.
The quality of this seal is critical to the performance of the car, and this seal is "completed" during the running in period.
When the little groves are put in the bore by the manufacturer, they are not perfect. Modern grooves are pretty good - they already have a little plateau on the peaks - but the rings still have to wear them in a bit to the perfect shape of the rings in that bore. Oddly, the best way to do this is to get the parts in the correct shape and size by warming the engine right up and then loading the rings up with plenty of pressure so they are evenly forced against the bore, causing them to evenly rub down the grooves to a perfect fit all round. If only a light pressure is applied (for a long time), then the rings will tend to press a little bit in some places, leaving other places not worn correctly. This can remove the little groves in some places leaving a smooth finish (glazing the bores), causing a poor seal for life.