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On 1 Oct 2003 at 9:36, Gamboa, Gary wrote: > I had a short conversation with a vw mechanic who mentioned blow-by. > He also said this would condition would require an engine rebuild. > > Can anyone give a definition of this? It's the condition when the rings aren't sealing well anymore. This allows more (which is a really relative thing) combustion gasses to pass from the top of the piston into the crankcase. There is always SOME blowby, but the term is usually applied when the amount is excessive. Any blowby has to eventually be vented from the crankcase. This is what the crankcase breather does. Part of the breather's function is to separate blowby gasses from the oil mist that is inherently present in any running engine. The oil mist must be collected somehow and returned to the crankcase. Once the blowby becomes excessive, this oil mist is often no longer collected effectively, ie the separation mechanism is overwhelmed, and you start losing oil. Blowby is usually a ring problem, but some cheap aftermarket cylinders have thinner than stock walls, which make them more likely to warp when warm. This prevents the rings from sealing, thus resulting in excessive blowby. This is a common occurrence with the smaller ranges of oversized cylinders which have oversized bores but stock sized ODs This makes it possible to install them in stock cases without machining the case for a larger cylinder. These are called "drop-in" PC sets. They are to be avoided. The "correct" way to install oversize PC sets is to buy "machine-in" sets that require that the case bores be enlarged to accept the larger cylinders. -- Jim Adney jadney@vwtype3.org Madison, WI 53711-3054 USA ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org