blow by

[sticky]

While looking at older tractors, I hear the term no blow by. What does that mean and how can you tell if a tractor has blow by?

 

[SkyPup]

Do a compression check on each cylinder, high blowby is from worn leaky valve seals and/or piston rings.

 

[AntrimMan]

Blow-by is a term used to describe compression ecsaping past the piston-ring-cyliner seal in the cylinder bore. There is always some compression escaping past the piston-ring-cylinder seal as it is impossible to obtain a 100% perfect seal in this area regardless of the engine condition. A compression test will evaluate each cylinder against any other cylinder or provide a comparison with know compression tables in a service manual. When blow-by is great enough there will be puffs of smoke and vapor emitted from the crankcase breather tube and perhaps also a milky whiteish deposit on the upper part of the dipstick tube and inside the valve cover. Leaky valve seals, either at the seats or the stems, do not contribute to blow-by. If you are unsure of your ability to make a determination and you are including this criteria in a purchase decision, you should consult a professional, which I am definitely not.
Best of luck,
Martin