Oil Fill Plug

[mark328]

On pressurisation, is a small amount of air meant to leave through the top of the oil filler plug? There is 2 'breather holes', not split or damaged hole. No air when fully pressurised.

The reason I ask is I need a new plug as not in the best condition but the newer model ones don't seem to have these 'breathers'

 

[ChrisBailey]

That would make sense, the piston will let compressed air past the piston ring/seal or skirt when its cold.

 

[brightspark]

On pressurisation, is a small amount of air meant to leave through the top of the oil filler plug? There is 2 'breather holes', not split or damaged hole. No air when fully pressurised.

The reason I ask is I need a new plug as not in the best condition but the newer model ones don't seem to have these 'breathers'

crank case breather . when its pumping its a bit of pressure loss through the piston rings when its full and stops there is no pressure in the crank case

 

[timgunn1962]

There's not just the pressure caused by air passing the rings: every time the piston goes up and down, it displaces the same volume on the crankcase side as it does on the compression chamber side. Obviously less pressure and it'll be cyclic, averaging zero, but there's usually some sort of breather.

 

[mark328]

There's not just the pressure caused by air passing the rings: every time the piston goes up and down, it displaces the same volume on the crankcase side as it does on the compression chamber side. Obviously less pressure and it'll be cyclic, averaging zero, but there's usually some sort of breather.

Thanks All, I can see a hole in the newer ones now, the photo just didn't show it well, of course yes, the piston will create compression on its down stroke slightly as well as the up stroke so the air needs to go somewhere

 

[brightspark]

Thanks All, I can see a hole in the newer ones now, the photo just didn't show it well, of course yes, the piston will create compression on its down stroke slightly as well as the up stroke so the air needs to go somewhere

if its a twin it evens out. if one is going down the others going up so negative pressure

 

[mark328]

if its a twin it evens out. if one is going down the others going up so negative pressure

it iis yes, its a tiny amount of air coming out so maybe not neegative but very little breathing

 

[brightspark]

it iis yes, its a tiny amount of air coming out so maybe not neegative but very little breathing

losses in the rings mine does it. slightly more now after over 40 years of use