I have just examined a new filter and it definitely has a pressure relief valve in the center. I believe its purpose is to relieve excess pressure when the oil is cold or if the filter becomes blocked.

Here I am depressing the valve. This view is from the outward-facing side.

The pressurized oil fills the filter cavity and flows into the filter through the pleated filter media.  The oil flows from the outside of the filter to the inside of the filter.  It flows through these pleats.

After the oil passes through the filter media, or past the relief valve, it exits the filter through this hole.  It then proceeds through the end of the crankshaft to lubricate the rod bearing and cylinder, and also through the channel in the engine case that directs oil to the top end and the gearbox. Note the perforated metal in the picture.  That perforated metal is there to support the paper filter media so it doesn't collapse and get run through the system (a fate worse than death).  :'(

So, Bokobob, I believe you said you reused the filter that was already in the engine.  If you are absolutely confident that you had the big hole in the filter facing the crankshaft, I think there is only one way the pressure could have built up to the point of blowing your filter cover to smitherines.  The relief valve didn't lift.  My recommendation is to get one of these and make real sure that its installed correctly, with the hole facing the crankshaft and the relief valve facing out.

DragBikeMike wrote on 04/10/20 at 20:10:27:


I was astonished to read that the oil flows FROM the outside of the filter to the inside through the pleated folds.  I had thought for sure that it flowed from the inside of the filter to the outside of the filter and exited the "cavity" through a fairly small hole elsewhere in the cavity casting.  
So I ask, where does the incoming oil from the engine enter the filter cavity?  

Please note that the K&N  filter that was in my bike when the side cover blew out while I was starting the engine appears to have experienced no damage whatsoever.  The heavy metal side cover itself that blew out  has a hole the size of a silver dollar or larger...Go figure.  Seems very strange to me.    :'(

DBM is 100% correct on the oil flow (not that I didn't believe him).

I went and looked at my parts engine.  

Look at the image. In yellow on the engine side is what I will call the oil pump outlet, and on the case is the inlet to the oil filter area. If you look at this and think about what is going on, oil has to flow this direction and it all makes sense.  The oil pump outlet fits snugly into the corresponding hole in the case circled in yellow, which is of course the oil filter cavity.

See the red "oil path"

Oil goes into this little filter cavity under some degree of pressure, flows into the outside of the filter down through the center of the filter (now clean, filtered oil), then up into the little oil passage that eventually carries oil up to the top of the engine and (I think) to the backside of the clutch case where it spits out clean oil there too.

So, if you have a gunked up filter and oil comes shooting into the oil filter cavity under pressure it has to go somewhere.

My red arrows might be off. I will take a peek tomorrow and get it right so I don’t lead anyone astray, and we can have an accurate reference.