KZ305 - 305CSR - GPz305 oil flow modification

Thought it might help someone in the future if this is its own thread...

The stock 305 engine has a plug on the left upper crankcase that covers an oil gallery - by about 5mm, I've found, which is roughly the diameter of the hole itself. By removing the plug and cutting it down, full flow can be achieved, hopefully doing something to remedy the 305's reputation for exploding. 

To start, remove the clutch cover and the timing cover. The clutch gasket covers the area you access the back of the tapered plug through. 

 

There's a gear under the timing cover that obstructs the plug - the gear slides out by hand, and can be put back without issue. 

 

I suggest heating the area around the plug for a couple seconds - I counted three with my MAP torch, I didn't want to risk any longer. Insert a long rod from the clutch side of the engine - I used a stud, as I'm working with a replacement crankcase, and then a 1/4 extension to pound on the stud. I've found that about 8mm is the upper limit of the space, there's a slight reduction in diameter in the center. 

 

A word of caution: don't use anything too soft for this. My first attempt, I bent a screw that I thought might work, and ended up in my current situation of swapping cases. This isn't a hard process, just use the right tools...or as close as you can get. 

Anyway, plug out. I cut away 1/4 of it, leaving as much as I could while making space for oil. This is about 5mm deep, a little less than half of the diameter cut out. It might be okay to just cut 5mm off of the end entirely, but this seems like a safe compromise. Deburr the edges if you need to. 

 

Then, with the orientation correct, it taps back in pretty easily, and you're done. Hopefully these photos are detailed enough.