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. 

This thread has certainly helped me out thanks so much.

I went ahead and cleaned that silly oil strainer that is not mentioned in the maintenance, mine didn't look too bad, ordered a new one because the gasket was shot.

I figured I might as went go ahead and try this oil flow mod and replace all the engine screws.

My problem is I can't seem to get the left cover off, I must be missing something. I've been tapping it with a leather hammer but I am afraid to smack it any harder.

Does anyone have any suggestions? I must be missing something. The manual doesn't say much.

Ok problem solved.

When I finally managed to get the sprocket cover off (4 screws), which took ALOT of wiggling and tapping, now with a rubber mallet....

It exposed 2 more screws on the left cover and was able to get that off.

Just food for thought.
Do we know this restriction is unintentional. Could it be there to properly distribute the oil. Will opening it up cause oil starvation in some other place?
Do we know 305 catoastrophic engine failures are common, or did it happen to someone, somewhere, and got blown out of proportion?
I'm considering doing this mod, but question if I'd be fixing what ain't broke.

It looks from doing the mod that the hole in the crankcase is totally blocked by the plug so something isn't right.
Either someone made an artery that goes to a dead end or...
Someone plugged an artery.
Dead ends don't seem very appropriate in a circulatory system to me.

I bought this bike with 500 miles on it and I've had to replace the rectifier and the main bearing. Has about 15,000 miles on it now and I am worried it has run this long
with that blockage. Better fixed late than never.

Here is the artery with plug removed.

 

Plug removed uncut, tapered end goes in.
 

Cut Plug
 

Marking for proper orientation to allow flow
 
 
 
 
 

ckahleer wrote:

Just food for thought.
Do we know this restriction is unintentional. Could it be there to properly distribute the oil. Will opening it up cause oil starvation in some other place?
Do we know 305 catoastrophic engine failures are common, or did it happen to someone, somewhere, and got blown out of proportion?
I'm considering doing this mod, but question if I'd be fixing what ain't broke.

I would suggest "extreme" caution with this. Looking at the below flow chart from the 250/305 manual in the file-base it looks to me like all you are doing is providing a "partial short cut" back to the sump which will rob oil supply and pressure from the head and crankshaft (it's only my opinion but I don't think the factory would "accidently" block a main oil supply)

Well shyte!

The nice thing about this mod is it would be possible to pop out the modified plug and reinstall it upside down to again block the hole.

I was getting oil into the cam area as evidenced by the leaking cam seal I just replaced.
It did seem to fit with the early main bearing failure is why the whole thing made sense.
It would be good to hear from the previous "oil seal modders" to see what happened afterwards.

Matratt3 wrote:

Well shyte!

The nice thing about this mod is it would be possible to pop out the modified plug and reinstall it upside down to again block the hole.

I was getting oil into the cam area as evidenced by the leaking cam seal I just replaced.
It did seem to fit with the early main bearing failure is why the whole thing made sense.
It would be good to hear from the previous "oil seal modders" to see what happened afterwards.

Ghostdive hasn't logged in since January 2023 so I guess "you pays your money and takes your chances" !!

I also used this as my information and guide. Attached.
 
 

Matratt3 wrote:

I also used this as my information and guide. Attached.
 

 

I'm not saying "don't do it" I have never seen one of these motors up close and personal. The filter gauze certainly looks like a bad idea on the face of it. However with the gallery - where does the mod allow oil to flow to other than back into the sump ? it appears the crank is fed prior to the bung so why does it cause big end failure ? just putting the questions out there, not trying to be a "smart ass"

Appreciate the input, what you are saying makes sense.

I just don't understand the design of it, and it appears I am not alone.