I have a 2009 KLX250SF. Glancing thru the manual, I notice it uses ball bearings at the crankshaft ends. The oil flow diagram seems to show these bearings are part of the oil flow (pump) circuit. My friend says that would mean they are pressurized, and argues this isn't practical.

Does anyone know if they are pressurized, or at least how they are lubed?

I could be looking at the oil circuit incorrectly, but I enjoy knowing how my machines operate.

 

Kawasaki and most others use low pressure high volume on all their ball/roller bearing bottom ends. Most singles have ball/roller bottom ends, simple to build and works well.

Some companies used pressed together crankshafts in multi-cylinders, but not many. Kawasaki was the one I definitely know did so. Kawasaki had that set up on the original Z1, the bottom end oil pressure seldom rose about 10 psi. A few friends had gauges on their Z1s back in 1975. It fed off the circuit to the crankshaft. We freaked out, then we got the explanation about the split circuits in Kaws from a friend who was trained by Kawasaki.

One reason was the flex and movement in the multi-piece crankshafts, racers would have the journals welded to hold and strengthen the cranks. Notice the 1 piece rods, needle roller bearings are inserted in the rod/crank pin before pressing together, ball bearings are the main bearings between crank and case.





Ball/roller bearings don't need high pressure to maintain the film of oil between a broad surface, the bearings roll on the surfaces, so if they touch, which they do, there is little friction heat. Not so with the broad fixed surfaces of plain bearings, where contact can result in seizure of the metals.

I would venture to say the head had a higher pressure feed due to plain bearing surfaces there. I would venture to say the pump feeds direct to the head with higher pressure in a separate circuit that maintains the pressure.

Plain insert bearings like on most multi-cylinder engines both motorcycle and car use a higher pressure oil feed that feeds all circuits since they are all plain surface bearings. I don't recall when Kawasaki went to the plain bearing engines, but probably back in the 80s sometime. I'm thinking it was when they went to their first liquid cooled multies, essentially ending the ball/roller bottom ends when the earlier air cooled KZ series eventually ended. I think the air cooled Zephyr series still used the ball/roller set up since the 550, 750, and 1100 were all based on the earlier KZ engine.

 

Excellent article. I think that is exactly the way they went with the Ninja 250 twin (my former bike). The KLX250S has minor differences.

The KLX250S (single) does use a crank assembly and solid rod, as you describe. Apparently, they chose to go with solid bearings (bushings) in the rod, instead of rollers.

Yes, I was surprised by the low oil pressure listed. At 4K, it is 11-21 psi, with the bypass kicking open at 11ftlb.

 

This is a pix from the D-Trakker manual (Europe's name for the bike). Oil from the pump (#7) follows 2 circuits. An internal circuit (branching down) feeds the crank. A second path (up) connects to an external Y pipe, which feeds the upper end and the tranny.

(The diagram wouldn't post.)

They chose to support the ends of the crank with large ball bearings. The above diagram confuses me in how these are lubricated. I would assume they, like the piston end of the rod, are lubed from oil sprayed into the crankcase, but this diagram confuses that idea. To the left, the diagram shows a circle at the bearing, which seems to show it's lubed thru the crank.

That was what I found confusing.

 

Sorry, I could post the picture. Page 282 of this manual is the diagram.

http://mototh.com/files/kawasaki/KLX...Manual_new.pdf

 

There is more than 1 thing wrong with that diagram. It shows oil from the left crank bearing going the wrong way & it shows oil going through the crank to the rod bearing. It doesn't seem to indicate what is going on with the right crank bearing.

 

I would venture to say that the oil pressure is far below any plain bearing engine made. Any car or bike with a plain bearing engine would need oil pump replacement if it showed that level of pressure at that rpm. Plain bearing engines will run 50-60 psi at normal engine speeds.

There is nothing in that manual or any other showing a solid bearing, or roller for that matter, on the big end. They only show the whole crank and tell how to true it up. But the thing is every other Kawasaki four stroke single model including the KLR and KXF four strokes have roller rod bearings.

Plain bearings require some pressurized flow to keep a good supply of oil to the plain bearing to keep the film of oil intact, where rollers only require a spray of oil on them - think two stroke, that small bit of oil in the gas does the job. A pressed in pin assembly will be difficult to have the oil feed ports aligned easily in a production assembly and any shifting of the pin could reduce oil flow significantly. It takes a solid crank with ports machined in them to provide the oil for the big end bearings. Not done with the three piece crank. Plus the three piece crank with roller bearings is relatively inexpensive to machine.

I would relatively confidently say the KLX uses a roller bearing on the big end. Part proof, it seems there have been some riders who have used XR250 rod kits to rebuild their cranks since Kawasaki only supplies whole crank sets and no company has a kit put together yet. for the KLX250/300.

 

I can guarantee that the rod bearing is a roller design. I was surprised that the cam journals survive at that low pressure but they do. The oil runs from the pump to the filter housing and splits-one feeds into the r/h end of the crank and splash feeds the three bearings across the crank via galleries drilled into the journals. The other branch heads outside the case and up to the head and trans via the steel oil lines.

 

Thank you for the help. The explanations make it clear, now.

With a roller on the lower end, splash feeding the 3 bearings makes sense.