Went to assemble my new top end on my WR250 sumo last night and ran into a minor snag. I never noticed that the Wosner piston kit didn't include a wrist pin bearing. Bah. More interesting is that the box lists part numbers for the piston, pin and rings but no mention of the bearing. My first thought was the OEM one should work, but a test fit shows the Wossner piston has noticeably more open length for the wrist pin bearing to slide left to right than the old Wiseco that was in there previously. The OEM bearing is 18 x 23 x 21.8. Wossner lists an 18 x 23 x 23.8 unit that would fit in the piston like the OEM does in my Wiseco. Question is, do I size the bearing to match the width of the rod, or the gap in the piston? If I go with the OEM should I get a couple spacer thrust washers made up to keep the bearing from walking out of the rod? Am I overthinking it?
Depending on the design of the crankshaft/rod/piston the rod will either be located by the crank or piston, generally referred to as piston or crank "guided" rod. If the rod is located by the crank, you want to leave adequate clearance on the small end at the piston interface to prevent inadvertent contact or rubbing. If the rod is located by the piston, exactly the opposite. Depending on the design, the bearing itself will not actually do the locating - thrust washers are located between the rod faces and the guiding component, while the bearing just floats in the provided clearances. This is not always true but relatively common. Not knowing the details of a WR250 engine, I couldn't tell you what your situation is, but you should verify to be safe.
Given my crank has big end rod thrust washers I'm going to say it's a crank guided rod setup. I'll snap some pictures tonight to give a better idea of what I'm possibly overthinking.
This is the old piston and wrist pin bearing. As you can see the bearing doesn't have much wiggle room between the piston pin bosses. This is the new piston, same bearing. Lots of room to slide around. My calipers don't fit in well so my measurement isn't spot on but I'm estimating about 4.5mm more width between the piston pin bosses on the new piston compared to the old.
The OEM piston and bearing look reasonable for a crank guided rod (just eye-balling it), but the Wossner piston with OEM bearing has significantly more end play than I would be comfortable with. If the dimensions you mentioned above are to imply a "longer" cage, it would only be 2mm increase (21.8 vs. 23.8) - doesn't seem to be enough to take up the 4.5mm you describe as the increase in end play. I have used thrust washers on the non-guided side in the past, but that was for a specific application where necessary for other reasons. What did they say when you called to inquire about the bearing? Is that piston intended for use with an OEM rod or something aftermarket? Another interesting difference is the lack of lubrication holes in the pin support as seen on the Wiseco piston. I have seen the lubrication provision done in many ways (as well as omitted) - is there anything in the Wossner piston not clearly visible in the picture?
The Wossner unit is a custom one for this big bore application, done up for Kustom Kraft. Kustom says to just use the OEM bearing and not worry about it. The tech I spoke with at Wossner didn't have any input. The Wossner has carve outs in the wrist pin bores, I'm assuming those are for oiling rather than using ports like the other piston.
Well, that is a bit discouraging. Keep in mind that the rod will be centered reasonably well in the piston simply due to the fact that the cylinder/bore is located and machined on center with the crank pin (within some tolerance) and the rod is guided by the crankshaft (based on your description). This fact at least takes excessive rod eccentricity (relative to the piston) out of the picture, leaving only the bearing to move freely relative to the rod. A go/no-go qualitative assessment I have used in the past, when no other direction is available, is to verify that in the worst case scenario there is no exposed rolling element outside of the rod land. If you center the rod in the piston and force the bearing to the extreme of its travel (touching one side of the piston) you should still have complete engagement of the rollers in the rod, if not you will risk locally wearing the rollers and eventually wearing through the cage and complete destruction (ask me how I know). Simply due to the fact that the Wossner bearing is wider, most likely makes it the better choice, but should be confirmed none the less. Hopefully I have explained this clearly enough?
