XR 250R Engine Rebuild

This bike was listed on Facebook Marketplace for a while, and eventually, it followed me home. The previous owner had fixed it all up with new plastics, a powder-coated frame, all new bearings in the rear suspension, etc. Then, a shaft in the transmission broke; he split the cases, lost interest, and put the engine in pieces into a couple of totes.

I had a lot of fun putting the puzzle back together. It wasn't too bad, actually. He had done a great job of keeping track of all the parts.

Here is the culprit: a broken output shaft. The tip snapped off, and the gears crashed together, sending metal fragments throughout the oil system.

The oil screen at the bottom of the frame had a significant amount of shavings in it. I rinsed the frame out thoroughly, filling it with gasoline and flushing it several times to ensure all "glitter" was gone. The oil pump was also scored up from the debris, so I sourced a used pump in better condition from eBay to ensure the new build has plenty of oil pressure.

The cylinder was scored up a bit. The previous owner had replaced the sleeve with a larger one and bored it to 80mm, bringing the displacement to approximately 300cc. After talking to some folks online and at XRs Only, I decided that 300cc on an engine designed for 250 wasn't ideal for long-term reliability. Parts are also more expensive, so I opted to return to a stock bore size.

This is the internal boss for the kickstarter. After seeing this damage, I found a complete bottom-end half for an XR250 of a similar year on eBay. I ordered that along with a stock cylinder (73mm bore). That cylinder turned out to have some corrosion pits that wouldn't hone out, so I picked up a 73.5mm Wiseco piston and ring set (4466M07350). The machine shop bored the cylinder to match the piston.

Onto the head: the intake valves were heavily cupped/worn - a common issue with these XRs. I picked up a new Kibblewhite stainless valve set and had the machine shop cut the seats and fit them. So much for this being a cheap rebuild!

Notice the head has extra fins welded onto the casting. This is a specialized mod by XRs Only to increase surface area and help the air-cooled engine run cooler. I'm definitely happy to have that.

Someone along the way also fitted an XR400R oil cooler to the bike. Hopefully with this plus the oversized head, I'll have an unstoppably reliable, trail riding machine!