301cc Big Bore Piston & Cylinder Set

It takes a lot of work to get all the body parts off to get to the engine.Just got the new cylinder on and am taking my time as I wait for the reflashed ecu. I will let you know when I get it back together.

How it should be!

If you own a CRF300L and haven't done the 301cc Big Bore Kit, you're seriously leaving power on the table. This kit is the real deal — a race-proven setup with championship wins and a lap record to back it up. The jump to 11.5:1 compression combined with the full Stage 3.0 package transforms what was a capable but underwhelming commuter into an absolute riot to ride. The throttle response is crisp, mid-range torque is dramatically improved, and highway cruising feels effortless in a way it never did stock. What really sets this apart is the attention to detail. Ari pre-gaps the rings, pre-installs them on the piston, and drops the whole assembly into the cylinder for you — so installation is far less intimidating than it sounds. The dyno numbers don't lie either: +25% torque and +34% horsepower with Stage 3.0, and even more if you go with the CNC ported head. For a platform that Honda deliberately detuned, this kit gives you everything Honda left on the table and then some.

Installation tips

First off, many thanks to Ari for all the remote assistance! He was great to tutor a “not ready for prime time mechanic!” I learned a lot. Be sure to read all the documents fully before starting. And watch his video. Also get a good manual! 1. I was able to not have to disconnect the air box from the throttle body, I used a cam strap to pull it to the rear which gave me enough room for the head to pass by. 2. Use a “magnet on a stick” to retrieve the head nuts and washers. Stuff a rag or paper towel in the cam chain tunnel so nothing drops down there. 3. Be sure to closely inspect the case where the stock cylinder sat, we missed a small piece of gasket that had broken off, requiring a round 2..the photo below shows where it was, just in front of the coolant channel. 4. Getting the rod connected to the 301 piston while it is inside the cylinder is not to tough. I used a 1/4” long extension with a 8mm socket to help the pin through. The wrist pin can be a challenge so practice on the stock one you removed to practice working it into where it sits. I used a small flat head (not micro small) to work it in. 5. The forward cam chain guide is a bit tricky, Ari has instructions. The trick is to get it to sit into the slots in the head, so look at the head carefully and understand what has to happen. 6. When placing the head nuts/washers I used a couple tricks a friend showed me. Place a piece of electrical tape inside the 14mm deep well socket, make sure it’s secure so it doesn’t fall off in the head. This holds the nut to the socket. Next apply a bit of grease to “stick” the washer to the nut. This works really well. 7. After torquing the head nuts. Place the 2 long bolts back into the cam chain tunnel. I broke one of these, requiring round 3. In the photo you’ll see that it’s best to use a 1/4” drive socket rather than a 3/8”. The 3/8” pushed the bolt head out enough that it damaged the threads causing the bolt to break, luckily it was in only finger tight! 8. Be careful when using a screw gun to put the body bolts back in. I got one in a bit off center and love those headaches. Print out Ari’s instructions and have a hard copy of the workshop manual. Lastly, I will remind everyone just how hard it is to get general nuts and bolts for a motorcycle these days, gone are the days of going to the shop and having bins of them. So take your time and have a system for accounting for which bolt/nut came from where.