kobra

Do you guys usually use a flex hone for prep before new pistons?

Yeah, I really think it was jetting or fuel starvation or something, but strange that it happened so suddenly after running great for almost 500 miles.

I would not attempt without the tool unless you want to break something. There is a surprising amount of force holding it on.

I used the oil pump, and tested it thoroughly on the bench before running it to get it dialed in perfect.

Hi guys, I wanted to get some more opinions on next steps since I reference this forum a LOT but it's my first post. Lots of great info here. I have an RZ350 which is the same motor as the Banshee but with some tweaks, the biggest being it has powervalves. I recently rebuild it from the ground up, all new seals, new crank, new 64.5 Wiseco's, fiber reeds, 1mm squish, etc. Trying to go for an OEM+ build to maintain reliability. Right around 500 miles I was really romping on it for the first time (on a closed course) WFO at about 100mph. It ran great. Then I let off and was cruising about 85mph, 1/4 throttle, and the left piston blew a hole in the middle and lightly seized. Took the whole thing apart, cleaned the bore with muriatic acid and can't feel any scratches. I also took some bore measurements. My questions are: 1. Where do you guys measure the bore diameters? In my manual it says to take 3 measurements both front to rear and side-to-side all above the exhaust port. I took some measurements of the entire length of the sleeve and they are both all in spec except for the region near the intake ports bridge below the exhaust port, which measured .005" and .0045" on left and right cylinders. 2. What kind of prep would you do before installing new pistons? Just throw them in? Scotchbrite? Flex hone? Wiseco hone brush? Sandpaper? I'm hoping I can get away with some new 64.5 pistons and it will all work. 3. Any ideas what other things I should check as to why this happened? The right piston looks fine. I pressure tested before and after and it held 6psi for 24hrs. Here's some pics to make this slightly interesting. After cleanup