Cylinder question from RZ guy

[kobra]

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

[97Screamer]

44 minutes ago, kobra said:

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

If the cylinders are in spec after cleanup I would run them. If you didn't have an airleak (which sounds like you did a leak test and it passed) then your jetting was too lean. You using the oil pump or premixing the fuel? 

[kobra]

16 minutes ago, 97Screamer said:

If the cylinders are in spec after cleanup I would run them. If you didn't have an airleak (which sounds like you did a leak test and it passed) then your jetting was too lean. You using the oil pump or premixing the fuel? 

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

[97Screamer]

I'm not saying that's the problem but I have read others that had that problem. If I was running an RZ at a track I would be premixing.

Sent from my HTC One M9 using Tapatalk

[kobra]

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.

 

[kobra]

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

[Yaxy]

I would get the cylinders looked at by professional. and let them decide if hone or bore.  The shop can then match the cylinders to the pistons you would like to use.  From what see it looks like your jetting is too lean.  A hole in the top of the piston tells me your lean and I see no traces of oil anywhere on the pistons. I would recheck the oil pump as well.

 

[trickedcarbine]

You are suffering the same fate many drag racers do. You send the piston to 9-10k rpm and the carb is wide open sending fuel and oil to the piston and it’s happy feeding off wide open throttle. Low you let out and the motor is still in the upper RPM’s while you’ve closed the throttle and forced the piston to starve. That piston is still up around 8,500-9,000rpm and all it’s got feeding it after getting hot is the lonely ol’ pilot jet. You need to fatten up the pilot to accommodate and possibly change some of your riding habits. You really have to make sure that the bike is fed in the rpm’s even if you’re letting off and slowing down. Try rolling out of the throttle slowly while really realing the bike in and bringing the rpms down as quick as possible so it’s starve time is much shorter.