Broken piston

[irk_miller]

I've been running TKRJ RZ pistons in my RD350. The bike had been running extremely well and reliable for a year until one day last week when I'm cruising down the highway about 5500rpm on the way to work.



On the good piston you can see where the edge of the cutout is rounded and stretching. Is it possible they didn't remove enough material to cause this.   

[Hawaii-Mike]

I don't know the answer to your question but that sucks!

[rodneya]

Were they installed in a fresh re bore job, or in worn cylinders?

[teazer]

That looks like the skirt has impacted the bottom of the intake port.  Check the barrels to see if there's any damage there and check that there is a generous chamfer on the intake floor.  And wrt the prior post, check for clearance piston to bore.  If it's excessive it may allow the piston to rock too much

[irk_miller]

Were they installed in a fresh re bore job, or in worn cylinders?

It was a fresh +50 bore. I did not chamfer anything myself. Sounds like ignorance on my part.

[m in sc]

easy mistake to make. radiusing the port windows though is absolutely necessary. now you know.   

[irk_miller]

I left town before I got the first reply to my OG post, but now that I'm back in town I'm able to follow up with pics and responses. The shop that did the bore did chamfer the intake port as indicated in the pic.

Am I having the groove cut out of the piston when I shouldn't? I assumed the groove is related to the crank, but I haven't found the reason why and I've never asked the question until now.

Should I also chamfer the piston skirt around the groove or chamfer more? I assume the groove is passing the bottom of the intake port on the compression stroke.

[1976RD400C]

Looking at how stock Yamaha does it, the angle of the chamfer on the bottom of the intake port is steep, more up and down rather than a 45-degree angle. There is a chamfer on the bottom of the piston skirt too.

[irk_miller]

Appreciate these replies.  The machine shop suggested I go with forged pistons over the TKRJ cast pistons.  Says the cast pistons are prone to vertical cracking which may have contributed to the mishap.

[irk_miller]

Just dropped the crank off at Ed Toomey's shop to get rebuilt and balanced and to have a couple of the weights repaired.  Ed gave me a place to be with this.  Mainly to stick with the cast pistons and make sure the bore is tight.  I'll double check the bore.  I'll also chamfer the groove in the skirt and steepen the chamfer on the intake as 1976RD400C points out in his post.  With it being an LC jug and then ported, good chance the piston is bulging into the intake.

[m in sc]

agreed, there's zero reasons for forged pistons on a street bike IMHO unless you like the noise.

[rodneya]

agreed, there's zero reasons for forged pistons on a street bike IMHO unless you like the noise.

And sitting waiting for the motor to warm up properly before riding