Topic: Broken piston after 50miles CB 750 K2

[Geoff Hastings]

I meant piston to valve clearance, generally .100" is a ballpark minimum for most automotive engines but you can get away with less if you have a light valvetrain, high spring pressure, etc. Running tight piston to valve clearance will cause problems if you ever get into valve float.

I'm guessing maybe you're referring to valve to guide clearance. If those numbers are inches that's super sloppy worn out, no way a valve hung in a guide. Likely be a smoke show after the seals gave up. If that's mm, that would be way too tight. Valves would probably be stiff to move by hand and would be pretty obvious on assembly.

005 and 008 would be the valve to tappet clearance, running it hard enough to get into valve float with a new build engine would be asking for trouble.

[dave500]

50 miles to melt a piston?im still set on crappy metalurgy?

[Ellz10]

50 miles to melt a piston?im still set on crappy metalurgy?

Same!

[Geoff Hastings]

Any further conclusions reached about the melted piston?

[Phoenix]

I'm on vacation and appologize for not reading each response.  I had two holed piston events on road trips of four hundred miles.  Both within 25 miles of each other.  I had the jugs bored and put in cruzin images 836 kit with my original cam.  This gave my original 736 a reak kick in the ass.  Maybe you need more.  I never found what caused mine.  Hondaman said he only knew of 3 holed pistons and mine was one of them.  On these old bikes, sometimes it's hard to aswer your "why's".  As someone said, summer is here.  Hope it all works out.