Any cat experts ? Got a few question about my c15 twins

Got it rebuilt 18 months ago, with aftermarket parts, wich i did'nt know til i was ask to pay for the job, anyway, the combustion valve keep breaking off on #6 ...4 time in a row, just bought a platinum kit from cat n intend to throw it on myself, what should i be aware of ?? Thx

 

Nope, they trew on a new head ,sleeve, piston,

 

I wonder if they d cut #6, i know they did on 3 4 5.....

 

Exhaust valve

 

Its the exhaust valve, sorry for the mistyping

 

The head is aftermarket too

 

Was worried about protrusion but , the whole work would o break , not only this one valve....