coolant overflow issue

[mxpimp2000]

my buddy just rebuilt his yfz and put the cylinder works 478 kit on and all was good until other day he was riding/racing and it started blowing coolant out of the overflow tank and coolant was all over the swingarm and rear end. we have swapped radiators caps and there is no coolant in the oil or anything like that. it wont do that if the bike is just idleing only when u ride it
:confused:

[miles Machine]

hate to say it but a bad headgasket allowing compression pressure into the system will cause that

[mxpimp2000]

thats the decision we have came to, talked to a few guys a tech and they had similar issues b4