hi all, i had just finished rebuilding my 2wd cossy engine on tuesday, i span the key it ran for about 7 mins then stopped with a big bang!
i checked it out and no 1 inlet valve had snapped off and mullered the new piston i had just put in. long story short is that i am trying to find out why, i had the machining work done by a guy who used to work for fords he bored the block to 0.5mm oversize with standard comp pistons put 9 new valves in the head as he said they were bent and skimmed it, i put the engine back together and i no for sure the cams were in the right sides and that it was timed up properly. i have had the cams measured and they are standard, i have had the head measured and it is 0.18mm shorter than fords minimum spec but am unsure 0.18mm would be enough to snap a valve? i have done piston pertursion measurements and it is 0.45mm but can find no specs for it can anyone please help. cheers dan

 

How did you set TDC on the crank?

 

as above
plus,did the valve drop,were the valve stem collets properly installed
did it jump on the timming belt (was the tensioner tight)

 

as above...

 

hi all, cams were set with the pointers facing each other and the crank was timed on the lug nearest the cut out on the pully belt was tensioned correctly and i wound it over with no plugs in with a ratchet seemed fine and didnt lock up, have sent the head back over to the guy who done it and he is measuring the valves, but i could really do with the piston height at tdc to see if the block has been decked, at present at tdc the piston sits 18thou or 0.45mm higher than the block, measured this with a dti gauge but have got no spec to compare it against.

 

my standard 4x4 pistons extend 20thou above the block, just measured them as the head is off
I'd have thought it unlikely that piston contact would make one valve head snap off, they normally just bend.. are there any marks indicating recent contact on any other pistons?

 

just thinking about it if the valve snapped completly as you say,its more that likely it dropped in to the bore,if the valve timing was out, but enough to let it run its unlikely that it was enough to snap a valve head
in general if there were no loud knocking sounds on start up and initial run then something catastrophic happened rather suddenly which is indicative of a valve drop