JD4240 head gasket query

Have a JD4240 and during other repairs noted 3 head studs brocken. Two were missing and one was still in the hole. There was some bubbles coming from one of the holes when the motor was running. Head needs to come off to remove the brocken bolts. However in the JD Parts web site it shows 2 different head gaskets. RE47336 thin black and R66065 thick with black and blue stripes.
Can anyone explain why there are 2 different thickness in the gasket. JD can't explain or even say which one to use. I want to get the gasket before I remove it.

I don't know but have seen that happen when they change suppliers during a production year. You'd think the dealer would be able to tell you which gasket if you supply the serial number. Otherwise you might consider buying both gaskets and return the one you don't use. Hopefully the difference would be obvious and once you have it apart you can tell which one you need.
Good luck with the repair and welcome to our internet tractor forums. I'm glad you found us.
Enjoy and post often.

To determine which gasket to use, you need to measure the height of the pistons at top dead center. If any of the pistons are more than .029" above the block surface, use the thick gasket. If they are all less than .029" above block, use the thin gasket. If a thin gasket is used when it shouldn't be, the valves can contact the pistons. If the thick gasket is used when a thin one should be, the compression will be lower than optimum and hard starting can result. I would recommend all new head bolts since you have three already failed.

Thanks for the suggestion. I intend to change all of the head bolts. Strange that JD can't give more details why 2 different gaskets.