They've been having head gasket issues on the engines from Saran for decades and nothing much seems to have been done. Even on here someone had 5 out of his 7 6400's suffer head gasket failure.

https://www.google.co.uk/search?q=john+deere+head+gasket+problems+site:thefarmingforum.co.uk&sa=X&ved=2ahUKEwi3x5z3rZXfAhWBzKQKHWF8B8wQrQIoBDAAegQIBhAM&biw=1140&bih=654

Claas were using DPS engines in their tractors and it seems they had the same issues so developed their own head gasket. There was a contributor who's a Deere rep and will be on one of the threads I linked to that told everyone head gaskets are a service item, they might be on a Deere engine, but most others they seem fi an forget.

I appreciate that lots of engines have had issues over the years, meeting emissions regs hasn't helped and Deere sell a lot of tractors, but they have more than their share of issues.

What seems to happen in the UK is a customer gets a problem with a tractor engine, Deere say it'll cost X thousand pounds to replace, but we'll do it for X minus a bit and the customer thinks they've won the lottery and goes on at length to marvel at what great service they'd recieved. I would imagine that just getting to the HG is a big job now before you've even changed it.