Using 4450 one day- ran great, no trouble. Went to start next day and it would not turn over. Took starter off and engine rolled backward fine, but when rolled forward it would come to a stop at a certain point. We took off the front hyd pump connection to isolate that- no difference. Pulled injectors to check for a hydro lock- injectors looked good and no difference. Cut apart oil filter and found nothing. Ran an oil analysis and it showed nothing bad in oil. We are going to pull the head thinking that the problem is in the top end... My question is- Are we on the right track and would there be anything in the transmission or pto that would stop the starter and prevent the tractor from turning over. It is a loader tractor with 14000 hours. It has had engine work at 10000 hours. Powershift and MFWD. Thank You for the replies.