I have a 1993 Nissan NS8B which has never been serviced but ran fine this year after being idle for numerous years in a garage. I decided to replace the water pump impeller and got the kit. I removed the shift shaft pin and unscrewed three of the four lower unit bolts to drop the LU, but the 4th one broke off at the head. I can move the unit slightly back and forth but the unit wont drop. The bolts don't seem to engage any threads in the LU but only up into the housing above it, so it's logical to think it should slide out. I've used penetrating oil and banged on the unit with a mallet but it won't drop. Any thoughts?