so these last 2 weeks i have been working on two identical rib boats for a whale watching company in Iceland, they each have two Yanmar 6LPA-STZP2 engines and ZT370 stern drives and the problems with these drives seem to be endless, 5 drives are already broken and just sit in the company garage, one of them bought new from the dealer lasted 3 days until it broke so it was sent to the dealer and they sent it back saying it was "unfixable" whatever that means, as far as i know they have always been lubricated properly with the recommended oil as specified in the manual, so im hoping to get some answers here, what can i tell the captains to do in order to lengthen the lives of these drives or are these drives just really bad and should i pitch them the idea to get mercruisers drives, and if so how much work would there be involved in swapping to those drives. And one other thing, i installed a brand new drive on one of the boats two days ago and it went smooth, really nothing that can go too wrong, checked the alignment of the engine beforehand and the drive went into the splines easily with all seals seated properly and all bolts torqued as specified, but when it shifts in i hear a loud knocking noise, only when shifting in, forward or reverse, otherwise its fine, Any ideas?