Received a PM with a request for info about my experiences with removal and replacing the gearbox in my O'Day 34 so thought that I'd post the same here.
As Hurth is no longer manufactured the suggested replacements is the ZF version. In my research I discovered that the ZF is not a direct replacement because the vertical drop between the input and output is different which means that adjusting the engine height and radical prop shaft realignment would be necessary which is something that I didn't want to get involved with... turns out that I had to move the engine anyway to get the gearbox out. I found a used Hurth 10 2R from a fellow forum poster so agreed on a price and delivery. A bit chancy I know, however the person was a fellow poster so thought it was worth the risk... so far so good. I used about a gallon of transmission fluid to fill and drain the gearbox to flush out any impurities, don't recall that there was much there.
So here's the story:
The Hurth is bolted from the inside of the bell housing so it had to be removed. I pulled Karma thinking that I could remove the gearbox without moving the engine by loosening the prop coupling and moving the prop shaft back far enough. Turns out that the gearbox bottomed out on the engine pan after moving less than 1/4". . If I do it again I'll not bother to put Karma on the hard and will just move the engine forward while still in the slip. Moving the engine was no big deal. Scribe the location of the mounts; remove the engine mount bolts; tie a line to the front engine bolt; run the line from the engine around the mast and out the companionway; rig a bridle between the headsail winches and crank away until you hear the engine move.
Many suggest to replace the damping plate while you have the gearbox removed. The cost is about $250 so I simply inspected it for broken springs and satisfied myself that replacement was not necessary. Karma's damping plate was all metal, whereas it seems that some use hard rubber instead of springs, perhaps that's why replacement is suggested?
Realigning the prop shaft was not too much of an issue as well. I'm glad that I learned how to do it. Simply use a feeler gauge to insure that the shaft coupling faces come together at the same time. There are some good instructions on the internet about the process...as well as some very complicated methods as well. I think that the challenge for shaft alignment is related more to those instances where the shaft goes through several cutlass bearings. The O'Day 34's shaft isn't long enough to be too much of an issue to realign. Check the gap using a gauge to insure that the mounting faces of the coupler are no more than 0.004" out of alignment. It helps to have a buddy adjust the mounts while you measure as you will have to go through a few iterations to get it right. Since I pulled Karma and aligned it on the hard, then again when she was floated, I checked and made a final adjustment after a few weeks back in the water. There was a bit of change, but not significant.
I took a lot of pictures and have posted them as an album on Google:
https://goo.gl/photos/Fe7tQ8VLxVjNiEGb8
