I would bet that either the mounts or transom are flexing under the torque. You need offshore engine mounts if you don't have them. Transom could be wet and flexing as well.

[QUOTE= I borrowed a good coupler, lined up the motor and ran it for another 6-8yrs without a problem.


[/QUOTE]

Dick, remind me not to loan you anything I might want back.

Not to overlook the obvious but do the drive rams have the correct stops in them and trim limit switch adjusted to prevent u-joints from binding? This was part of a swap from Bravo to Bravo XR.

+1 If the transom isn't thick enough (2"+ something) the driveshaft would bottom out in the crankshaft and put a huge stress on the u-joint.

everytime I fixed it I put a new coupler in just to be on the safe side because I didnt know what was causing it to break. I just checked the stringers and the transom both are solid and didnt see any flex in either of them. Both motor mounts are solid and no movement. Everytime it broke I was just cruising along about 60 or so but not under any torque or anything. What do you think the possibility of maybe the flywheel having a defect with either the coupler mounting holes or just the casting of the flywheel itself ????? I am running the Dart Big M block now compared to the GM block I was running in the old set up. Does that block have different specs or need to use different parts ?????

If you can put the alignment tool in and out with 2 fingers, alignment is ok. When you say you replaced the whole motor, is the flywheel housing new. The new rear mounts don't require the lock washer type spacer on top of the mounts like the old ones did. If they put the new engine in without knowing this you would have an alignment problem. But alignment problems usually are compensated for by the rubber coupler, which should fail before the u joints. Also are you replacing the entire u joint shaft and yoke complete or just replacing the joints. I would replace the entire assembly, not cheap but these are not like the old joints that you could smack in with a hammer.

Each time it has been new from mercury shaft, u joints, ect.

http://www.offshoreonly.com/forums/g...p-coupler.html what about something like this causing your problem???

you replaced several gimbal's,couplers and ujoints..the aleignment is spot on,, my next strp would be to start looking into the drive.their may either be a bearing starting to go bad then locks up after a few hours of use or mabe a broken tooth off of one of the gears getting jammed wher its not soppose to be and locking up then shering the ujoint..i had a volvo do this years ago and it turned out to be the drive..

If the alignment is right, then the hull as it is is junk, stringers and or transom flexing under power killing the parts....just like anything else when that boat was built they never intended it to handle that amount of power...not much extra built in...no different than sticking a big H/P engine in a street car and wonder why the rear expired...you may run without hatch, and have someone watch the engine for movement under power..transome flex going to be tuff to see. good luck, Rob