There is a general consensus that because of all the changes ,many of the years models and gear cases do not interchange.
A lot of the changes occurred in mid eighties,some crossflows still being made until later dates such as 87.The 120 and 140 being of mind there,however the 120 used the v4 gearcase and the 140 the bigfoot or v6 gearcase in 85 and in 86 sometime went to the smaller v4 gearcase.
According to bom any v6 gearcase providing it has the same bolt pattern will interchange with other years of mfg as long as not only the shifter rod is reused from old gearcase but main as well.This includes even the new lightning gearcase.
Failures have occurred but due to only the shift rod being reused,what happens is it will shift but only one direction meaning forward no back,or back no forward,reusing the old main normally cures that.
I know this as did switch mine out ,did have that problem and bom wrote me back how to correct it.
I didnt buy it new yet bom took the time to inform me of what was needed on my 94 model,to correct the problem.
Long since gone ,Im still awaiting a reply from merc concerning my 79 model inline six.
A lot of the changes occurred in mid eighties,some crossflows still being made until later dates such as 87.The 120 and 140 being of mind there,however the 120 used the v4 gearcase and the 140 the bigfoot or v6 gearcase in 85 and in 86 sometime went to the smaller v4 gearcase.
According to bom any v6 gearcase providing it has the same bolt pattern will interchange with other years of mfg as long as not only the shifter rod is reused from old gearcase but main as well.This includes even the new lightning gearcase.
Failures have occurred but due to only the shift rod being reused,what happens is it will shift but only one direction meaning forward no back,or back no forward,reusing the old main normally cures that.
I know this as did switch mine out ,did have that problem and bom wrote me back how to correct it.
I didnt buy it new yet bom took the time to inform me of what was needed on my 94 model,to correct the problem.
Long since gone ,Im still awaiting a reply from merc concerning my 79 model inline six.