I ran an inline 6-cylinder Mercury 140 with a 5" extension kit from Bay for several years on a Checkmate 17. I loved it. Follow the directions and you'll have no problems.
Only caution I have is to exercise a bit more care than usual when re-installing the lower unit to ensure the proper alignment of the extended water tube. I never had a problem, but could see how it could be misaligned if you weren't careful.
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury
|