Re: 1968 Merc 350 short shaft
Older motors had smaller wristpins, Merc did upgrade to larger ones in the later-style 90ci Sixes and 66ci Fours, apparently same thing was done with your 35hp.
It's very possible you could find rods that would work out of an older Four or Six, there's certainly a lot more of them than 35hp Twins.
There were a lot of changes to the model line in 1966, including a change in the "350" Twin's bore from 2.875" to 3". Same year Merc introduced the 1100 Six with a 2.937" bore. In 1967 the "650" changed to the same 2.937" bore and stayed with that until the end of the production run in '71.
741-2696 A2 pistons were used in '67 thru '69 models and used 617-2768 A3 rods. I bet these are the big-wristpin ones.
'65 used a 732-2054 A4 piston which superceded to 732-2735A2 along with a 611-1885A2 rod.
'66 used the same piston with rod #'s superceded to 611-1885 A3 or A4 depending on the Ser #.
It would appear you got 732-2735 small-pin pistons instead of the ones you need.
I suppose you could use them if you could find earlier rods but there was a reason why Merc changed the wristpin size and I expect it's due to durability. Of course this is a classic motor and if you're not gonna be thrashing it severely, the wristpin size is probably not as significant as having new pistons and ball bearings with all the balls intact!
Or, maybe you can go back to the vendor and ask them to send the correct pistons!
If you end up using these pistons, I'd also check the piston rings to make sure they fit (if they were ordered separately from the pistons).
HTH & G'luck...........ed
My turn to ask a question, picked up this motor for a quick rebuild, needs 2 pistons and upper and lower crank bearings. The upper bearing lost 3 balls into the top cyl and beat the piston up etc. Ordered new pistons which arrived and had the correct part number according to the merc parts lists. The piston pins however that came with them are smaller than the original, .650 vs .765. Needle bearings are the standard .090. Is there a connecting rod available with a smaller small end??