canadianfcat Posted October 5, 2012 Report Share Posted October 5, 2012 So I'm doing a swap on my boat of my 40 hp merc to a 75 hp merc. Does anyone have advise on sealing the holes,setting the height..ect? Link to comment Share on other sites More sharing options...
DaveC Posted October 5, 2012 Report Share Posted October 5, 2012 I would coat some carriage bolts in silicone and install them into the old holes. That should be ok...as far as setting hieght Im lost on that one. I thought you either had a short shaft or long shaft motor? No clue on that part... Link to comment Share on other sites More sharing options...
canadianfcat Posted October 6, 2012 Author Report Share Posted October 6, 2012 My current 40 isn't sitting right on top of the transom. There is some sort of calculation with set back and plate height above the keel bla bla bla But all makes a big difference in performance..It's got me confused! Link to comment Share on other sites More sharing options...
jesse Posted October 6, 2012 Report Share Posted October 6, 2012 You might be lucky with swapping 2 mercs,the bolt holes should line up.You have to put a good sealent on the bolts when reassembling,the motor bracket has a ledge/lip that sits right on top of the transom.If you have any issues let us know,if it is on a trailer & you want to bring it to forterie i can give you a hand sometime! Link to comment Share on other sites More sharing options...
canadianfcat Posted October 6, 2012 Author Report Share Posted October 6, 2012 Thanks justasnag! It's on a trailer but the new motor isn't. I borrowed a engine picker to save my back and help line things up. See my current one isn't sitting right on the transom.It's bolted about an inch up. The plate needs to run right on the top of the water I believe. Link to comment Share on other sites More sharing options...
jesse Posted October 6, 2012 Report Share Posted October 6, 2012 If the new merc will bolt in the same place & they are both longshafts maybe you will be good?Did you use the boat with the other motor on it or did you just get it? Link to comment Share on other sites More sharing options...
canadianfcat Posted October 6, 2012 Author Report Share Posted October 6, 2012 Ya I used it all summer but was just underpowered...The old 40 is a 1991 and the new 75 is a 1992 so both are same era engines. I've got to put the battery back in the boat and drop the engine to measure the lengths. I know the new one is a bit longer but maybe only from below the plate to the skag. Link to comment Share on other sites More sharing options...
DaveC Posted October 6, 2012 Report Share Posted October 6, 2012 Thanks justasnag! It's on a trailer but the new motor isn't. I borrowed a engine picker to save my back and help line things up. See my current one isn't sitting right on the transom.It's bolted about an inch up. The plate needs to run right on the top of the water I believe. Thats bazar that your current motor is sitting an inch up? Do you mean its sitting an inch higher than the transom? As Snags said, it should rest on the top of the transom I would think. Is the engine you have a long shaft ? Is the one you are replacing a long shaft? Do you currently have a long shaft sitting on a transom that is sized for a short Shaft? Thats wierd its sitting up like that...an inch. Link to comment Share on other sites More sharing options...
jesse Posted October 6, 2012 Report Share Posted October 6, 2012 Ya,thats a strange 1.My guess now is that it is a shortshaft transom with a longshaft motor bolted up on the side of the transom which does not seem safe without that top lip holding as well? Link to comment Share on other sites More sharing options...
canadianfcat Posted October 6, 2012 Author Report Share Posted October 6, 2012 This video will show what I'm talking about. I measured today and the motors are the same shaft length above the cavitation plate so should bolt in the same place. Link to comment Share on other sites More sharing options...
fishkid Posted October 9, 2012 Report Share Posted October 9, 2012 What kind of boat is it? Hull shape will determine how high to mount it. The way it's measured is from the bottom of the keel (or pad) to the centreline of the prop, but a lot of people go by the measurement from keel to the anti-cavitation plate. I would mount it with the anti-cav plate just above the keel, whatever hole that ends up being. The engine mount sitting on the keel, an inch up, 4 inches up, makes no difference what so ever. Link to comment Share on other sites More sharing options...
canadianfcat Posted October 9, 2012 Author Report Share Posted October 9, 2012 It's a 1991 Spectrum blue fin 1606. After watching the video and reading some on line I figued your suggestion is the line of thinking. It just surprised me how many just strap it on and go without really doing much research.My buddy thats my goto people on all things engine related could figure out why my engine was mounted this way. Link to comment Share on other sites More sharing options...
fishkid Posted October 10, 2012 Report Share Posted October 10, 2012 Yeah a lot of people seem to think that you get what you get and there's no changing it. But what you have to remember is that every transom and hull design is different so one engine won't fit the same on everything. Setting the hieght will help improve top speed and the control if it's done right. Too low and it's dragging more water and will slow you down. Too high and you can draw in air and over rev, and you can get prop torque from the prop blades surfacing. Since your installing it on a hull that already has holes, just mount it so the anti-cav plate is riding just above the keel and call it a day. For not doing any on the water testing, that's the best that you'll get. Link to comment Share on other sites More sharing options...
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