Help with Merc 1250 frankenstein

[merc1250bayliner]

I have a merc 1250 with a blown lower unit. I disassembled the lower unit and discovered that it needed a total rebuild but decided not to rebuild it. The case threads were gone, and to get it apart I had to destroy the carrier. The clutch dog and forward gear were slipping. Also it had water intrusion from every seal imaginable.


So I went ahead and found a restored lower unit that was pressure tested, new skeg, the whole nine yards. This brings me to the first problem. The lower unit I bought and would like to keep is from a 78 115. It will not bolt up to my leg. The bolts are slightly off and the driveshaft on the 1250 was 20" but this one is 21" if you do not include the preload pin. My plan all along has been to find another powerhead because mine is in only fair condition and I would like to use this motor for many years. I would like to find a 115 powerhead. I do not think I will lose much power between the 1250 and 115.


So I just ordered a 79 20" leg that I am hoping will bolt up to my newer style lower unit. My first question is, will the 78 new style lower unit with the swept back skeg and more square style cavitation plate bolt up to the 79 leg? My second question is, will that leg bolt up to my transom bracket? My third question, if I buy a 115 powerhead will I be able to use some parts off my 1250? I know I need a distributor. Will the carbs and flywheel work?



I am already invested in the motor and lower unit that I have, so any help with my frankenstein project would be great. Thanks.

[rhardwick]

Anything after 1972 is pretty much interchangeable for the inline engines. Depending on what year 115 you find you will see a big increase in power. The later ones (after 1982) were prop shaft rated, the 1250 is powerhead rated. I personally would use anything from the 1250 on a later 115.

[CrayzKirk]

Very odd that the lower doesn't fit, I haven't seen every year however I don't recall every seeing an inline six lower with a pre-load pin. Maybe it's off of an inline three? As far as fitment goes, things changed drastically in 1972. The 1250 shares the power head mounting of the older 1350, 1100, etc. and the later 115 shares the mounting of the 1400, 1500, etc. They will NOT interchange. As far as internal parts, the rotating assembly is the same except for the pistons. The 1250 has the old deflector style and the 115 has the direct charge style. The 1250 external parts will mount on a distributor 115 however the 1250 distributor has a unique advance mechanism which reduces the advance at full throttle.

Parts for the later models are more readily available and I personally prefer the later model 115 without a distributor. Very similar to the distributor 1500 in performance. Most parts for the early models are NLA (no longer available) and when you find them, can be expensive.

[merc1250bayliner]

Yeah I just found out that the lower unit is not the correct one. Too bad. I ordered another one that looks in good shape but the new one has not been pressure tested and I have no idea if the clutch is still good. I did receive my 115 midsection. So when my 115 power head and lower unit come in I should be good to go. Hope the new lower unit doesn't slip or leak. The carrier threads for the spanner nut were gone when I took my old lower unit apart.

[merc1250bayliner]

So I can still use my 74 115 powerhead if I change the midsection to the early 80's 115 midsection I have correct? The midsection will accomodate the powerhead mounting bolts of the 74 powerhead. But will the midsection bolt up to my transom bracket???