1973 115 hp Mecury Outboard LU swap with 1971 135 hp Compatability?

What I have 1973 115 hp Mercury Tower of Power serial 3525854

Donner motor is a 1971 135 hp Mercury Tower of Power serial 3115690

Will the lower unit from the 135 hp be compatible with my 1973 115 hp?

I have read that all 6 cylinders from 1972-1988 will swap out with do problems, however I can not find anything about 1971, it looks like a lot of the parts are the same when looking them up but not all of them are compatible. Please help I have lost reverse on my lower unit because I broke the shift rod off the connection in the lower unit when trying to reconnect my LU in a hurry. I do most of my work myself but I decided to bring it into the mechanic and they said it is non repairable

Please let me know if this situation will work or what years and models will be compatible with my powerhead.

Re: 1973 115 hp Mecury Outboard LU swap with 1971 135 hp Compatability?

It'll bolt right up with no issues.

HTH.........ed

Re: 1973 115 hp Mecury Outboard LU swap with 1971 135 hp Compatability?

emckelvy said:

It'll bolt right up with no issues.

HTH.........ed

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Thank you much Ed

Re: 1973 115 hp Mecury Outboard LU swap with 1971 135 hp Compatability?

It will bolt up but you will have to change props as the 135 is a 1.78 ratio and the 115 is a 2.1 ratio

Re: 1973 115 hp Mecury Outboard LU swap with 1971 135 hp Compatability?

Faztbullet said:

It will bolt up but you will have to change props as the 135 is a 1.78 ratio and the 115 is a 2.1 ratio

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Thanks for the responses guys!! However I missed out on that Lu and am looking at a lu off a 1964 85hp 6 cylinder. Will it work?

Re: 1973 115 hp Mecury Outboard LU swap with 1971 135 hp Compatability?

dngbt5 said:

Thanks for the responses guys!! However I missed out on that Lu and am looking at a lu off a 1964 85hp 6 cylinder. Will it work?

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Nope, the '64 L/U has no provisions for an exhaust tube, also the shift shaft bushing sits higher on the gearcase and you wouldn't be able to make the reverse lockout mechanism work.

Any Inline Six lower unit from around '65-up will work. Later L/U's are better. '79-up Inlines came with a redesigned gearcase and swept-back fin and are real nice. I had one on my '71 1350 and it worked great.

L/U's from 4-cyl 66-cubic-inch models 65hp, 75hp, 80hp, 85hp will also interchange. Some 65hp models have a 2.3:1 gear ratio as do all of the larger 4-cyls mentioned. You'd need a prop with about 2" more pitch. If you're skiing or pulling tubes, etc, this gear ratio works well as you can swing a larger diameter blade at a lower prop speed for better hole shot.

About mid-70's Merc went to a stainless driveshaft and these are much better, as you don't get the corrosion around the driveshaft oil seal area, which is a big problem with the older mild-steel-shaft L/U's.

If your L/U doesn't have a groove for an O-ring right below the driveshaft splines, you can use an older L/U with a groove, but you should fill-in the groove with JB Weld, Marine Tex, etc.

The later motors had an O-ring in a stainless carrier assy, pressed into the bottom of the crank. The O-ring groove of an older motor could possibly damage the O-ring in the carrier, as the L/U was installed.

Check your local Craigslist, there are probably a lot of parts motors available.

HTH.........ed

Re: 1973 115 hp Mecury Outboard LU swap with 1971 135 hp Compatability?

Thanks a ton for the info. I have found a 1978 doner motor. It's sound like the lower unit is in great shape and the engine will run and has good compression on all cylinders except one. My question is now whether or not the electrical components will be compatible with my 1973?