[Mercerman]

I have a '69 Merc 1000. Does anyone know what years of 6 cylinder lower units will fit this?? I thought I once heard '66 thru '85 Inline 6's or is it any 6 cyl. after 1966?? Also what year did they start putting stainless steel shafts in?

[emckelvy]

Anything in Six-Cyl flavor '66 and up would fit. Later models don't have an O-ring groove (which yours does) for the crankshaft splines but it's easy enough to cut a groove with a grinding wheel.

Stainless shafts started showing up somewhere around '75-76 and the later-style gearcase was introduced in '79. Stainless-shaft units are definitely worth looking for, considering the problems the older mild-steel driveshafts had with corrosion in the oil seal area.

I used to have a '71 1350 with PT&T and later-style gearcase and it was a Sweet motor. Wish I had it back!

HTH.............ed

p.s. if you can find a nice stainless-shaft lower from a 4-cyl 850, 80hp, or 75hp, your 100 will like the 2.3:1 gears.

[Mercerman]

Does it need that O-ring? What is it there for and why did they stop using it?

[emckelvy]

It seals against the inner diameter of the crankshaft when the driveshaft is inserted, in order to keep water out of the splines. Especially if you're running in salt water, if you don't have this O-ring the crank and driveshaft will rust together and you'll need small incendiary devices to pry them apart!

Later models did away with the O-ring groove and instead have a stainless-steel carrier assy pressed into the end of the crankshaft. This assy holds an O-ring which seals against the smooth body of the driveshaft to accomplish the same thing as an O-ring in driveshaft groove.

I imagine the O-ring in the carrier stays in there better and is less likely to deteriorate. Probably does a better job of sealing in the long run.

At any rate, you could always pull your powerhead and press the later-style O-ring carrier assy in place. Or just grind an O-ring groove in the later-style driveshaft, a lot less effort unless you already have the powerhead off.

When I rebuild my old 1350 and installed a later-style lower unit at the same time, I pressed the assy into the crankshaft and it worked great.

HTH.................ed