outdrive (lower unit) compatibility question

I apologise if this has been asked and answered. I did a search but I must be using the wrong terms.

I am interested in purchasing an older boat. It's a '72 20' Thompson with a 165 mercruiser. The current owner tells me that the lower unit has an issue but wasn't specific about what the issue was and I didn't ask because I am still in the spousal negotiation stage of the purchase.

I am new to I/O's as I have only owned outboards. If I am understanding correctly the upper unit of the outdrive is where the gear ratio comes into play. If I am correct in that understanding are the lower units compatible provided they are from the correct year? On a website I was looking at for stern drives the drive from this boat is compatible with other 165's from 1970 through I think 1978. Would the lower unit be compatible from other sterndrives of different horsepower and greater number of cylinder engines from the same year range?

Sorry if this question is just stupid. My consideration was to get a lower unit and just do a swap so I could get it out to water test it and then to rebuild the drive and swap it out over the winter so I have a rebuiilt drive readu to go for next season before I get out on Lake Michigan for some salmon fishing.

Re: outdrive (lower unit) compatibility question

You are not quite right in your compatibility assumptions. All lower units from about 1967 to the last of the Alpha One (Gen I) are interchangeable, as complete units. Internal parts are a different story. The unit on you 165 will be an 'MC-1'. They ran to 1983, then there was an 'R', an 'MR', and finally the Alpha One. You will heard/read the term 'Pre-Alpha'. THERE IS NO SUCH THING AS A 'PRE-ALPHA'. If it's not an Alpha One, it's either MC-1, R or MR. After 1991 Merc completely re-designed the leg and we currently have the Alpha One (Gen II). No parts or casings are compatibly with the MC-1 to Alpha One (GenI). The different ratios for the different engines is determined by the tooth count in the upper housing, known as the Drive-shaft housing. The lower housing is called the Gear-housing.

You have a few options available. You can try and find a lower second hand. Dubious and up could end up with more problems than you started with. Either of the other options are good. 1. buy a complete gearhousing from SEI (available to purchase on iBoats) or buy a complete stern drive unit from SEI (also available here). Either option gets you and NEW unit with a 3 year warranty, and the best part, they are so cheap I don't see why anyone would be rebuilding boxes anymore....

Chris........