Identifying my lower unit

Got an 86 Searay cuddy cabin...

Mercruiser 4cyl in board (B407574) TRANSOM ASSEMBLY # OB423625

ID on the LU is faded, how can I determine what it is...

MR / R / ALPHA1 GEN 1 / GEN 2????

I'm very confident it is the original if this helps.

Thanks in advance!

Re: Identifying my lower unit

it's probably an MR. But to be certain you should drop the lower and see if it has a springy pin on the top of the driveshaft or not.
No pin = MR
pin = R
Is there a reason you need to know? parts?

Re: Identifying my lower unit

Your engine number puts you at the end of the run of 3.0 litres used between 1982 and 1986. IMO a very good engine indeed (I have a small Sea Ray in the stable with an earlier example from the same series). Durable, economical, and easy to source parts for right now. Its a General Motors block with 181 Cubic Inches of Displacement. It makes about 140 HP from the factory. In addition to the marine offering, there is an industrial variant of this same engine.

If your drive unit is original to the boat as you suspect, its a 1.98 ratio drive. I can't tell from your transom assy. what drive you have on there though. This can be accomplished with the number off the drive unit. Assuming it IS the original, I suspect you have either a late MR or an early Alpha on there based on your model year and engine serial number. I am sure if I am wrong about this someone on here will shortly correct me.

Re: Identifying my lower unit

Ayuh,... You've got an Alpha 1...

Re: Identifying my lower unit


look here for a stamped serial number

Re: Identifying my lower unit

JustJason said:

it's probably an MR. But to be certain you should drop the lower and see if it has a springy pin on the top of the driveshaft or not.
No pin = MR
pin = R
Is there a reason you need to know? parts?

Click to expand...

Yes, I'm going to replace the water pump and lower unit seals.

Re: Identifying my lower unit

if your doing pump and some seals then drop the lower. If you have the preload pin you need the parts for an R. If you do not have the pin and a flat driveshaft (vertical shaft) you have an MR. An MR is the same thing as an alpha 1. Try not to get lost in the terminology crap of it all. There are some differences between R and MR drives, but the easiest way to tell what you have is preload pin or no pin.