Alpha 1 ss water pump number?
 

Going off memory from my old SS...... It's a Gen 1 Alpha water pump setup.

Correct.

Note: The plastic tube that is re-used is shorter and in the pocket in upper ha;f that has the upper brass tube, has a gasket and plate that is roughly half open vs the regular Alpha having it fully open.

Oh, you'll need a different set off muff's if you start boat out of water. Give me a bit to remember/find where I bought them. I do remember it was from the guy who makes all the Merc racing ones.

It is best to buy the complete kit -- the lower base and all - the kits can be bought 4 or 5 different ways - I would buy the complete kit as that is cheaper then buy say half pieced out as a kit. Let me know if you need help with that - have to watch out which kit you buy cause the base can be different and so is the base O ring.

Word of warning - the Alpha 1 impeller is the VERY short vane impeller -- with that said it performs as a high water pressure - less water flow volume impeller than a GEN 2 Alpha impeller. So the composition of that impeller is made with harder compounds --- Do not buy one that has been sitting on the shelf for a year or 2. The impeller becomes harder / stiffer sitting over time even on the shelf and then inreturn flows even less volume of water and less water pressure.

Years ago my shop was part of testing impellers for a aftermarket marine company. One of the tests was water volume output in which is very important. . We tested Globe - Merc - Jabsco - Johnson - Sherwood - And many China and even ones made in Taiwan and Italy brand impellers. Italy brands did fairly well but the best were OMC OEM and Merc OEM impellers both made in the USA.

This Alpha 1 impeller needs to be changed MAX 2 years but I really recommend ever year for that one especially if the boat did not get used very much within that year..

I remember reading the half plate was designed to lower water pressure to an acceptable level due to the LWP of the SS lower.

It is not part of an Alpha (SS) water pump replacement. I was just mentioning it as being one of the few 'water flow' parts different than the regular Alpha.Again, this piece is in the upper drive where the tube bolts too. I'll be in my garage working on some sleds in a bit, if my Alpha SS is reachable I'll take a pic as it is in half right now for, a drum roll, impeller/impellar housing, and etc replacement. :)

​​​​​​​Yup. That is what the manual states too.
Sorry to throw you off, my way of explaining things can suck thru typing.

It should help. Most I know that installed one gained 6mph +.

Using same prop, mph killing mufflers/same tune, the 20ft boat with approx 350hp-370hp I work on went from 65-72mph., just changing the drive. :) We'll work on more speed when time permits.

Okay, here is the upper half with tube. Under where is the 4 bolts, is a metal piece that is 1/2 closed for the Alpha SS. As Ryan mentioned, this is supposedly to cut down water pressure as the Alpha SS has low water pickups which can create a bunch of water psi at higher speed. The regular alphas have this gasket/plate under this tube totally open to water flow.

Sorry for the bad pics. I'm not so good at taking them, but atleast I took them. Usually I forget. LOL.

https://cimg2.ibsrv.net/gimg/www.off...c850129589.jpg
https://cimg6.ibsrv.net/gimg/www.off...c32d4d8966.jpg

One last thing, with Alpha Gen1's and Alpha SS, when you split the drive into halves, the oring at the top of the vertical shaft will get stuck in the oil seal in the top half. Very easy to get with a pic. The oil seal is in the pic's I provided above. Just slowly stick your finger in their, the oring will rotate enough that you can get with a pic or small pliers. If the oring is still on vertical shaft, I have never seen this. LOL.

Gen II's got rid of this oring and groove for it as the groove for it is where the vertical shaft breaks, if it does.

Here is a good step by step on replacing the Apha Gen 1 waterpump. 'A' on Water Pump Removal step 2 is the oring I was talking about.
MerCruiser Outdrive Water Pump Replacement Instructions - MerCstuff.Com