See full version: How to tell which mercruiser outdrive you have
The water pump base, gasket and o-ring is not the same. One way to tell the year of the lower units 1970 to 1983 is by mercruiser vertical drive shaft end has 1/4 pin about 5/8 long and Alpha drive shaft has no pin on the end. [links]
Example: let say you have this upper gearcase showing and your boat is years 1970 to 1983 our GLM Marine water pump kit is number 12110 how ever if the lower unit was replaced with the newer Alpha One that came out in 1984 up the GLM Marine water pump kit is number 12120. here
It is the year 2020 now so your Mercruiser outdrive lower unit may have been change after all its 40 plus year old. If your still know sure just send us a picture to omcparts@aol.com or call the order desk for the tech service number. [links]
That said the replacement water pumps are not the same for both sterndrive units. [links]
Please note : that the Mercruiser, MC-1 lower unit and the Alpha One lower unit can be used with this upper gearcase. [links]
Mercruiser Alpha 1 parts upper gear-case drawing outdrive parts main index page for upper and lower gear-case technical breakdown. here
R atio determination is VERY important.
The size and horsepower of the engine, to a large extent, determines what ratio the drive should be.
The drive ratio is determined in the upper unit by the number of teeth on each gear.
In general, the smaller the engine, the higher the ratio.
S ometimes the only sure way to tell what ratio a drive is, is to access the upper gears and carefully count the teeth.
This SHOULD be done by a certified mechanic.
Small block V-8's usually have ratios of 1.50:1 and 1.47:1
V-6 engines and older in-line 6 cyl engines use 1.65:1 ratios.
In-line 4 cylinder engines will use a 1.98:1 or a 1.84:1 ratio drive.
Before the Bravo drive was introduced, Mercury mated the big block engines to an Alpha with a heavy-duty 1.50:1 drive (labeled HD 1.50:1) which really didn't hold up too well.
They changed the ratio to a 1.36:1 which held up better. more
Bravo Drives
T he Bravo drive saved the day for the big block puppies.
It replaced the need of using a transmission as the TRS drives needed and took the place of the weaker Alpha Drives that couldn't handle the torque.
T he Bravo uses a shifting cone mechanism much like the proven Volvo shifting system.
Big beefy gears and thick shafts make the Bravo tuff.
I t can be identified by the removable rear plate on the back of the upper driveshaft housing. The plate is held to the drive with three twelve-point bolts.
B ravo's come in three basic types and a few different ratios. Bravo One is the most common. Bravo Two has a larger lower housing and prop-shaft so it can throw a larger diameter prop and the Bravo Three is a dual prop configuration which is great for medium size cruisers for better handling.
here
TRS / TR Drives
L ots of TRS drives are still operating out there!
They are very large and very tough. The main draw-back of the TRS drive was that it needed a transmission as no shifting occured in the drive itself.
N ewer and faster boats needed less weight and more room so the TRS has been discontinued by Mercury Marine and it's market replacement is the Bravo series.
T he TRS initially was mated to Mercury Marines TRS transmission. This was Mercury's own design which used some Ford inner parts. The tranny had an internal oil cooler and an aluminum housing.
Mercury then changed to the Borg Warner Transmission which required that the TRS Drive input shaft be a splined male shaft instead of the older, hollow female splined shaft.
When ordering TRS stuff, make sure you specify what tranny set-up you have.
T he TR drive used the same upper driveshaft housing as the TRS but it's lower housing and propshaft were much larger for larger props. [links]
Thanks for that. I've ID'd it as a Bravo2 with 2:1 ratio. I've asked your sales dept for a quote.[links]
Hi, I'm looking at replacing a Mercruiser Alpha 1 gen 2, but I don't know what gear ratio it is. It's fitted to a Yanmar 220HP Diesel. How do I figure out what gear ratio I need?
115 S. Forest Lakes Blvd.
Oldsmar, FL 34677
Thanks for the reply. Hmm, you could be right. I've just seen it today for the first time as the boat has just come out of the water for the first time since we've owned it. The leg has now been removed, and it's starting to look very like a Bravo 2 to me, though I'm no expert. Here are a couple of pics (complete with barnacles) to help ID it. So the question then is what it is, what do I need to replace it with, and how do I figure out the gear ratio? Sorry for so many questions! And thanks a bunch for any help.
I don't know of any applications that used a 220 hp diesel with an Alpha drive. Are you sure you do not have a Bravo drive?
Re: Which Mercruiser outdrive do i have?? [links]
If it's original to the boat it's a gen1. Gen2 didn't appear until 1991 or 1992. More info here:
you will need the proper oem manual and special tools in order to shim the gears properly, not to mention the housing may be damaged internally making rebuild impossible
Re: Which Mercruiser outdrive do i have??
You may want to gather some info on replacing the upper gear set, as I understand with out proper tools and experience the chances of success are slim to none. I have read many recommendations that say if the upper gears are bad to replace the upper drive unit and not try to replace the gear set yourself. Just passing along what I have read..