View Full Version : Trim Pumps

12-08-2011, 12:18 PM
Okay, so I know I probably should know better but I don't. My understanding is there are two Merc alpha setups, at least as far as trim pumps go.
1. Low volume high pressure (prestolite?), and
2. low pressure high volume (oildyne?).

I have been told I have the wrong trim pumps for my drives. Anyone know how I can ID my drives and then sort out the proper trim pumps?

All advice welcome, and thanks in advance.


12-08-2011, 12:28 PM
Get a Clymer manual for the years of production.
Not only will it identify the pumps, but tell you how to rebuild or service them.

And actually, there are three (according to my manual);

12-08-2011, 12:36 PM
I think the Prestolite is the oldest and has a metal (cast) rectangular sump.
The Oildyne/Eaton has a cylindrical metal sump.
And the Oildyne Singer has the white translucent plastic sump we see so many of.

12-08-2011, 01:51 PM
My setup is complete Frankenstein. don't know the drive years, engine years, anything. And I have reason to think all the major bits were acquired at different times and dropped in. (New, the boat had 250 inlines with drives, circa 1972. Later got 292s, in the 80s I think. Then got alphas, maybe in the 90s. AND the skegs are different. So, I don't even think the drives are from the same year.)

So, I assume I need to start by finding some ID info for the drives, and go from there. Not even sure what I'm looking for...

On the decals of the transom assemblies, they have serials stamped.
A694141 for sure on the port drive, and maybe 08713969 on the starboard.

EDIT: I found these tables online, but so far I don't see anything that clearly fits the numbers above. Unless they are Alpha One SS drives, and the hard-to-read number is other than what I guessed. But I thought Alpha SS drives had those pointy noses, and mine look like plain-Jane Alphas to me.

EDIT 2: The only numbers I have found are on the top section of the transom assemblies, stamped into the decals. Is there some other place to look? Like, do you have to remove the props or something?

12-08-2011, 03:55 PM
Mike, the drive serial # is stamped into the upper gear case (or drive shaft housing) on the port side. If you don't see it, sand on it a little. I'm sure I can find some pictures if necessary.
Now, back to the pumps, I think the Prestolite pumps were used on the "R" and pre-Alpha drives, and the Alpha I's used the Oildyne/Eaton pumps. I think the Oildyne/Singer is the same but w/the larger plastic sump.
You can't go wrong w/either of the Oildyne pumps.

12-08-2011, 04:08 PM
Question do the drives trim under power? If so why do you think you hve the wrong pumps? If the drives are struggling it may not be the pumps, I have seen many that were stiff due to the pivots binding from lack of maintenance.

12-08-2011, 04:14 PM
Thanks George--I will give it another look when the sun is up tomorrow. (I think the paint must be really thick or something--I didn't see squat and I looked where I think you mean.)

12-08-2011, 04:15 PM
The drives move. Mechanic said they were the wrong pumps. And I think they leak a good bit.

I suspect he's right because I bet the boat has the same pumps it had in '72, even though it got much newer drives later on.

12-08-2011, 04:28 PM
I know there are different cylinders but the pumps though different in series and style all do the same job. I am running my original 1986 Alpha 1 pump to trim my new Bravo Diesel X drive, it works perfectly. Your pumps probably need kits, they are very simple to rebuild even if you have little knowledge. A manual and digital camera can make you an expert!!!!!


12-08-2011, 04:48 PM
My understanding of the issue is that my trim systems are supposed to operate at significantly lower pressure. And that my trim pumps are making the whole thing high-pressure.

Would that, in your experience, be likely to cause leaks, poor performance, or even failure?

12-08-2011, 06:54 PM
The cylinders can get stiff and do the same thing, pressure wise.
I bought a set of used cylinders and 135 PSI of air wouldn't move them.
They can be rebuilt, but if they are scored internally, move on.
New cylinders at great prices (probably cheaper than rebuilding) are available from SEI.

12-08-2011, 07:07 PM
Here's a picture of the Minx's cleaned outdrive......
See the number?

12-08-2011, 08:10 PM
Oh wow, thanks! I can't wait to check in the sunlight. I think that's hopelessly not visible right now, between paint and decal.

12-08-2011, 10:02 PM
I don't think that the ultimate high pressure that a trim pump can actually develop really means anything here.

As long as you can trim your outdrives while your boat is underway in a reasonable amount of time & the drives stay in that final fixed set trim position while underway at speed I think you are OK~

Think of about a ten ton style hydraulic floor bottle jack.
It may be capable of moving ten tons at it's ultimate high pressure ~~~but when you are only actually lifting one ton what does it really matter ?

Speed of trim operation is really the key here; not ultimate trim pressure.
As long as the pumps do not leak internally or externally & your final trim settings do not drop down, drift ,or change.

Phil (Mop) suggests you re-build these existing pumps with new rebuilding kits ; which I think would be a good idea after all these many years.

I would also rebuild the trim cylinders with new seal kits & hone the trim cylinders also.
Or replace them.
New trim hoses might be in order now also.

12-08-2011, 10:59 PM
It's not just high pressure vs low. It's high pressure/low volume vs low pressure/high volume. They seem slow to me. Slow enough you feel very lost trying to trim underway.

And that might make sense if the trim cylinders are larger or something. I'm thinking, like trying to blow up an air mattress with a bicycle pump. You may only need about 10 psi, not the 100 that the pump will do. But the volume/time is so slow it'll take a year.

Again, I don't *know* the answer, just looking for what people do know.

12-08-2011, 11:20 PM
If your trim reaction time is indeed slow this would be a case of pump output volume VS the cross-sectional size of the trim cylinders.
Or the pumps & their internal valving could be internally leaking.

Trim cylnder operating speed is a factor of pump volume vs cylinder diameter.

You must however match the pump's output volume to the ID diameter piston size of the trim cylinders.
Pressure here is really meaningless.
Pump volume is the key.

Do you know the cross-sectional size of the early Merc cylinders vs. the cylinders you currently have on your drives ?

Are your new trim cylinders much larger than the originals that came on the boat with these same existing pumps ?
This will tell you IF the pumps are the correct volume output or if they just possibly need rebuilding.

Larger diameter cylinders need a higher volume output pump to move the larger diameter cylinders at the very same speed as earlier smaller cylinders.
The key here is finding out Merc Alpha trim cylinder diameter size info Newer Alphas vs. Old Alpha cylinder styles.

How many trim cylinder diameter sizes were used on the Alpha drives over the years ?
I do not know the answer to that question.
Perhapps someone here can tell us?

12-09-2011, 09:03 AM
SG, Good questions, but I don't know what the originals were to compare.

12-09-2011, 02:09 PM
Cylinders aren't all that interchangeable w/o some modifications.

If you take my example of the used cylinders I referenced above, and it took 200 PSI to move them. Also assuming the maxium pressure developed by the pump is 700#-800#, you would have only 500#-600# available to work with after overcoming the cylinder resistance. No matter how well the rest of the system is working, you'll always be at a disadvantage and the system would be required to develop more pressure than would otherwise be needed.
W/both lines disconnected, you should be able to move the cylinders rods in and out fairly easily.

Just a heads up to check everything in the system.

12-09-2011, 04:06 PM
The old prestolite had the reverse lock separate, mounted up on the engine. If you switch to new pumps they will have the reverse lock built in.

12-09-2011, 04:10 PM
Gradually getting closer. George, thanks in particular for the direction on the drive serials. I could feel them under the decals. A little peeling and a little sanding gave me the numbers.

Port 0B871992, Stbd A516468 B

Now that I have those, I still don't see a definitive answer on what drives those are. But I *think* they are essentially just Alpha 1 Gen 1, mid-late 80s most likely. Though the skegs are different, so I still wonder. And the trim cylinders are longer on the starboard motor. Measured from end to end without the cap, the port cylinders seem a little under 15 inches, and the starboard a little over 15. The shorter ones still have me confused, and don't quite line up with any specs I've found yet.

My best reading of web pages so far suggests I DO have high pressure low volume pumps, and that my drives and trim cylinders call for low pressure high volume pumps instead. And from the discussion so far, it sounds like that might account for what seem to be very slow-trimming drives. Maybe increasing the likelihood of leaks or maybe not.

That's where I am so far. I very much appreciate everyone's thoughts and insights.