Logo
FULL SITE MENU

250 to 280

J

jeffnick

Regular Contributor
I currently run an Aquamatic 250. What would I need to change to put a 280 on instead. I hear the 250 is a little weak for my 260A motor?
 
Jeff, here's the problem with the 250, if there is a problem..... and generally there is not!

1..... the male yoke shaft diameter (the yoke that splines into the main drive gear) is much smaller than it's predacessors.
I've not seen an AQ260 snap one off, but it is much smaller.
All else seems to be well within durability limits...... i.e., the sliding sleeve, cups, driven gear angular contact bearings, main drive gear bearings, etc..... including the main gear case.


2...... the AQ series drives underwent three driven gear angular contact bearing dimensions over the years.

200/250 uses the smaller of them all.
This makes the transmission and the Intermediate housing proprietary for the 200/250.
IOW, the 270 and later transmission will NOT mate to the 250 Intermediate housing and/or visa-versa.
You will need at least a second generation 270 Intermediate housing if the 280 transmission is to be used.

The 270, 280, 285, 275, 290 275A and 290A, etc, all use the mid diameter driven gear bearing diameter.
These must remain within this model designation in order to interchange with one another.

The later C drive (and all that follow), use the larger diameter driven gear angular contact bearing..... actually, it's integral collar that makes the difference here.
These too must remain within this model designation in order to interchange them with one another.


No matter which way you go, the shim value must be re-calculated and an adjustment must be made when swapping any of these parts from one Intermediate to another...... lower units included.

Jeff, there will also be an issue with the two different PDS (primary drive shaft) spline count that may affect the female yoke that is required.
Your original 250 PDS will be course spline.
The course splined PDS and female yoke remained for quite some time, up until the mid 80's when it became a fine spline.
Of the universal drive shafts that use the internally clipped bearing crosses (aka U-joints), these female yoke can be interchanged.

There is a course spline female yoke for the externally clipped bearing crosses, but will be difficult to find.
I've seen only a few of them over the years..... but it was a V/P p/n at one time.


When swapping a lower unit, be sure to bring the vertical shaft "Tapered Roller Bearing Race" over with it.
We keep these paired throughout their use! (often this is over-looked and the race stays with the Intermediate housing)

There will be a shimming procedure shown in the OEM service manual..... and this MUST be done for both the transmissions and the lower units!

Post back with your proposed idea..... be glad to help you.

.
 
Last edited:
Wow! Thanks for all the info. Sounds complicated.
How about if I just get a whole new/used leg - would it bolt to my transom shield?
 
Correction to my previous post:
RicardoMarine said:
the male yoke shaft diameter (the yoke that splines into the main drive gear) is much smaller than it's successors.
Click to expand...

jeffnick said:
1..... Sounds complicated.
2.... How about if I just get a whole new/used leg - would it bolt to my transom shield?
Click to expand...
1... not at all, and it would be good if you learned how this is done.
IOW, you could check the previous installation shimming..... not everyone gets this right, including some marine mechanics! :mad:
Besides, you'll want to pull the replacement drive apart for several reasons:
  • to renew all of the O-rings.
  • to leave the transmission OFF when installing the Intermediate and lower. Much easier this way, and it opens up the area for the shift cable and water neck S hose attachment. Drive shaft Bellows are also much easier to re-connect. IOW, install the transmisson last!
2... yes, that is an option. The 250 transom shield will allow the 270, 275, 280, 285 to bolt onto it.
When we get to the 290, the suspension fork changes..... but it too can be interchanged.... but is rather involved.

Be very careful when removing the suspension fork hinge pins.
The ears on the transom can be heated, ever so slightly expanding them so that the pins can be removed without damage.
These pins are soft, and should not be driven on with anything harder than an Italian bread stick. :)

You will also want to replace the PDS bearing(s).

When you have time, take a look at this thread that I put together for some Q's and A's pertaining to the AQ series Volvo Penta drives.

.
 
I'm not a fan of running 350 CID class engine exhausts thru the Volvo lower unit. The MERC manual for the 260HP 350CID (5.7L) engine calls for a single exhaust port diameter of 4", if duals, 3" each. I measured the narrowest point of a 280 drive's path thru the transom and it's about 3" (including the relief ports). The MERC manual also has a specified upper limit for exhaust back pressure of 4 psi. I measured the back pressure in my 280 and came up with 6.5 psi, more than 50% more. Regardless of who made the engine, all 350 CID engines @ 260 HP have the same flow requirements.
see
http://home.comcast.net/~rfierro/si...t/&PHPSESSID=768a38d5f9fdbc39cad135dd717023e5
I got another 450RPM out of my engine @ WOT when I converted to thru transom w/mufflers (NJ law).
 
I knew you meant successor. And as far as 350 exhaust thru the leg - not really an issue...it's a houseboat.
duck.jpg

But I do have a spare 4cyl leg with a set of 6cyl gears for it so with Ricky's instructions and encouragement I'll venture in to a reshim regear reseal.
 
if you aren't working your motor hard enough for exhaust restriction to be an issue, then you don't need to worry about the 250 drive strength
 
Back
Top