I would like to clean without dissassembly some old mercruiser trim cylinders. Would it be a good idea to use ATF ? I thought this would help free up these old rams and help soften the seals and piston cups and such. Any ideas?

I don't think ATF is the solution. If you looking for an oil that softens old rubber seals an clean out old deposits, you should rather try an ester based, or may be a vegetable hydraulic fluid. I think those fluids will clean out your cylinders and make your seals distend and softend. (may be too much?)

not sure what "trim cylinders" are, but maybe this will work:
http://www.schaefferoil.com/datapdf/131.pdf
It is Schaeffer Oil Company's Neutra 131 Fuel Stabilizer, which if you scroll to the second page of the linked information sheet, you will see it can be used as an equipment purge. Maybe just what you need (I don't know) and you can use the leftover in your fuel tank.

http://www.mercurymarine.com/uploads/images/1145/sideView.jpg

What you see here out front are "Tilt" cylinders. Used primarily for cycling the leg in and out of the water. These high speed cylinders have a stroke about 8 inches or more.

You see the stainless hydraulic hoses? Behind the hoses hidden in the casing there are two more cylinders which are sometimes called "trim rams" They are a pair of cylinders having a stroke of around 2-1/2 inches. Trim ram control the leg's arc, cycling about 4 inches, thus to control the level of the boat in the water while at speeds higher than trolling. These cylinders have no rod end or heim joint, only blunt rod ends that push against the leg.

When lifting the leg complete out of water, the rams push the leg up slowly then when they extend fully, a valve shuttles to make the tilt cyls take over lifting the leg complete out of water.


Leg is nick for "lower unit"