How To Replace ALL Piston Cover O-Rings In A 722.1 Transmission While On Vehicle

[Squiggle Dog]

My 1980 W116 300SD with 722.120 transmission had leaking piston cover O-rings. I read that while it is possible to replace the O-rings in the B2 and B3 piston covers simply by pressing in with your hands, the spring pressure in the B1 piston cover is too great.

A special factory tool is recommended my Mercedes, but the transmission must be removed from the vehicle since it takes a lot of room, and there is only 70mm (3") of clearance between the transmission and the tunnel. I have heard that it may be possible to use this tool with the transmission still in place if the exhaust pipe and driveshaft are removed, and then the transmission is lowered down as far as it will go. This is a lot of work.

I don't have a lift and doing the above would have been difficult. Feeling that I'd rather just pay someone else to do it, I called an independent Mercedes-Benz repair shop asking how much it would cost. The shop said that it would be too difficult for them to do, and they referred me to a place that specializes in German transmissions. I called the transmission shop, and the mechanic said that he wouldn't do the job without completely removing the transmission from the vehicle. This would have been expensive and would have put me without a car for a few days.

So, I decided to make a tool that would allow me to replace all of the piston O-rings with the transmission still in place and without having to lower it or remove any other parts, because that is just too much work.
CAUTION: If you make your tool any less stout than I have made mine, it will probably fail and you will not be able to finish the job. Use your ingenuity and good judgment.


I bought 3/4" hollow square steel tubing at my local hardware store. Solid aluminum bar would work as well, but it is harder to find. You need at least 14" of it--more if you make a mistake. I cut the long piece to the length of the transmission (235mm), and the short piece 120mm.

I got some 30mm long bolts the same thread pitch as the ones for the transmission pan and drilled a couple holes for them in the tubing. The holes need to be offset all the way against the edge on one side. This is to clear the valve body, and you may still need to grind a little material away at one edge (it starts at about 80mm in from the right side to about 90mm in, if you look at the picture above). The center of the bolt hole on the left side is 12mm in front the end, and the center of the right bolt hole is 21mm from the end. The center point of each should be spaced 202mm from each other.

I used a 4" long bolt (about 3/8" thick) with a thin locknut going through the center of the tubing. The center hole for it is 107mm from the right. The stronger the bolt, the better. Even this one started to bend under the spring pressure.

I drilled a hole through the short tubing so it would go through the bolt. The tubing will crush during use and cause a big disaster if you do not reinforce the hole with round tubing. I had to buy a long piece of 3/8" ID round steel tubing, cut, and grind it just small enough to slide into the square tubing. It keeps the tubing from collapsing and from it catching the threads and binding. You must do this or just use solid bar stock for this piece.


Then I drilled a long, slotted hole at the other end of the short square tubing so I could attach a 23mm socket. I put a thick washer inside the socket and then used a bolt going through it, with a locknut on the inside of the tubing. I tightened it just enough for it to stay in place, while still being able to slide as needed.

[Squiggle Dog]

The first step to the piston cover O-ring removal process is to put the transmission in park, set the parking brake, chock the wheels, lift the car as high as you need, then put a stand underneath.

Clean the transmission very well. Compressed air is useful. The slightest piece of dirt can cause mechanical problems if it gets inside, not to mention it can get caught in the sealing surfaces.

The control pressure rod clamp will be in the way of the B1 piston, so completely remove the nut and bolt and then slide off the clamp and set it out of the way.



Loosen the bolt holding the dipstick tube to the intake manifold. Put a container underneath the transmission and remove the drain plug at the dipstick banjo bolt. Let the fluids drain out. Also remove the plug in the torque converter to let it drain (you'll have to turn the engine over until the plug is at the bottom, either by using a socket on the crank, or on the power steering pump pulley bolt [some advise against using the pulley bolt]). Once drained, remove the pan and continue to let the fluid drip out into the container.


The exhaust pipe clamp was in the way, so I loosened it and slid it out of the way.


Now the tool can be bolted to the transmission. Make sure to put the socket through the new O-ring before compressing the piston, otherwise you won't be able to get it in.

[Squiggle Dog]

I started with the B1 piston (the trickiest one, for which this tool is required). The socket can be positioned to the ideal place.


There is a washer and bolt on each side of the arm. Each one should be snug against the arm. The inner one is loosened, then the outer one is tightened. This helps keep the arm from tilting. Have patience and make sure that the piston is being compressed evenly on all sides. The socket can be repositioned with much care. Don't rush it and make a mistake!

Once the piston is compressed just enough to remove the circlip, pry it off at the notch with a flatblade screwdriver.


Turn the arm nuts the other way, loosening the outer nuts while tightening the inner nuts. Be very careful to not let the tool go off center or slip off. This could very easily become a disaster if you are not patient and meticulous. Turn out the arm just far enough to reach the O-ring with a pick. If you go too far, the metal piston ring further in could pop out, and there is also a pushrod which could pop out of place. Fortunately, my tool stops against the tunnel before going too far. Remove the old O-ring and install the new one. Don't get them mixed up or let dirt get inside.


Before pushing the piston cover back in, take a look inside the transmission by the corner of the valve body to make sure the pushrod is still in the hole. You can't see it in the picture, but it's right where the 8 is. If it's popped out, you'll have to stick something up in there to push it back in.


Start turning the arm nuts inward to compress the piston cover, doing whatever is necessary to make sure it goes in straight. If it tilts, it will bind up and the O-ring can get cut on the sharp hole edges. Once it's in far enough, push the circlip back in place. I put the gap at the bottom, so it has a weep hole effect.

Then slowly let the arm back out and make sure that the piston is rested back up against the circlip and not tilted. Congratulations, you have done the first one!

The tool can also be used on the B2 piston, which is under much less tension. Same rules apply.

[Squiggle Dog]

I did not use the tool for the B3 piston on the other side. I pushed in the piston cover with my hands, removed the circlip, then let it out to rest against the tunnel. It can easily fall out of place! Once again, make sure that the pushrod is still in the hole.


Push the cover evenly back into the transmission being careful to not cut the O-ring on the sharp edges. It may require a helper to put the circlip in place while you hold the cover.

Assemble the parts you removed with a new filter, pan gasket, and crush washers, then add transmission fluid, following the procedures in the manual. If all goes well, you won't have any more piston cover leaks.

[Squiggle Dog]

Part numbers for piston cover O-rings:

B1: 000 997 47 48 64

B2: 000 997 48 48

B3: 008 997 35 45

[karmann_20v]

Sweet tutorial, Squiggle!

Do You happen to know if the selector shaft seal (behind the neutral switch) can be changed with the transmission on the car and did you do it?  The one on my 79 450 sel is leaking like a sieve...
Thanks!

[gavin116]

Hi Squiggle Dog


Brilliant work as always.  You really do have the patience of Job, I know I would have lost my cool long before the first hole was drilled in the custom fabricated tool...


Keep on 116'ing

[ptashek]

You should totally work for Haynes. Maybe then their manuals would actually be worth the money they cost
Would this procedure apply to other 722 models? They don't seem to differ much externally at first glance.

[polymathman]

This is why I check this Forum daily.

[TJ 450]

Great idea! I recall attempting this on my 6.9, but with the cover being a lot larger on that version it's even more difficult.

Tim

[rob280sel]

Wow we could have done with a tool like that when I was in the auto trans shop.

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