Landseer

B2 piston can gall, jam and eventually break. The piston has been redesigned apparently several times. The new piston has a smaller diameter, but it is made up by a thicker, wider piston seal, which you will see in the pictures.

My early 84, 85 and 86 cars have original B2 pistons. They are identical transmissions, BTW. Will post part numbers later.

The replacement can be done in-situ. I'm fortunate to have an 84 on the benchtop to show the parts clearly. But the new piston is actually going into an 85 that is opened up for surgery. So this is practice.


The puller is needed for the early 4 spd cars like mine. Why? Because there is an aluminum bushing that the inner bore of the old B2 piston rides in as a guide. It also is upgraded, to a plastic bushing. I needed to make a puller to remove the aluminum one, along with the lip seal (that part hasn't changed)

Here are some pictures of the parts and the puller to do this job.

(Search also excellent posts by Tails https://rennlist.com/forums/928-foru...placement.html and by Steve Cataneo (Post #5) https://rennlist.com/forums/928-foru..._-_-_-_-_.html on the B2 piston for more details. The extraction of the aluminum guide is perhaps specific to pre-S4, so I made this post to augment their work).

UncleMaz

Long before a 928 was even a twinkle in my eye, Tails and Steve C. provided me with the info and guidance I needed to do this job on my Mercedes.

Landseer

The B2 Piston actuates the B2 Band via a little metal rod that looks like a dog bone. Different lengths are used to achieve the clearance per the WSM.

(Tails also covers this in his write-up, but the part numbers I found were different than his.)

Here is a picture of a few.

The smallest was in the 84 transmission.
Next largest was in the 85.
Longest in this picture was bought in case I needed it to account for wear. I do not want to remove this 85 transmission unless absolutely necessary.

I bought several others, even longer, in case I needed them on reassembly.

Landseer

For the archives:

I took the show under the 85 car to attempt to remove the aluminum sleeve on a transmission in-situ.
It worked easily, but I had to modify the puller slightly.
I replaced the bushing tool, shown on the puller, with a thin washer of similar diameter.
The bushing tool was too thick to snake it in past the B2 band.
Easy to remove the aluminum and replace it with the plastic sleeve.

Also, I reused the same dog-bone pusher. WSM shows to measure for 6 to 7 mm of play. Mercedes manual says 5.2 to 5.5 mm, measured the same way. Mine was about 5.2, so I left it. Looks like the B2 band had plenty of friction surface, though its hard to gauge in the car. Slightly longer dog-bone took it to just over 4 on a trial, that was too-little play. I could have used a shorter dog-bone, like installed in the identical 84 transmission, but just left it. I don't think its a problem.

My transmission seal kit happened to came with the wide seal for the new B2 piston, just FYI. That means if you dive into an early 4 speed, you will need to upgrade the B2 piston.
So, I couldn't reseal my existing B2 piston, even though it didn't look broken. The outermost surface of the new B2 is smaller diameter to accomodate the bigger, wider seal. But, my old B2 was scored, especially on the shaft end, the one that contacts and is guided by the aluminum sleeve.

This baby wasn't moving near as smoothly as with the new B2. This upgrade is recommended for anybody with an early car with miles on it, at least from a theoretical perspective.

Reinstallation was simple. Will see how it works in a week or so.

Last 2 pictures show where the Bowden cable actuator is located, plus, where the governer is on the DS side of car, just for fun.




Good to go, I hope.