B2 Piston Failure - PeachParts Mercedes-Benz Forum

savagetom · #1 02-04-2004, 10:23 PM

My son has an 82 300D with the classic B2 piston failure symptoms. I hate the thought of pulling the transmission. Has anyone successfully replaced the B2 piston without removing the transmission? I've seen posts indicating that it is possible to replace it in the car easily on 126s and with difficulty on 124s. Any tips?

Mike Murrell · #2 02-04-2004, 10:51 PM

You may have seen this already. I'll post it in case you haven't.

http://web.archive.org/web/20041026013432/http://business.baylor.edu/Richard_Easley/autofaqs/b2piston.htm

savagetom · #3 02-05-2004, 08:28 AM

Thanks. Yes, I have seen it. His car is exhibiting symptom number 2. Hopefully, someone has some experience changing this part.

savagetom · #4 02-14-2004, 09:32 PM

My son did drive the car 600 miles without incident, it did just fine once it got out of first gear (only had to run through the gears three times). Currently up on stands, we've got the old one out and with the transmission in situ, it looks like the new one should go in without much of a clearance problem.

One question, though: Concerning the new bushing in which the 'nose' of the new piston rests, should it be RTV'd or otherwise secured in place - it doesn't seem tight enough for a friction fit to be safe? The old bushing is soft rubber (more of a seal than a bushing) and is glued in place; the new one is rigid nylon and more of a guide. Any special sealants recommended for use inside a transmission?

A pic of new and old is at http://savage.mbz.org/IMG_1125.jpg

Thanks.

shane83SD · #5 02-15-2004, 02:38 PM

From the manual "Automatisches Getriebe 722.3", it outline the following steps:
1. Insert Teflon ring with grease in groove of brake band piston B2. (looks like it is already on the new part).
2. Insert thrust pin.
3. Install new O-ring.
4. Insert brake band piston B2 with thrust pin and at the same time make sure that the thrust pin of brake band piston B2 is installed in brake band B2.
5. Press in brake band piston cover B2 andd install locking ring.

There is no mention of any sealant.
Good luck with your project.

savagetom · #6 02-16-2004, 06:06 PM

Shane, thanks for the feedback. Did you see anything in the manual about renewing the piston guide?

shane83SD · #7 02-16-2004, 09:39 PM

For removal:
After removing brake band piston cover B2:
1. Remove brake band piston B2 with thrust pin.
2. Remove o-ring.
3. Remove plastic guide ring with lip sealing ring.

To install:
1. Insert plastic guide ring for piston B2.
2. Insert lip sealing ring with the shop-made drive-in drift.
3. The sealing lip must point in the direction of the brake band piston cover.
The rest are described as before.

So I would guess that the sealing ring is functionally similar to the wheel bearing sealing ring. Before you remove it, you may want to note the orientation first.

Would you do a summary report with pics if possible when you are done? Thanks.

savagetom · #8 02-23-2004, 12:34 AM

Well its done. I was able to reassemble the unit yesterday and it is working fine.

So to answer my question, it can be done in place. It is possible that it could be done without trimming any of the sound insulation if the tranmission is shifted to the left. I can't verify that but it can definitely be done if you remove some of the insulation. We removed a triangle shaped piece about 2 inches on a side.

The hardest part of this was getting the snap ring out. We fashioned a pusher out of a short piece of copper pipe and washer, bolt and nut. We cut away the sound insulation and used the pusher to push against the transmission tunnel and the cover. That held the cover in the depressed position and and allowed my son to pry out the snap ring. The spring isn't really that strong but you don't have much leverage on your back on a creeper. Getting the old seal or guide out was difficult and not a pretty operation. I stuffed a piece of cloth in the hole to the transmission with some vaseline on it catch the pieces then went at it with a long screwdriver. I eventually gouged it all out.

It went back together more easily than it came apart. The bushing is a little thicker than the seal so it doesn't fit all the way down into the opening that the seal was in. I guess I should say than it bottoms out in the opening but the flange of the bushing (or guide) isn't flush with the opening. Hopefuly it is the right part!!! Anyway it is siliconed in place.

The trickiest part is getting the dogbone (thrust pin) into position. You can't get the piston into postion with the dogbone inserted in it because of clearance issues. I used some cold vaseline to hold the dogbone in position in the transmission with the end of it postioned in the center of the bushing opening. I was able to carefully square up the piston and push it into place.

With the cover cleaned up and a little silicone grease on the edge I was able to push the cover in and get the snap ring in place without much difficulty. The transmission is working great now and the shifts seem to be at reasonable speeds and are nice and smooth.

I'm contemplating whether to do this as a preventive measure on my younger son's 84 300D. It is a euro model and I don't think it has the insulation and the transmission may be a little narrower. Allegedly, it is easier. I think I will at least order the part and have it on hand. I wonder how many transmissions have been overhauled when this was the only thing wrong with them?

How long did it take? I'd guess about 4 to 6 hours spread over several days. That doesn't count the head scratching, analyzing, research, etc. I think I could do the next one in 3 hours plus drying time for the silicone. If I did it for a living and had a lift, you could get it down to 90 - 120 minutes including changing the transmission filter. Maybe less than an hour if you didn't mess with the seal/guide or taking the filter pan off.

I've attached a picture of the old and new parts.

brewtoo · #9 02-23-2004, 09:47 AM

Always makes you feel good when you accomplish something like this, on many levels...

savagetom · #10 02-23-2004, 10:19 PM

Thanks, brewtoo.

I does feel good. A sense of accomplishment and a sense of saving a lot of money and perhaps a car. We spent maybe $150 including a filter, pan gasket and fluid. If we had faced a $1500 to $2000 repair estimate we would have had to think long and hard about putting that kind of money into a car with 293,00 miles. You can't under estimate the effect of the internet in improving the efficiency of the economy.