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Old 12-14-2003, 01:43 PM
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Join Date: Dec 2003
Posts: 37
Question for Techs: M103 rear main seal

I am going to replace the rear main seal in my M103 engine.
The seal it has now is not leaking, but I am thinking of replacing it as a preventive maintenance.

I was looking at the online parts catalog of Fast Lane and they have two seals. The standard and the service size. Which one should I get?

Thanks in advance.
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Old 12-14-2003, 02:37 PM
it leaks, its german
Join Date: Mar 2002
Location: raleigh nc
Posts: 1,111
Personally I'd go for the standard one as you dont have a leak.

Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
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Acme Automotive Inc.
Raleigh NC
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Old 12-14-2003, 02:53 PM
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Join Date: Jul 2003
Location: Montreal, Quebec
Posts: 508
Out of curiosity, why would you want to replace the seal if it is not leaking??
Preventive maintenance you said ?? I don't get it.
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Old 12-14-2003, 03:31 PM
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Join Date: Dec 2003
Posts: 37
Cause I will be replacing my transmission and I will be in this area, so I might as well...
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Old 12-14-2003, 06:38 PM
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smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,126
There is a risk of tearing the oil pan gasket when you remove the rear crank seal carrier. The shop manual casually says, 'Do not damage oil pan gasket. If necessary, replace oil pan gasket.' In most applications the M103 has to be lifted to replace the oil pan gasket.

FWIW, when I was shopping for an indy to rebuild the transmission in my 91 300SE, none said that they recommend replacing an M103 rear crank seal just because the transmission is out.

95 S420
87 300SDL
83 300SD ... $950, see cars forum
91 300SE ... sold
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Old 12-14-2003, 07:46 PM
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Join Date: Jul 2002
Posts: 1,141

I'd replace the seal if you've got the transmission out. I think the earlier M103's, there is an updated seal carrier. You need to talk to a good MB parts guy.

Service seal is for a damaged(worn) sealing surface. Chevy's they use to sell a seal where the lip was moved about 0.063", so it was on a new part of the crank hub. A little bigger works too.

No reason you'll need it. best of luck,

Michael McGuire
83 300d
01 vw A4 TDI
66 Chevy Corsa
68 GMC V6 w/oD
86 300E
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Old 12-15-2003, 04:13 PM
Posts: n/a
If it ain't broke don't fix it.

The rear main seal in my 103 engine now has 265,000 miles and doesn't leak a drop.

If you disturb the seal, you may very well end up with a leak.

Good luck,
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