It could be the B1 or the B2 piston cover. The B1 is the large circular cover up front by the filler tube and the B2 is slightly larger one toward the back. Both on the passenger side.

The B2 is the easier to remove and replace as the B1 piston has a MUCH stronger set of springs behind it. I had to make a special tool to press the cover in to remove the C-clip.

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Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)