On the 560SL my wife used to have, I did exactly that -- used a long extension bar.

I recall it was either 18" or 24" long. I bought a set of three from Harbor Freight that had one which was perfect. You can't go wrong with a 18" or 24" extension and a couple of universal joints. I recall someone telling me you CANNOT get those bolts out without that extension.

Once I had the extensions and universal joints, the job was really ease. Be REALLY careful to not drop the starter on any body parts.. it's heavy! I'd make sure to be clear of it, just in case you end up dropping it when the last bolt is pulled out. Might want to have something for it to land on (plywood, foam, etc.) rather than having it hit asphalt or concrete if you're returning the core.

Neal

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'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi) SOLD
'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
'14 SL550 72,000km (43k mi)