I'm considering replacing this myself on my 450SLC 5.0 and have a couple of questions about the guide rails.

There are a bunch listed by the parts suppliers, but from tearing an M117 down, I know that some of these rails are impossible to get to unless i tear the engine down itself (or are they). I do not want to replace these unless I absolutely have to.

Can anyone give me advise on which ones I need to replace and the part numbers?

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With best regards

Al

Al:
3 upper rails and 1 tensioner rail are all you need to do. These are "easily" accessed from the front. You will need to remove the PS pump to do the LH side, and the alternator to do the RH side. These should be your parts, they're the same across all the m117s:
http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1985@@1985&makeid=63@@MERCEDES-BENZ@@63&modelid=6291%3AMBC|1540%3AED|10000058@@500SEC&catid=240154@@Engine+Mechanical&subcatid=240293@@Timing+Guide+Rail&applicationid=W0133-1622449&mode=PA
http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1985@@1985&makeid=63@@MERCEDES-BENZ@@63&modelid=6291%3AMBC|1540%3AED|10000058@@500SEC&catid=240154@@Engine+Mechanical&subcatid=240293@@Timing+Guide+Rail&applicationid=W0133-1640070&mode=PA
2x http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1985@@1985&makeid=63@@MERCEDES%2DBENZ@@63&modelid=6291%3AMBC%7C1540%3AED%7C10000058@@500SEC&catid=240154@@Engine+Mechanical&subcatid=240293@@Timing+Guide+Rail&applicationid=W0133-1639568&mode=PA

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2021 Charger Scat Pack Widebody "Sinabee"
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1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k