i am doing just this right now. my 420 lunched a rocker arm that gaulded the cam. now what is probably the sticking point for some will be removing the rocker arms. there is a special tool to compress the valve/spring. this tool goes for about 275+- $$. you will also need the ball stud height checking tool about 80 bucks . this tool you can't go without. there is no other way to check for the proper clearance. so for 350$ you've got it made. now i did talk to phil on the tools and he had me check a few others on the price and his was right. so i kinda left the car for six months or so sit. but now i'm back to it.
anyway, there is a guy by the name of samstag sales. i found him on ebay. look for samstag in seller search. i got both pieces for just under 200$.
i purchased the cam for 233$ a ball stud, rocker arm and a vlv cover gasket for 271$ from http://www.europeanautomotive.com
as for difficulty the hardest thing i'm finding was taking the damn air clearner off. 5 or 6 clips, 1 wing nut and 2 bolted studs. i guess they don't want the air cleaner walking off. the vlv cover comes off with 4 bolts. with the vlv compressing tool about a minute to get the rocker times 8 for each cyl. and then factor in at least one beer. you'll also need either a helper to bang the starter to rotate the cam to the proper position. lobe away from the rocker. or you can get a hand switch so you don't have to share the beer.
once all the rockers are gone look at the cam gear and chain. this is important. mark the exact locating of the chain on the gear i also would suggest using either wire ties or nylon ties this will help to secure the location. next is taking the bolted cam gear off. first you will need either another special tool to hold over one of the lobes so you can unbolt the cam. i would use either a larger open end wrench with leather around the cam or a rubber strap wrench. no need to spend the big bucks here. now the next thing is to remove the oil tube. no big thing but they are plastic i believe so for another 10 -15 bucks buy a new set. now all that is needed is to remove the 5 bearing towers. you should mic out the bearing i.d. for tolerance with the new cam. if they are close good enough leave them. the reverse to put them back together. there are other issues do i replace the upper rail while i'm there you need to answer those questions. but if the car is absolutely clean otherwise ???
my 420 was absolutely perfect. this problem happened about 4 days after i bought it so i was stuck with it. i only paid 5500$ but now i'm glad cause the cherry is off the tree and i don't see this car any harder than some of my others that i've worked on. though maybe a bit more expensive. i've been taking fotos of the job so i might be able to send some photos later this week.
good luck. and don't discount those knocked off cd's. they do help.
1972 350sl Red/Blk 117k
1988 420sel charcoal/Blk 140k
1987 420sel gold/tan 128k
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