this forum is such a gold mine. you guys rock!
I've done a lot of reading and I'll try to summarize it here for future chain replacers (and to improve google's pagerank score for the sources by linking to them).
first, check your chain stretch (actually, "wear-induced elongation")
[1]
next, decide to either use an offset woodruff key, or replace the chain. your call. chain stretch past 6 degrees can cause smoke, idle misses, loss of performance, etc.
[2]
replacing your chain? you'll likely want to replace more than just that. the safest thing to do is replace everything in this diagram
[3], but that decision will likely be made based on your budget:
parts:
the chain:
don't buy the WIPPERMAN chain
[4]. the IWIS chain is higher quality
[5][6]. order here:
[7].
you'll also need a link. order here:
[8]
you'll need a tool to crimp/rivet the link into the new chain. either rent one of the nice tools from a board member
[12], or try the harbor freight tool
[13][14].
make sure you get the chain crimping right. don't let this happen to you
[23].
chain tensioner ("a" in the diagram):
I'm unsure if I need to replace the entire tensioner
[9], or just its spring
[10]. either way, you'll also need a gasket
[11].
infrequent oil changes can possibly clog the tensioner's oil gallery with sludge. no oil pressure in the tensioner = no chain tension = broken guides
[25].
my understanding is that's all you can do without removing the timing cover from the front of the engine (still searching for a write-up about that, I'll add a link when I find it).
if you are willing to remove the timing cover, you should replace all of the guides. if you don't and they fail, they can get eaten by the chain, which can cause it to jump a tooth, which is catastrophic
[24].
tensioning rail ("85" in the diagram):
sounds like the best one is made by SWAG
[15]. order here:
[16].
I'm unsure if I should replace its pivot pin. the link in the parts catalog seems to be broken
[17].
slide rail ("89" in the diagram):
order here:
[18]. again, not sure if its necessary to replace its pin. its link in the catalog appears to be broken as well (part 186 052 14 74).
130mm and 120mm guide rails ("93" and "93a" in the diagram):
I'm unsure which is which, but if you're replacing both, its irrelevant. order here
[19] and here
[20].
again, I'm unsure about the pins for these, and couldn't find the pins in the catalog.
replacement procedure / advice / notes:
dieselgiant.com has a writeup
[21]. from this and another forum post
[28], I had the idea that you might use (a lot of) zip ties if you don't have the proper vice grips on hand.
remove the rail tensioner before rolling in the new chain
[29]. you may also need a thermostat housing gasket?
you may have to remove the cam sprocket and the vacuum pump to replace all of the guides/rails
[22] (however, that post was about a 240D).
how much would all of this cost if a shop did all of the work? at least $500 for just the chain and tensioner
[26], and $900 (or possibly more) for all of the guides as well
[27].