i'd like to suggest you try to find posts from stevebfl and Thomaspin. there are also some others that i am no doubt forgetting.
my reasoning is that it has been said quite often that the bottom end of these 4.2's can expect a life of 350 to 400k with out replacing the rails. so that mileage would include the lower chain sprocket.
the upper sprockets are very easy to do and you should not have any problem doing these yourself. caution and care on how the sprocket fits in the chain is needed but beyond that it is a fairly simple task.
by looking at the keys you can determine if you need to use an offset key or not. 4 degrees is not that much lash but you could take that up if you choose.
the lower gear i believe sits in an oil bath of sorts is my understanding on the rails life span so i think you could extend that reasoning to the gear as well. also don't forget to replace the valve cover gaskets and the small lock washers on the vlv cover screws.
1972 350sl Red/Blk 117k
1988 420sel charcoal/Blk 140k
1987 420sel gold/tan 128k
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