I thought I'd start a list. I wish I had known some of these things. So the criteria is it has to be something which isn't totally obvious, oil change, change the filter. Ill start with the ones I know of and add from there...

If you're:

-repacking front bearings, check your front brake rotor thickness
-R+R rear axles, check your rear brake rotor thickness
-R+R master cylinder, check your brake hoses and booster
-removing manifolds, replace or check the starter
-removing turbo, check the turbo drain tube for leaks
-dropping lower oil pan, check/replace oil check valve, turbo drain tube
-R+R brake booster, check or replace front sway bar bushings
-removing injectors, check your prechambers
-R+R rear springs, check subframe mounts, spring pads, control arm bushings
-R+R diff mount, its easier to R+R fuel lines and tank screen
-R+R rear sway bar links, check the sway bar bushings
-R+R radiator, check transmission cooler hoses, coolant hoses
-R+R motor mounts, check oil cooler hoses
(Jeremy5848)
-- Replacing the battery, check underneath for rust.
-- Changing the oil filter, check the two shaft o-rings and replace if they've turned to plastic.
-- Changing the secondary fuel filter, check the o-ring seals and replace them if they've hardened with age.
-- Replacing a lamp in the instrument cluster, change all of them while you're in there.

Additions??? There must be lots of em!

dd

PS R+R engine, check xyz is another thread unto itself.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car