1984 300DT: RS motor mount measures one inch - time to replace?
 

Take a look at a photo of a new motor mount. The center metal bit is elevated from the metal "ring" that is attached to the subframe. The center bit should still be elevated if the motor mount has any life left. If it has sagged to the same level or close to it, it's probably time for a new mount.

My new mounts compressed significantly upon installation. Not sure of the exact height but enough to make it visible with the naked eye compared to an unloaded mount.
The failure of my right mount was the rubber tearing away from the metal components allowing the engine to rise up and PS belt to hit the oil cooler line.

The new mounts will definitely sag with weight on them, but you should still be able to tell that the center section is elevated above the outer section.

The motor mounts in my SDL hadn't torn away, but they had compressed to the point that the center section was resting on the subframe crossmember. Metal on metal basically. No idea how old they were, considering so many other things I've found on that car - possibly from the factory (yikes!)

Look at how close to the oil cooler hose the pulley is then decide whether you want to change the mount. I converted 1 SD oil cooler hoses to AN fittings and made slightly longer hoses that give more clearance. There are lots of pics and discussions on this site. I forgot whose idea the AN was but it is a good one for those who intend to keep the car.

I judge from underneath by how far the center allen bolt head (8 mm wrench) is from the lower sheet metal. I just put a new motor mount on the R side of my 85 300D. My thumb to the bolt head puts the joint at the sheet-metal, i.e. 1" which is what I recall before. When I have had sagged mounts, with the rubber bulging out, the bolt head is about at the sheet metal.

BTW, the new rubber mount made a world of difference vs the poly-filled one I had on both sides before. It is like driving a different car. If you replace your oil cooler hoses, cut the new upper one 1" longer (15") as I did, to keep it further from that evil p.s. belt.

Is it Time?
 

Husky...If you're even wondering, it's time. lol

With motor mounts I've always waited too long when wondering about it. Then in the end find myself "wondering" why the heck I waited. Same thing goes for a lot of other maintenance items.

fyi...I'm finding that "pondering" is the same dang thing.

Swap 'em out with some good ones, otherwise you'll be swapping them out for some good ones sooner than later.

@ 86: and waiting too long lets one learn how to deal with cut cooler hoses and possibly an engine R&R with requisite gasket replacing before install.

Then you run into whileimatits like trans rebuild, engine compartment cleaning, hose behind the false fire wall changing, drive shaft refurb..............the list can go on all because of a motor mount.