My brother-in-law and I replaced the loop glow plugs in my 1978 300D yesterday, thought I'd share the experience.

Peach parts did not have the adapter plugs when I went to order, so I ended up getting a "kit" from another site. Just the plugs, a fuse and some premade wires.

For several days before, I sprayed some WD-40 or liquid wrench at the threads, hoping this would help free them up. I drove around for a while till the engine was at operating temp, then parked it and started work

I decided not to remove the injector fuel lines, figured I could work around them. Turned out to be the right call.

Used a 7mm wrench to remove the little nuts that hold the wires on. Removed all the wires and insulators. Then used a 21mm wrench to remove the glow plugs. They came out pretty easy, it was just very very time consuming as there was not a lot of room to turn the wrench between the injector hard lines

The loop plugs were very clean! Just a little blackened, otherwise they looked almost new. My brother in law is a VW diesel mechanic and he was shocked both by how easy the job was and how clean the plugs were. One of the loop filaments looked a little thinned and almost twisted, which may explain why one cylinder was always "late" when doing a cold start.

Since there was zero carbon or anything on the plugs I did not ream the plug holes.

Sprayed the new plugs with anti-sieze, threaded in by hand, then tightened with wrench. Put the new wires on, put nuts on over them finger tight, then gently tightened with wrench. I did not put the old insulators from the loop plugs since they are not needed for this set up; you want the wires together on the end of the glow plug. I connected the "power" wire that comes from the firewall but did NOT reconnect the ground wire at the other end of the chain of glow plugs.

Tried it out, wow, HUGE difference in starting. With the loops it would be three glow plug cycles and about 5 seconds of cranking while the "late" cylinder catches up. With the new plugs it's one cycle and it fires immediatly.

Anybody with the old plug system should do this conversion ASAP. It takes an afternoon, and two wrenches, plus whatever you buy the plugs for. I paid a bit of a premium, but then again I don't have to monkey with making home made wires. The job can be done with two wrenches, as long as you are careful about positioning the wrench and not banging up the metal injector lines

One thing I did notice is that for the first ten seconds or so of idling from a cold start there is an almost unnoticeable "miss" that disappears as soon as the temp needle starts moving. That one slightly thinned loop plug may be a symptom of something else rather than a cause.