What size allen wrench on block heater plug?
 

My '84 300D does not have a block heater installed. Got one ready, but the plug in the block takes a huge allen wrench or something similar. Maybe 18mm, my 17mm is too small.

Anyone know the size and where I can get one?

This job ought to be a real hoot.

I was getting ready to post the exact same question, but you beat me to it. :mad: ;)

I'm thinking I may want to install one for this winter too, so I am interested as well......

19mm - I have the snapon Allen with a 4 foot breaker bar, and I couldnt get it out.

There are lots of posts on doing this. Mercedes would install them at the dealer within the first 3 months for free, but after that, I think that that plug just solidifies in the threads....I could not get mine out. Its not worth cracking the block trying to put in a block heater. I just keep my 300Cd in the garage. I tried a lower radiator hose heater, but it never worked quite right.

It's a real bear to get out. Yes, 19mm is the right size. I couldn't even get mine out with an impact wrench. I had the engine out of the car, finally 6' of breaker bar broke it loose.

With the starter out it appears it can be gotten to from the bottem with the car up in the air a few feet. I'm hoping that with my car being from Texas, it hasn't been subjected to as extreme conditions and might not be quite as "stuck"

I will be doing this soon also. But I am going to take off the manifolds, clean them out inside if need be. and do it from above. Already have the gasket set to do it.

Well, if it couldn't be gotten out with those massive-tools-I-don't-have, I think I will make some sort of "invention" to warm the engine a little, such as a modified hot plate that magnetically sticks to the bottom of the oil pan or something, to keep the whole thing warm, that'd certainly make some difference. Its time to be creative and perhaps spend less money on the setup. :D

First get some "Kroil" and get it onto the threads several days in advance. Do this every day for several days.

Then on game day:

Get the engine up to operating temperature. Then get a bit of dry ice and a suitable pair of gloves. Press a chunk of dry ice on the face of the plug and hold it there for 6 or 7 minutes. If the chunk evaporates before 6-7 minutes are up, then get another chunk. Make sure all your tools are ready to go. You need to quickly put some torque to it after you remove the dry ice.

Remove the dry ice, install the allen wrench and use a large pipe to quickly apply the torque. Bring the torque up fast. Don't just slowly add force to the bar. You need to "crack" it loose.

If you followed all of the above, it has to release.

Thanks for those tips! If I decide to put one in sometime I will definatley try it that way, I think that'd definatley work. I never had a problem getting it started last winter, just had to crank about 15-20 seconds on the coldest of days (like -5 deg out)....after its valve adjustment in january it started even easier, and also the rack damper pin was screwed all the way in real tight at that point, it has since been adjusted to optimum position, and I can start it at 33 deg's within 2 seconds of cranking, and it runs without shutting off once its started no matter what, I was quite happy with that. We'll see how winter goes, if it gets bad, I'll probably go with a block heater, I'd rather not spend the $$ on the tools/dry ice/penetrant/heater itself if I don't have to.

Dont take the starter out, thats a PITA.

Take off the air cleaner, and the brackets and then you can get to it.

Use a good quality hex/allen bit on it. Smack it hard with a hammer a few times (the reason to use a quality allen bit) and then use your breaker bar or impact on it.

If you can't get it out and need something in the meantime, run a droplight under the hood with a 100watt bulb and leave it on overnight (hood closed). It will keep it warm enough to make starts a little easier, but may run that electric bill right on up.

Heat the plug with a butane torch. Keep your fire extinguisher close. It will come out.

I think Brian has the right idea with regards to this. You need to have everything working in your favor on this one. I've wondered if it is possible to use a pipe wrench on this plug, engaging the OD that sticks out from the block. I would try the allen first, but offer the pipe wrench as an alternative IF one can get it in there. I'm glad I removed that plug and put in a heater when I rebuilt the engine.

The technique I used.
 

I'm not sure how easy it is to find a 19mm allen wrench, but a 3/4" allen wrench works just fine. I consider myself a pretty decent shade tree mechanic, and this job isn't especially easy. Don't even consider it if you cannot get the car up off the ground safely. I used a friend's lift to get the car up nice and high, pulled the starter (a challenge, but not too maddening), and then went after the plug in the block heater hole. We ended up breaking a couple of 1/2" drive accessories before getting a standard "L" shaped 3/4 inch allen and about a 6 foot pipe. Both of us were putting as much force as we could muster on the pipe before the plug finally broke loose. The block heater went in very easily, and I had the starter rebuilt while it was out (my buddy's dad runs a starter/alternator shop).

This job is well worth the effort once it's done. However, it will be an exercise in frustration if you aren't set up to do it right. You've got to be "In it to win it" to make sure the job gets done.

SteveM.