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Block Heater Removal

on the 1981 300D (OM617.912)

by franklynb


Having acquired a tired 257K 300D, which I drove from Arizona to Milwaukee after a cursory inspection and a few minor repairs; AND a spare 617.912 engine with 2 degrees of stretch in the chain, but an apparent leaking head gasket...

I decided to chase that vaunted 500 psi "spec compression" mark by rebuilding the leaking, but "younger" engine. Since I'm a fan of block heaters, this gave me a chance to check my "CPR" <coolant plug removal> setup and method one more time, before the ultimate challenge.


OK. Since its NOT a turbo, as evidenced by the lack of plumbing when stripped of manifolds and starter: its not the ULTIMATE challenge. But, if I do the "right design" of tool, I think, my setup will also be capable of accessing the plug for turbo-diesels. Ideally, with the turbo and its plumbing still in the car. Ideally.

By the end of the weekend, I hope to remove this plug, using the tool in this orientation:

.. using the knowledge gained, and some re-design from Trial #2, the subject of this thread extension. <This is an EGR car. Just look at that gook! A power wash is in order. We just need one mild day, so that i don't end up with an ice puddle in the unheated shop bay.>

While my Trial #1 setup proved adequate, there were two things I wanted to improve:

1) use a shorter bar. Certainly less than my 6' long tube extension.

The tool setup is shown below:
This shot shows the funky gas weld between the two sockets, which is actually a repair, from my effort to:

2) "harden" the tool setup such that I might consider loaning it for someone else to <ab>use.

During Trial #2, I decided to use the "old school" heating method <see post #1 and that yada-yada about machine tool factory heritage>, and figure out how to REALLY heat the plug.

So I hauled out my "gas axe" Oxygen-Acetylene rig, with a Victor #0 tip to allow me to spread the heat out to a "brazing" type of setup. My concern was overheating, and possibly softening the hex, or distorting the plug, such that the hex would break out before the plug would loosen.

The plug is tight for a couple of reasons that weren't apparent to me until I removed one:

The plug is a straight thread 38mm diameter, with a 1.5mm pitch. There is a sealing flange, but no "gasket" or sealing ring to speak of. Which means that the prevailing torque, and thread form must be tight enough to seal against leaks, slightly stretching the plug in the hole. I could probably look this up in Machinery Handbook; or if anyone knows where to find it in the service manual, point me to the page -- I have a hardcopy and will scan and add it to this thread.

Anyway, i started Trial #2 with the previous "propane" setup, only to shear my homebrewed tool, almost immediately. it broke at the weld, probably from poor penetration, and proceeded to "corkscrew crack" around the welded joint.

I decided to "fix it right", so after some lathe trimming to improve the "mating" between the two surfaces, and shorten the overall length; and making an "inner shaft" structure from a piece of 13mm hardened hex stock that now goes through the two sockets, and is welded to the 13mm x 1/2" base socket, with a pressed fit into the 1/2" hole in the 19mm hex:
<note to self; take camera off "spread auto-focus" mode!>
and gas tacking, straightening on the lathe, and then fusion welding the seam in a spiral overlay -- I'm happy!

The application of "gas axe" heat surprised me. I thought, as I was spiraling the heat around the periphery of the plug, that I'd not have a problem getting the plug cherry red.


The heat soak capacity of the block is clearly HUGE. As fast as I could spiral, the plug would absorb it, draining the 1/4" diameter spot of heat into a smoking gray, hot plug. I didn't think to borrow a non-contact pyrometer for Trial #2, but this is in the plans for this weekend's "Ultimate Challenge"; so that I can record the temperature of the plug "at release".

"At release"? YES! To my amazement, after applying the gas axe for about 20 minutes
the plug unscrewed! Using ONLY the service tubing extension:
and MODERATE torque input -- I'm guessing perhaps 150 ft-lbs, and I've done a LOT of torque wrenching in 40 years -- the plug came out without a hitch.

Another observation: even after breaking loose, I had to use about 50 ft-lbs of torque to unscrew it! Which tells me that its probably an interference thread, which makes sense in its "gasket-free" design. And partially explains why, when seized by time, acids, heat cycling, and galvanics -- its an ULTIMATE CPR challenge!

Discuss this DIY here.


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