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Universal M-25 glow plug replacement
Finally got around to replacing the glow plugs on my M-25 (the ones I paid $6.05 each at sparkplugs.com) and the toughest part of the job was doing the middle plug. In a rather weird piece of engineering on this engine, it is very tight getting an open-end wrench around that middle plug to remove it. Also, the manifold is just close enough to make it very difficult to remove and install the glow plug. In any case, I was able to do it without removing the manifold by using my fingers to get the round lock nut onto the end used to secure the wires and catch the first couple of threads. Once you do that, use pliers to move it down the threads and clear the side of the manifold.
With the new plugs, it only takes 25 seconds to warm them up to starting temp. By the looks of the ones I removed, I think they were the original ones from 1983.