My '06 Metro 6/1 was getting reluctant to start when the temps got under 30F. Found out about glowplugs (thanks Bruce ) on this forum and ordered up one from Utterpower hoping for a 10 minute fix to my problem. It arived quickly and looked to be a nice piece of work. Unfortunately Metro changed their head design recently and the standard plug protruded much too far into the precombustion chamber. George had heard of this and offered a refund if I wanted, and said that some had been successfully installed by making a spacer to shim it out to the correct depth. So much for the 10 minute fix.
A quick check with a caliper convinced me that there would not be any handy spacer material in my scrap inventory and machine shops are a long ways away. Also, the top end hadn't been checked on this engine and I'd read some pretty low compression ratio numbers in this forum and maybe this was an opportunity to improve things a bit. More fun than cleaning house anyhow.
Pulled the head . The casting was much nicer than my older Metro (part of the "redesign"?) clean, generous cooling passages, everything lined up nicely. Checked the deck height -.070"-damn. Pulled the barrel and found 2 1/2 gaskets! A .007 gasket had been torn almost perfectly in half perpendicular to the crank. (Has anyone seen that before? Please tell me it's not the way they true their block decks.) The stud holes were cut to the outside and I'm guessing (hoping) that they use tear aways during assembly and the lunch whistle blew at the wrong time.
Measured the head volumes and did the math and came up with a CR of just over 14:1. Dismal. The spherical cut in the stock plug held 5.5 cc's and if it was eliminated, it would still only bring me up to just under 16:1. Decided to reduce the plugs volume but keep a spherical shape. A 2 inch circle fit nicely in the stock plugs cut, I'd use a 3 1/2 inch circle for my sphere (or about the size of a wornout 4 1/2 inch grinding disk).
Real machinists should find another thread at this point. Everyone else should understand that YOU SHOULD NOT DO THIS! At least not this way.
I needed to spin the plug to make a reasonably accurate cut, the idea being that I'm going to take the Utterpower plug and cut it back to the dimensions of the stock plug and then give it a shallow spherical face. I am without a lathe so the trusty drillpress and a mandrel seemed like the way to go. The Utterpower glowplug setup uses a CS valve plug that has been machined to accept a glowplug. Because the glowplug sits deep in the plug they use a brass rod that screws onto the GP as an electrical extension. A fiber composition plug fits snugly over the brass rod and into the bore hole of the plug. A nice setup. Probably not designed to be a mandrel though. Tiny threads, soft metal. I knew that. What we need is a 12X1.25 bolt to go in the GP hole but, alas, that's not real common and the nearest one is probably as far away as the machine shop. So..if we're real careful, (and we were) and it spins reasonably true (and it did) maybe it will work (got lucky). Took an old glow plug that I'd saved for just such an occasion
, cut the nose off and reassembled the plug and extension. Chucked it in the drill press and ran the table up til it was just touching to try and stabllize things as much as possible. Spun it slow and gently put a hacksaw to a line I'd made and, by golly,it worked. Cut the last bit by hand to avoid binding. Got the table out of the way and held the angle grinder, precicely positioned by eye, with the edge of the wheel to the center of the plug. Carved a lovely sphere. Polished it with a stone and the darn thing looks fine. Capacity is just over 1 cc so thats a reduction of 4.5cc which puts the CR at 14.86:1 Maybe I screwed up the math or measurment someplace.
Anyhow, engine runs well, but then it did before too. Maybe a little less smoke. The glowplug is great, first compression stroke fires.
Bill