An M.G. Locost Build
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October 6, 2011
Tapping

We've been cutting, grinding and filing more A-arm hardware, and accumulating all of the cuts, scrapes, bruises, and burns that go with it. We understand that all of this tedious metal work has to be done, but it doesn't feel a lot like we're actually building a car right now. We got the first of four upper A-arm tubes threaded for a rod end, which is good start, but it turned out to be a much bigger chore than we would’ve thought.

To make our one threaded tube, we started out with 3/4" DOM tubing with a 7/16” hole in the middle. The hole had to be drilled out to 29/64” for the tap, and it turns out that unless the drill bit is exactly on center, the drill fights you all the way. Hand drills. Not the best. The good news is, once you’re about an inch into the hole, you’re pretty much forced to be centered, and after that the drilling goes well. Or not well, but you know, fairly smoothly.

  The Joys of Tapping
click to enlarge

Likewise the 1/2"x20 plug tap has to be exactly centered and perfectly lined up with the hole or it'll cut a couple of threads and then bind. The best way we found to center the tap is to drill a half-inch hole down the tube about 3/8” deep, which forces the tap to be already straight by the time it starts cutting threads. When you’re done you just need to figure out what to do with that first 3/8” of the tube. One option would be to cut it off. We couldn't think of any other options so that's what we did. Luckily we made the tube a little longer than we needed.

Tapping just one tube took over an hour. That's 35 threads in a 1-3/4" long tube. We thought that after we got our hole drilled that this part would at least be a little more fun, but the tap fought us all the way, especially toward the end. A taper tap might've been a little easier if we could've found one. The plug tap we used was brand new when we started and looks now like it's been through a war. Not sure it's up for another three tubes.

Suspension Bracket Blanks  
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To avoid tapping more tubes, we cut our 12 suspension brackets out of 2” square tubing. These were a little more encouraging. Each bracket will have to be ground smooth on the sides, drilled for the suspension bolts (3/8"), and cut off at the top. It’s optimistically about two hours per bracket, but we're thinking it’ll be easy work. Somewhere down the road we'll need another eight brackets for the shocks, but I think we can make those with 1-3/4” square tubing, 1-1/2” inside dimension. Or maybe even narrower. We really should try to get a hold of a shock and measure it to be sure.

So far our PowerPoint suspension design seems to be holding up, although it's probably too soon to tell whether it’s actually going to work. As we earlier suggested we might, we did decide to go with 12-gauge tubing for the A-arms, instead of the recommended 16 gauge, because we'd prefer the suspension didn't fail, and the extra weight of the 12-gauge tubing is hardly a factor when you're bolting on giant cast iron MGB suspension hubs and solid steel kingpins. Which we're really looking forward to doing someday.

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Comments:  
posted May 14, 2016 at 17:46:20  
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