An M.G. Locost Build
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November 6, 2011
Tap Lessons

We're feeling much better these days about our Locost suspension. We have both of the upper A-arms complete and ready to weld. The new jig provides for plenty of camber adjustment at the rod ends, and all of the tubes are perfectly lined up. Or at least it looks that way. I'm sure they'll be fine.

  Tubes loaded into 2.0 version of the UCA jig
click to enlarge

Tapping the last two threaded tubes went fairly well, after totally destroying one unfortunate tube, once we discovered that cheap hardware store taps were never intended to cut threads. They'll cut a few, maybe five or six, but then the tap gets dull. You can keep twisting the dullified tap, but it's no longer making real threads, just sort of gouging spiral grooves. It takes a long tap handle and a mighty effort to keep turning the tap when the upper part of the tap tries to work its way through those imitation threads, and starts to bind.

We did manage to get one tube threaded, but it took an hour and we had to run the tap through several times in both directions before we could thread in a bolt. We tried using the same tap on the second tube, but it just mocked us. So we bought a second cheap hardware store tap, naturally expecting it to work better than the first one, which of course it didn't. So we bought a third cheap hardware store tap, and it was around this time that we figured out the true nature of cheap hardware store taps. But we did find a way to make them work.

Hardware store tapsónot the best  
click to enlarge

What worked is this: Cut four rough threads with an old tap, then chase those threads with a brand new tap. Cut four more with the old tap, chase with the new. The old tap, with a little effort (okay, a lot of effort), can still knock out a few fake threads, and the new tap puts a nice crisp edge on those threads with almost no effort at all. By doing it this way, the old tap never binds, because the new tap has made real threads behind it, and when youíre done you can thread a bolt or rod end all the way through. So that worked pretty good.

What works even better is this: Spend more than $4.99 on taps. Our local Ace hardware store carries your basic carbon steel plug taps, and yours probably does too. These can be used over and over again to chase rusty threads, but that's about it. Because they're plug taps, they don't really have enough taper to cut new threads. So it's a challenge. But there's a certain amount of satisfaction in tapping holes with the cheapest tools possible, so consider that before you go online to buy those high-speed taper taps from McMaster-Carr.

Work continues slowly but steadily on the lower A-arms, and we're starting to see a light at the end of the suspension tunnel (thatíll be a much better metaphor when we're working on the transmission). Suspension brackets are starting to take shape and we're pretty sure the original LCA jig is going to go the distance. We're hoping in a week or so weíll be able to get everything welded up, and thatíll be one major hurdle out of the way. Welding the frame should be a snap by comparison.


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