Tap and die for trailer hitch

Question

I am trying to install a trailer hitch on my 2010 Honda CRV LX to tow 2 bicycles behind my CRV. The trailer hitch set that I bought included 4 bolts that bolt directly into the frame. My honda has 4 holes on the frame that are perfectly lined up to the holes on my trailer hitch mount.

My issue is that the threads for all 4 holes in the frame of my CRV are rusted. I tried soaking them in wd-40 and brush out the old rust but I am still not able to get the bolts in the hole.

I figured my next best course of action before giving up and going to a shop would be to drill out a slightly larger hole for each of the 4 screw holes and then use a tap and die set to rethread the frame. I am a complete newbie to this. I was looking to see if there was a chart or some guidance on what size tap and die you would use for a specific diameter that I drill out.

From what I can tell, I need to use a 3/8" drill bit to drill out the entirety rusted threads. After I have a clean hole, I was wondering how would I learn which size tap I would use to create the new threads in the newly drilled out 3/8" hole? After that, I was wondering if I could just go to a hardware shop to get a new larger and possibly stronger bolt with the same threading to match the new hole.

Thanks for the help!

Answer 1

An easier / better approach would be to restore usable threads to the existing holes. Don't drill anything.

Get a tap that exactly matches the bolts that came with your trailer hitch for both diameter and pitch. Use that tap with oil to slowly, carefully work the tap into the rusty threaded hole. Be careful not to cut new threads. Use the existing threads and clean them up.

Turn the tap handle no more than 1/4 turn at a time, back it out to clear out debris, then turn it back in till there's resistance and go another quarter turn. Keep it up until the hole is fully threaded.

Use plenty of oil.

Answer 2

Try cleaning up the thread

I tried soaking them in wd-40 and brush out the old rust but I am still not able to get the bolts in the hole.

Well, that's not going to do anything at all. I gather you're not a restoration machinist? :) Yeah, WD-40 isn't even penetrating oil, and is also inferior as a lubricant. You'll need to run at least thread-chasers through it to get that rust out of there and see what kind of material is left. Get some Liquid Wrench which is actually cheaper than WD-40, and save that WD-40 for the off chance you find yourself with a Titan missile to polish, because that's what it's designed for.

The thread appears to be M10-1.25. That means 10mm nominal OD on the threads (actually slightly smaller), and thread pitch of 1.25mm per thread.

I'd get an M10-1.25 tap - they're like four bucks - and run it in there very gingerly after learning how to use a tap. Make sure to squarely load the tap shaft - apply ONLY rotational force and not leaning force. Don't go at it with a ratchet, use a T-handle with equal force both sides. DO NOT break the tap off in the hole - they are brittle. When it gets stiff, back out and come back in. 3/4 turn in, 1/4 turn back or even 1/2 turn back if the going gets tough. Lots of lubricant, doesn't have to be great lubricant (hey! A role for WD-40!) Using a tap to bash through rust is quite hard on the tap, but again, they're $4. Still, treat it with respect - a tap's revenge is to break off in the hole, and they're hard to get out.

The tap will clean up the hole. Once that's done, you can see how loosy-goosey the bolts fit in. If they're snug you're probably fine.

Or, drill out and tap a larger size.

If they're woobly the threads may have rusted away. Now let's talk about tapping. You don't just pick your favorite hole size and go "what tap will fit this?" because there aren't that many screw sizes. No, you decide what screw thread you want, and then you decide which tap to use. In this case since you're enlarging an existing tapped hole, you should aim for a tap drill slightly larger than the Outside Diameter of the existing threads, which itself is slightly smaller than the nameplate bolt size (10mm).

So what tap wants a hole just about 10mm? That'd be a 25/64" drill hole for a 7/16"-14 * coarse thread tap. If the 25/64" hole is more rust than shiny, step up to a 13/32" drill and use 7/16"-20 *fine thread tap.

Thinking about it more, 10x1.25 is a fine-thread fastener. I'm not sure why Honda chose it, but it makes sense (if you have to drill out and re-tap) to follow Honda's lead and go fine-thread as well. Alternately, use the coarse 7/16-14 and then cap it with another nut for more thread purchase.

7/16" is an oddball size but it gives you the best chance of preserving the remaining "meat" in those welded-on nuts on the frame. Do not hop up to the next metric size of M12, because that's a big leap and won't leave a lot of OE nut to hold your trailer on. And certainly not 1/2" (M12.7).

Or, drill out and don't tap

Depending on your access, if you can get to the top of the welded-in nut, one option is to drill it out (or 25/64" to keep your options open) and then run a 10mm bolt of appropriate length and strength through it, and cap with a nut or nuts up top, stacked on the OEM nut.

* 7/16-14 is the same as M11.1x1.8.

** 7/16-20 is the same as M11.1x1.27.