0

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!

Justin Todd
  • 588
  • 1
  • 12
  • 29
  • You match the tap to the thread dimensions of the bolt needed. Drill bit size matches the tap size and there are tables online for this. A tap and die set will probably be more handy than just getting a single tap and tap wrench. Not sure if honda uses standard or metric threads. Check if the rust is just on the threads or has eaten into the metal. – crip659 Jun 22 '23 at 00:28
  • 1
    I’m a little surprised that they screw into the frame instead of bolt through the frame… – Jon Custer Jun 22 '23 at 00:58
  • CURT 13555 Class 3 Trailer Hitch, 2-Inch Receiver, Fits Select Honda CR-V https://a.co/d/7UTfSku that is the link to the product. I was surprised it wasn't through bolted either – Justin Todd Jun 22 '23 at 01:03
  • 2
    I’m voting to close this question because It is not about home improvement. – RMDman Jun 22 '23 at 02:13
  • I'd vote to close this question because I do everything except this... and garage doors. You're a noob and you want to drill, then tap threads into rusty metal, upside down and in the dark? Naw man. RTFM ("Weldnut cleaning" @JonCuster ;) – Mazura Jun 22 '23 at 02:53
  • 1
    Those are weldnuts. do not drill and tap into what you think is car body from the 70s; it ain't. – Mazura Jun 22 '23 at 03:12
  • 1
    I’m voting to close this question because this question belongs on "mechanics" – Jasen Jun 22 '23 at 10:47

2 Answers2

6

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.

MTA
  • 11,238
  • 1
  • 17
  • 37
  • So a comment on the hitch that I bought on Amazon said they used a 10mm x 1.25 tap to chase the threads which worked for them. How would I find the specific pitch? – Justin Todd Jun 22 '23 at 01:02
  • 2
    I believe the 1.25 number is the pitch. 1.25 mm, or approximately 20 threads per inch. – SteveSh Jun 22 '23 at 01:18
  • @JustinTodd see: https://diy.stackexchange.com/a/215225/18078 – Ecnerwal Jun 22 '23 at 02:59
  • 1
    M10x1. 25 is the standard way to denote that thread, so is the tap you'd need to buy according to the amazon commenter. I'd buy a thread gauge too, and measure it. Some tap and die sets include them – Chris H Jun 22 '23 at 07:00
  • Just wanted to let you know that the tap worked perfectly and got almost all the rust out and made a perfect surface for the bolt to go in! – Justin Todd Jul 31 '23 at 20:36
  • @JustinTodd Excellent! Nice to know that it worked out. – MTA Jul 31 '23 at 21:21
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.

Harper - Reinstate Monica
  • 300,628
  • 26
  • 286
  • 734
  • The problem with retapping only a little bigger and a different pitch is that the threads move in and out of phase. So you lose material in some places meaning the effective thread engagement is less than you think. It also tries to pull the tap around. So it's doable but requires more care than you might think. On a removable part I'd mount the tap in my drill press, to hold it square to the workpiece (turning it by hand). Cleaning up the existing thread would be far better, and if it worked in a similar place on my 15-year-old Transit in a wet climate, there's hope for this newer Honda – Chris H Jun 23 '23 at 13:48
  • @ChrisH that's exactly why I picked 7/16... the thread ID on 7/16-14 is a tick bigger than the thread OD on M10-1.25. – Harper - Reinstate Monica Jun 23 '23 at 18:23
  • Ah OK, you didn't mention comparing the ID of one to the OD of the other in the answer, and inch fractions don't compare readily to metric as I only use the latter. There are a few pitches of M11 that might be interesting too. It's a shame it's not M10×1.5, because then you could open it up to M11×1.5 (in a similar way to using a slightly bigger thread of the same pitch with a helicoil). M11 of course is rare; I don't easily available your options are – Chris H Jun 23 '23 at 18:35
  • @ChrisH M11x1.25 thread pitch is a tough thing to find. In America it's actually much easier to find M11.1x1.27, better known by its other name, 7/16-20. :) And the difference of pitch 1.25 vs 1.27 isn't going to make a difference on a 10mm high nut. – Harper - Reinstate Monica Jun 23 '23 at 19:11