6

This is to do with the screw that goes into the seat rail of my car. The screw is a Torx screw and someone managed to cross-thread it and they then replaced it with a regular screw of a larger size. I now have mismatched screws for the seat rail and I would like (purely out of OCD) to go back to the factory size screw. The problem is that the screw hole is probably too big now.

My question is: How do I refill a metal hole and tap the correct sized hole for the factory screw? Can I do this with epoxy or perhaps something like https://www.doitbest.com/products/701351?

I've never DIYed before so please forgive the stupid questions I might ask!

migrated from diy.stackexchange.com Mar 5 '18 at 15:19

This question came from our site for contractors and serious DIYers.

  • 2
    that's going to need welding and will not be a beginner's diy job. – ratchet freak Mar 5 '18 at 9:35
  • Is this a machine screw that is inserted into a threaded hole, or a sheet metal screw that forms its own threads as it is driven into sheet metal? – fred_dot_u Mar 5 '18 at 10:24
  • I believe it's the latter @fred_dot_u. – user1936752 Mar 5 '18 at 10:30
  • This has been answered for you here : mechanics.stackexchange.com/q/52187/10976 – Solar Mike Mar 5 '18 at 15:22
10

Allowing for the answer that the screw threads into sheet metal, a helicoil will not provide sufficient grip as suggested in Plan B above.

One option that is likely to work reasonably well is to use a product called a rivnut. It is similar to a pop-rivet in that it is installed from only one side and also distorts upon installation.

rivnut installation

One would exercise caution during the draw-back portion, as it is possible to strip out the threads of the rivnut (also known as nutsert) making it necessary to drill out the insert and hope to not enlarge the hole.

An advantage of the rivnut process is that an existing stripped hole is likely to be smaller than the hole necessary for the rivnut installation, allowing for a clean hole to be drilled.

rivnut

You may be able to find a body shop to install a single rivnut for you at minimal cost. Even a Harbor Freight tool is an appreciable expense for a single hole repair, although having one on hand for future projects is often valuable. I had in the past purchased the HF tool, but eventually wore it out and purchased a more expensive higher quality version.

harbor freight tool

Post migration edit:

I realized hours into the previous evening that my answer does not properly solve the problem. The original screw is a sheet metal screw, while rivnuts are designed for machine screw threads. If the OP has a minor OCD issue with mismatched screws, this solution will not correct that aspect.

On the flip side, one could drill out the other holes and place rivnuts in those locations, providing matches if desired. I've done this on projects in order to keep the fasteners alike across the board.

  • Thank you! Again I learned about an item which I didn't know before. – glglgl Mar 5 '18 at 14:26
6

seat rail of my car

I'm just going to stop you right there and hopefully help you understand that this is a critical safety fastener, and you should not consider this lightly. During a vehicle crash your seat must stay fastened to this location for the entirely of the crash for the rest of the vehicle's safety systems to have their full intended effect.

Should this particular fastener fail, you have 3 more backing it up, but the wrong forces could cause a sequence of failures that results in greater injury to the occupant of that seat than would have occurred in the original design.

The seat bolts aren't just hard to remove by design, they are designed and use materials that are different than other fasteners in the vehicle. Seat bolts, for instance, should typically never be re-used, and they aren't meant to be removed and replaced in an unlimited fashion, eventually the receiving nut/rivet also wears out with subsequent replacements.

For instance, I've run across several seat bolts from a big-three automotive manufacturer with a self-tapping, tri-lobular bolt such as this:

enter image description here

The rivets they fasten into are not pre-tapped, the bolts create the necessary thread during initial installation. They are famously difficult to remove, and replacing them with the correct bolt and doing so without cross-threading them is also a challenge.

If you cannot take it to an authorized manufacturer and have them supply the correct repair I wouldn't consider "fixing" the issue you're facing without doing significant research into how your solutions are equivalent to the original fastener. I can tell you that typical fastener solutions are going to be inadequate for this specific usage.

Understanding bolt grades, pull strength, sheer strength, vibration tolerance, bolt force at the correct torque, etc is no small thing.

Also, be aware that the correct repair may have already been done. The way a vehicle is repaired when body damage occurs may have a result that isn't the same as the original design at the factory. Such differences occurs mostly for cost savings, but the end result is that the vehicle must still function according to their design and safety standards. In this case it may be that the correct fix is replacing the bolt with a larger bolt since the original rivot/nut cannot be repaired to the safety standards within the cost target of that repair.

As such I do not recommend seat and other safety systems repairs on vehicles as a "diy" project, particularly if it's just an aesthetic issue.

4

Plan A

  • Disassemble nearby parts, remove them (including carpet, cabling etc)
  • Cut out the part containing the damaged hole
  • Read about stick, TIG and MIG welding
  • Go on a welding course
  • Buy large amounts of welding equipment
  • Upgrade the power supply to your garage
  • Weld in new steel
  • Grind welds flat
  • Drill a new hole at correct location
  • Tap hole for correct thread
  • Prime and paint

Plan B

  • Buy a helicoil insert of an appropriate size.
  • Drill the hole out to the larger size needed.
  • Tap the hole for the larger thread.
  • Insert the insert.

Plan C

  • The horrific kludge you mentioned which will annoy your OCD every time you sit in, look at or think about your car or any car.
  • $100,000 of hypnotherapy to help you forget.

Plan D

  • Replace the car. You've worn that one out.
  • You can simplify Plan A quite a bit if you have already have a welding setup to use, or a friend who can do the welding for you. A helicoil insert (Plan B) is a pretty cheap effective option, provided the metal with the screw hole in it is fairly thick - if it's not, I wouldn't trust it to hold my car's seat in place. – Sean the Bean Mar 5 '18 at 14:59
0

I would mainly want to make sure the connection was made properly. Force-threading the wrong size screw into the hole doesn't give much confidence; the new screw may have been damaged itself when inserted like that.

I would drill out and tap the hole to the next larger size up, and purchase the correct type (grade, finish, etc.) of screw for that size, and go with that. It won't be worse than what you have now and by using a larger fastener may actually be stronger than the original.

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