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I purchased a used jointer. The fence should've been held by two M6x20 bolts (according to the manual) to the body of the jointer. When I went to attach the fence, I noticed two wood screws and the holes were filled with some unknown material. After I cleaned out the two holes I noticed they were stripped, that explained the missing M6 screws.

To which size can I re-tap the holes? I think M8 is perhaps too big, so what about SAE? Is there something close to M6 size?

By the way, I'm confused by the tap sizes, especially the SAE, UNC, UNF, etc. Besides that, how do I choose the right drill bit size for the new tap? I've spent a whole day trying to figure it out - I'm now frustrated.

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    I'd like to see the parts. You assert that M8 is too big, but is it really? A re-tap to that size might be the dead-simple solution. – isherwood Dec 28 '20 at 15:26
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Tap size chart and Drill Size Chart are the useful search terms. Most any useful tap size chart will have the drill size listed. A drill size chart will help you sort out the relative sizes of drills in fractional inch, number, letter, and metric sizes. Taps you can get at the hardware store may simplify the process by being packaged with the correct tap drill. Ordering on line from a "real metalwork supplier" (often less expensive, and far more selection) you'll likely have to order them separately.

What will work depends just how stripped out it is.

1/4-28 or 1/4-20 are the "next size up" with 1/4-28 offering a bit more allowance for hosed threads since it's a finer thread (and thus a bigger tap drill - #3 = 5.4102mm) but if the holes are stripped all the way to 6mm that's still a bit precarious. 1/4-20 uses a #7 tap drill = 5.1054mm)

The rather unusual but extant M7 is the first size that will work if the holes are ripped out to 6mm - 7x1 uses a 6mm tap drill, 7x0.75 uses a 6.2mm tap drill.

Next up from that is 5/16-18 (F tap drill = 6.5278mm) or -24 (I tap drill = 6.9088mm)

  • UNC = Unified National Coarse
  • UNF = Unified National Fine
  • UNEF = Unified National Extra-Fine

SAE (society of automotive engineers) is an older standard that has been incorporated into the Unified Thread Standard.

You can also repair the threads to M6x1 by using a Heli-Coil® thread repair kit (or possibly generic knockoffs if their patents have expired) where the hole is drilled out larger, larger threads are made in the larger hole, and a Helical Coil that fits in the new threads on the outside and has M6x1 threads on the inside is inserted in the hole. [I have used this product rather extensively, but am not associated in any other way.]

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  • One advantage of the heli-coil is it comes with the correct drill, tap and insert. – Platinum Goose Dec 28 '20 at 4:13
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    +1 for the helicoil repair that is the way you do it. Putting a larger bolt in there may have problems with the bolt holes for the fence. You can find Helicoil kits on Ebay where some of the inserts have been used for very cheap, buy the kit, fix the threads and if you still have inserts left then sell the kit on Ebay where it will go to the next guy. – Ted Mittelstaedt Dec 28 '20 at 9:03
  • M7x1 is rare, and a pain to get hold of, but is certainly real - I bought some nuts on eBay recently. If you've got room to drill out the mating parts and room for the heads, that's a good solution. You may well be able to put an M7x1 tap straight in without drilling, but you'd need to drill for the even rarer M7x0.75 – Chris H Dec 28 '20 at 15:37
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    I can sort of understand why someone in the USA would want to repair a device built with ISO standard size parts using some parochial unit system like "inches" (whatever they are!) but that is pretty useless advice for anywhere else in the world. – alephzero Dec 28 '20 at 19:38
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    +1 on helicoil , remember to use red loctite on the helicoil outer threads because vibration will loosen the fastener over time , also ask the manufacturer for what thread pitch the bolt is , ie m6 x1.25 – binaryOps20 Dec 29 '20 at 0:33
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Depending on the construction of your jointer, a potential fix is to adhere a small plate to the table on one side of the stripped out hole, and drill and tap into that plate.

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  • Ecnerwal's Heli-Coil solution will very likely be much cleaner and easier. – Tim Sparkles Dec 28 '20 at 18:15
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    Not if one doesn't have a Heli-coil laying around. I'm a big fan of the "use stuff you already have" approach to living. – isherwood Dec 28 '20 at 20:33
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    If one could use a plate, it might also be possible to use nuts, washers and longer screws. – Dave X Dec 28 '20 at 22:54
  • @DaveX certainly might be possible to do that, though constraining the bolt/screw by screwing it into a properly fit and tapped hole seems much more secure than constraining it via the friction of a washer and the screwhead. – Tim Sparkles Jan 11 at 21:49
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Consider tapping to M7 if M8 is actually too big. If you're in the US you can get M7x1 20mm socket-head cap screws or hex-head screws, as well as a tap, from McMaster.

If you're in a metric country, it will probably be as available and cheaper.

The size of a stripped-out M6 hole (~6mm) is about right for an M7 tap (6.1mm)

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  • I live in a metric country and I consider M7 rather exotic. I know where to find them if I really want, but I am pretty sure the nearest hardware shop has none of them. – fraxinus Dec 29 '20 at 13:24
  • @fraxinus We have mixed here in Canada, but 'odd' SAE sizes like #5 (~3mm), #3 (~2.5mm) are significantly more rare than #4, #6. Same with AWG (wire) odd sizes. But when you need it, it can be ordered. – Spehro Pefhany Dec 29 '20 at 16:43

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