Im trying to help a neighbor fix a stripped bolt hole. A friend of his took the old bolt out, it's M10x1.25. The threads came out with it. So, he grabbed a bolt that was the next size up that he had, 7/16x14 and threaded it in. Worked great, the bolt tightened down and held. Then he went to put the spacer in, and it wont fit. Its hardened steel and can't drill it. How can I fix the original hole back to M10x1.25? Will a Time-sert work for this? The hole is to big to heli-coil it back to the original size. Any ideas?

would it be possible to get a special bit that will drill it or a less hard spacer that can be drilled? (assuming a less hard spacer would be adequate for that situation).

A good heli coil kit will enable you to get it back to 10mm, with the drill and tap that comes with it...being a good kit.

It currently has a helicoil in it for the 7/16x14. The 7/16 is slightly larger than the 10mm. That's what I'm trying to fix

Hardened 7/16 washers ???, Carbide tipped 7/16 bit ( think masonary )

Here it what I did when the bodyshop effed up a stud they were told not to touch on my superbird wing. I drilled and tapped it bigger 1/2" in this case, threaded a piece of aluminum rod on a lathe, screwed it in , tig welded it, then drilled and tapped that back to original size. It will work on steel or cast iron too.

I like the Time Serts, but the Kits are expensive. They have the normal thickness and double thickness if the hole is really messed up.
I think the Key Serts have both thicknesses. main difference is how the inserts lock. Key Sert you lock be driving pins down the side of the threads which may bot be good if the hole needs to seal, like going into a water jacket.
The time sert uses a counterbore bit so the insert has a lip on the top that keeps the insert from going down into the hole when using the thread forming tool. On the insert, the bottom few threads are fully formed, so when the thread forming tool is ran into the insert, it forms the last few threads while locking the insert into place. These work really good if needing to seal the hole. Because they are held in place by forming the last few threads, you do need to make sure to get the correct length insert.

Look up a guy abom79 on youtube just showed fixing a motorcycle head with a over sized insert the other day

Yup.

Thank you. This is more of what I was looking for. While making deliveries today, I asked a machinist customer and he was saying to just have a machine shop mill the hardened spacer to the correct size. Minimum $50. I like this better. Now to look if I have enough material in the block to go to that size. Glad I asked here before making a final decision.

as well. i have used these hundreds and hundreds of times over my 42 1/2+yrs as a machinist.
as to your spacer, drilling with a carbide masonry bit works like a knife through butter on grade 8 washers. however, most guys use WAY too high an RPM drilling any and every thing. slow with lots of oil will amaze you what you can drill through ! it even saves your bits from burning up. when you spin them at 10k RPM with all the force you can muster forcing the drill through the work piece, it becomes work hardened, and you will need at LEAST a carbide cutter to cut [or chew] through the [now] super hard, burned, surface.

Unfortunately its not a washer, but a 2" thick spacer. On the lowest speed on the drill press and plenty of cutting lube, I barely made a dent. Its the bushing that the timing belt tensioner pivots on.

OMG, these are a LIFE SAVER. I finally was able to get back to work on the car after ordering the tools and going on vacation. These are amazing. This is exactly what the job needed.

happy that worked out for you !