Proper technique to enlarge a hole

[dutchgray]

If you have already drilled tap size for 5/8 18 you will be oversized for 5/8 11.
You could still tap it fine but you wouldn't be getting all the extra strength you think you were getting.

 

[mreisner]

I have ran into the same problem trying to drill hubs on dump trucks for new studs for wheel wedges. Turned out the problem was I was using a cordless drill that had just enough movement in the chuck because it was also able to be a hammer drill. I switched to an old fashioned corded drill that all it did was drill and had no problems after. Before with the cordless it grabbed badly.

 

[manoweb]

If you have already drilled tap size for 5/8 18 you will be oversized for 5/8 11.
You could still tap it fine but you wouldn't be getting all the extra strength you think you were getting.

yeah the original stripped hole was already too big to tap drill for 5/8-11. I could drill it out again for 3/4-10...

 

[manoweb]

I'd suggest tapping 5/8-11; these threads will be far more resistant to pulling out and loosening due to vibration, esp. if the cast iron is of less than stellar quality. You'll get a lot more shear area in the cast iron with the coarse threads.

But fine thread is often recommended for vibration, and yes fine has less shear area but has more threads per unit of length so it should compensate? I understand that the base material is the limit here

 

[LopezBart]

Try the fine thread, I guess. I'd avoid too much torque.

 

[manoweb]

OK I placed an order for a 21/32" drill and a 3/4"-10 tap set. The fine threads in unknown quality cast material were bothering me after I did some more research so I'll size up and go coarse. In fact I'm going to save some money as 3/4 coarse hardware is more common

 

[dnschmidt]

Or take the easy way out and use something like an ASTRO Turbo drill bit which has a step drill on the end of it. Harry J. Epstein sells Norseman bits (that actually infringe on ASTRO's patent) individually. Look up Norseman Vortex bits on their website and you'll go to the various sizes. The largest of these is 1/2" so if you got a monster bolt you're out of luck. The best lubricant is Anchor Lube which is a paste so it doesn't run out all over the place.

 

[larry_g]

I did not think about using a stud, I am not sure what would be the advantage over bolts. These four bolts are just used to fix a plate in place, that it not very unwieldy; the bottom of the front loader attachment then bolts to this plate.

At the time of the suggestion I thought you you were trying to salvage the existing hole and the stud can use 100% of the hole depth maybe using unmolested threads at the bottom of the hole. Even going over sized to 3/4 use the longest bolt you can without bottoming in the hole. Good luck.

lg

 

[rustyzman]

The move to a coarse thread in cast iron is wise. Fine threads are generally not ideal in cast iron.