I finally had some uninterrupted time to work on the truck today so I remounted the loose spring mount. I first secured the mount tight to the frame using 3/8” bolts and bolts. On the first hole, I tried using progressive drills in slow speed,starting a 32nd of an inch larger and working up, but the bits would only dig in and stop the drill. If I ran it at high speed with light pressure it would drill, then start to chatter.
A trick I used as a machinist was to flatten the cutting edge of the bit slightly by either grinding or using a a small stone (so it scrapes rather then cuts). This would stop the bit from digging in, especially in soft materials like brass or copper but would also work on soft steel. Not having any sacrificial bits like I did when I was working and I not wanting to have to resharpen numerous bits, I decided to try to enlarge the holes using the just reamer.
This worked out nicely, fairly easy to drill and left very clean holes. I had previously bought grade 5, 1/2”-20 x 2” shoulder bolts, nuts and lock washers figuring on cutting to fit. Although the reamed holes were to size, the bolts measured undersize by about .002”, so I added some Loctite 660 to fill the gap.
This may spark a conversation on rivets vs bolts. The tensile strength of a single 1/2”x20 grade 5 bolt is 19,200 lbs. and the sheer strength is generally 60% of the tensile strength at 11,560 lbs. There are four of these bolts in two different planes working as both tensile and sheer function. I don’t think it will ever pose a problem. I will however check the torque on these bolts on a regular basis.
