I was torquing down the lower transmission bolts that mount the transmission to the engine on my 2001 Camry V6 and one of the bolts broke. It is the middle of the three smaller lower bolts. I have been trying to drill into the bolt first with Titanium coated HSS and then with cobalt HSS bits with very little success. I was curious if this bolt is that necessary if all of the other seven bolts are in place and torqued down. There are five larger bolts supporting the top portion of the transmission/engine interface and the two smaller on the bottom. I have ordered an Irwin left handed cobalt drill bit set to see if the Irwin cobalt bits can make any better progress than the Milwaukee cobalt bit I got from Home Depot. Any insight would be greatly appreciated. Thanks.

 

Photo?

Suggest a carbide tipped drill, slower speed or variable speed drill motor (used a slow speed) plus some kind of lubricate.

Make sure the hole is as close to the center of the bolt as possible..

 

There is a ridge on the bottom that makes it difficult to get a bit centered. The punch doesn't seem to have much trouble making a mark but I don't get much material coming out when I drill. I picked up a carbide tipped masonry drill bit 1/8" but it didn't do much after a few minutes of drilling either, just some very small bits on the end of the bit. I could here it "clicking" inside the hole as it slid over the ridges of the punch marks but didn't do much cutting. I don't know if this is the type of bit you to which you were referring.

 

A few thoughts.Are there any issues with accessing a drill motor into the area to drill a straight hole?

You might find a drill the same diameter as the hole in the transmission case. This drill will center on the trans hole and thus the point of the drill center on the broken bolt face. You need only drill out enough bolt material to clean up the broken bolt face and create a centered dimple from the angled drill point.

The type of drill noted can be described as typical spiral fluted drill used to drill into metal but having carbide inserts brazed in at the tip for cutting. Think of it as a circular saw blade having carbide inserts. One can get a saw blades with and without inserts and the same for drill bits.

Carbide is tough enough to cut but the drill body being a cutting steel is flexible enough not to break if abused. There are solid carbide drills but these are brittle (can't take side loads) and more expensive.

If you can make a mark with a punch perhaps a decent drill made of high speed steel (HSS) will do the job.

You need a drill made to cut steel as it will have correct bit cutting teeth geometry. You want to rotate the drill at something like 2000 RPM and use a lube such as oil.

If you have the time might be able to find a drill bushing to use. The OD would be the same as ID of the transmission hole. The bushing hole would be the size of the drill you are wanting to use to create a hole. Since you are only using the bushing only once to drill one hole can make it out of plastic or even wood.

Make the hole in the broken bolt the same size as recommended by the tool maker for the size of extractor being used.

 

There was an issue getting a drill in straight but I bought a HF right angle drill to overcome that problem.
I had the same thought on trying to find a bit to see if I could use the bolt hole in the engine as a guide to try and normalize the surface of the broken bolt to some degree.
The carbide drill bit I purchased is as you describe I think. I have been trying to use HSS drill bits and HSSCobalt but they only seem to take a tipfull of tiny bits after several minutes of drilling. I am getting a higher grade Irwin HSSCobalt left handed set today at some point and will see how those do.
The bushing idea is great. I will see what I can come up with.
I just cant figure out why i am not getting more material out, maybe my expectations are too high but it will take hours to drill the bolt out at the rate I am achieving presently.

 

This image is close to the 1/8" Carbide tipped bit I purchased to try and get a hole started in the bolt.

 

If the bit is just riding over the top of the bumps on the bolt face you may need to apply more pressure. This is where a bushing helps to center the drill and keep it from walking all over the place. If the surface is irregular the bit will walk around and not cut.

As to actual cutting don't know the specs on the bit you have. Unless some special hardened bolt, then even an high speed drill (HSS) should be able to cut the bolt reasonable fast. You will know its cutting because of the chips generated.

Check the cutting surfaces of the bit for damage or wear. Too much drill spinning and not enough cutting leads to overheating of the drill. The drill hopping around could cause damage.

My two cents is to try to clean up the surface of the bolt using a larger dia drill. This clean up will allow the smaller drill to fully contact the bolt face instead of moving around. If the bit is centered and stable so it can't move around, apply pressure to get it to cut. Use oil as a lube.

Below is a link to various drill types. Look at the carbide tipped 1/2 inch reduced shank photo to get an idea of the drill construction writing about.

The keys to success are to get the bit centered on a smooth surface where it can't jump around. Then apply pressure at slower RPM's using lube. You might try drilling into the smooth face of a unbroken bolt to get experience and feel. Don't have too high of RPM or you will overheat the bit.

When the bit starts generating chips and the hole starts to get deep, from time to time pull the bit back out to extract said chips. Drilling a deep hole means the chips may not self extract but pack up and possibly break the drill.

 

Success!
The Irwin HSS Cobalt drills made an incredible difference. The Milwaukee cobalt drill wouldn't do anything but the Irwin bits ate right into the bolt. The ground down 3/8" ID plastic pipe fitting was my attempt at a bushing. As you can see I didn't get rid of the ridge because there wasn't enough room for me to get my right angle drill in with the length of a bigger bit. The Irwin bits were considerable shorter but the diameter of the biggest bit wasn't enough to completely fill the hole but the bushing kept the bit in position enough that I could normalize most of the face of the bolt and only get a bit off center. Probably would have had an issue if the bolt had been fused. The extractor I used was a size smaller than what was recommended.

 

The replacement bolts I got from AutoZone are harder than the originals that broke and stretched. (rated 10.9). The recommended torque is considerable more than the 34 ft-lbs (55 ft-lbs). Will that cause an issue? They didn't have the correct size in the 8.8 hardness which has a recommended torque of 40 ft-lbs. I could put a lock washer on these bolts if that torque isn't going to be enough to hold them. I don't want to torque the bolts to 55 ft-lbs for fear of ripping out the threads...

 

.....which leads me to this. The bolt on the left is one of the two bolts that holds the cover that covers the access to the torque converter bolts and provides the bolt hole for one side of the strap that supports the exhaust pipe. (You can see these bolts in the picture in entry #3 above. They are to the right of the hole where the broken bolt was located.) The bolt on the right is the broken bolt that I pulled out used for comparison.

The manual refers to a "hole plug mounting bolt". Singular, it doesn't say bolts and it says to torque the "hole plug mounting bolt" to 15 ft-lbs, the same torque as the other side of the bracket which has studs that are a size bigger than the bolt you see above. 15 ft-lbs seemed a little excessive for this small of a bolt. I thought maybe the manual was referring to the bolt that goes into the mount that holds the exhaust support strap but the manual shows the strap bolts need to be torqued to 24 ft-lbs. I was leary about the 15 ft-lbs so I went slow as I was afraid I was going to break another bolt. I never got close to 15 ft-lbs when the bolt pulled out the threads. This bolt is quite short and there are quite a few threads that are left unstripped in the bolt hole. I am going to see if I can find a longer bolt to replace this one.

Followup: This is a 6mm bolt with 2 dashes (6T). Service specs say a 6T 6mm bolt should be torqued between 69 (no flange) and 78 in-lbs (with flange). This bolt has a washer so I am not sure which one it should be. Definitely shouldn't be the 15 ft-lbs listed in the Automatic Transmission section. 15 ft-lbs is for an 8mm 6T bolt with a flange so this section has an error here.