Drill Guide for Broken Bolts

A friend has a vehicle with a GM 6.5 L engine in it. Prior to his ownership, one of the two starter mounting bolts was broken off. The hole for the mounting bolt is unthreaded for about a half-inch or so, and (of course) this is where the bolt broke -- recessed in the bottom of the engine block.

He had tried some left-hand drill bits but was not able to start drilling in the end of the bolt. The break was at an angle, and the drill bit kept skating off to the side.

I offered to make him a drill guide. I also recommended a long-reach end mill to use with the guide to make a flat starting point for the drills.

I made the guide mount from a piece of scrap material. The slotted hole was already in the scrap when I acquired it. I added the clamping hole, slit, and clamp screw. I made guides for three different sizes of drill bits. The two smaller guides have a nose (turned down to the diameter of the starter mounting bolts) to aid in aligning the mount to the hole in the engine block.

Attachment 46380 Attachment 46381 Attachment 46382

The design of the mount and guides allowed for the use of carbide-tipped drill bits where the tip is slightly larger than the drill shank. The drill shank is inserted through the guide and into the drill chuck. The guide is then clamped into the mount. The guide is also easily removed to get the chips & chunks out of the bolt hole.

Here are a couple pictures with the mount in place on the engine block.

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Using this tool and the end mill, he was able to remove the bolt with some effort. The damage to the engine block (from the early drilling attempts) was significant enough that he had to install a threaded insert to be able to mount the starter.