The MGA With An Attitude

At 03:40 AM 8/13/03 -0400, Russell Goebel wrote:
>".... oil leaking from around the sump. Finding a couple of bolts not tightened properly, I tightened the first one only to have the bolt head break off. It looks as though the bolt was fractionally too long, and had bottomed. On removing the sump I discovered the broken bolt is in the main bearing end cap. Not looking forward to drilling a hole then using an extractor (easy-out) from beneath the car."

Absolutely do not try to use an easy-out to remove a bolt that broke after bottoming out in the hole. This is where the tool becomes known as the "easy-break". If you break the tool in the hole, then only recourse is to have it burned out with an ELOX machine (or EDM). Electronic Discharge machining uses a carbon electrode and high voltage alternating current to generate an electrical arc to burn the metal away at a rate of about 0.001 inch per sec. This takes several minutes to burn to a depth of 1/2", but it is very effective, very clean, and does not harm the surrounding material. You might find a shop with this capability in any large metro area, but expect to pay minimum billing of at least $25 for the service.

"I am thinking of removing the main bearing end cap and completing the job on a bench. Would removing the no3 main bearing cap, with the engine still in the vehicle, cause any damage ...."

No problem. We do that all the time to change bearings without pulling the engine. Just remove only one bearing cap at a time.

Then at 10:06 PM 8/14/03 +1000, Russell Goebel wrote:
>"Rereading your e-mail response, do you think it best to drill out the broken bolt and re-tap the thread - rather than use an easy-out?"

Yes. But it's a little tricky. And you really don't want to destroy the bearing cap, as replacing one of those requires complete disassembly of the engine for line boring the main bearing journals, which is quite expensive as well as labor intensive.

I have personally drilled out many broken bolts, and I would not hesitate to do it, but there are some problems involved. First, definitely remove the bearing cap and get it to the workbench for processing. If you should perchance break a drill bit in the hole, you might still be in for a trip to a shop for ELOX service, so do try not to break a drill bit.

When you drill out the bolt you have to have the drill bit very straight and very close to being dead on center of the screw. So start by grinding the exposed end of the screw flat and make a center punch mark as near to the center line of the thread as you can. Some skill and finesse is involved here, as the thread may not look entirely symmetrical from the end view of the broken bolt. The closer you can drill the hole to the center of the thread the easier the job will be.

Then of course a drill press will be a much better tool than a hand held drill for keeping the bit running straight and not breaking it. I don't have a drill press, and have done many of these with a hand drill, but It's a challenge to say the least. If you bugger it up with a hand drill it's not the end of the world, just more time involved to get it properly sorted out in the end.

One approach often mentioned is to use a left handed drill bit. Then when you get some depth to the drill, and especially when you get close to the root diameter of the thread, the rest of the broken screw has a good chance of simply unscrewing with the torque of the drill. As fate would have it, not many people have a good assortment of left handed drill bits on hand. I'm doing good if I can keep a full set of standard drill bits in good condition. Being squeaky tight, I could never bring myself to pop for the cost of a full set of left handed drill bits, just for the occasional chance of trying to drill out a broken bolt. And I'm never in the mood or the proper time or place to run out to hunt for a left handed drill bit when the opportunity occurs. Besides, it's in keeping with my tradition to keep the hobby as inexpensive as possible, and those things are not cheap. So I provide instructions for use of standard drill only.

Start with a small diameter drill bit, and drill a pilot hole all the way through the length of the broken screw until it drops out the bottom. Use plenty of oil on the drill bit and take it easy. Haste makes waste (and much more work). Then use a larger drill bit to open the hole to a diameter approaching the root diameter of the thread. When getting close to the thread, sneak up on it one drill size at a time until you can see the thread being exposed at one side of the hole. The only way you might see the thread all the way around the hole is if you hit it dead on center. That would be very rare, unless you know the exact location of the hole and use a milling machine to position the drill bit.

As soon as you see the thread appear on one side of the hole, stop drilling. Hopefully you would have been close enough to the center that the drill hole then would be close to the tap drill size for that thread size. Then you use the standard thread tap to carefully clean the remains of the broken screw out of the original female thread.

If you had the drill hole a little too far off center, then you might have to drill slightly larger to allow use of the tap, in which case you would be drilling more into the root of the female thread on one side. This may not be too detrimental to the final function of the screw, if the application is not a critical load bearing application. Cylinder head bolts and bearing cap studs are critical, but an oil pan bolt is nothing special. If you end up with 80% of the original surface area of the female thread remaining, it will be okay to just install a standard screw and use normal care not to over tighten it.

If you happen to bugger up the thread pretty bad in the process, then you might need to install a Helicoil thread repair insert to renew the thread. These are cheap and easy as long as the hole is not too far off center. If the hole is buggered up so bad that the HeliCoil installation doesn't take, then you have to drill the hole out large enough to tap a much larger thread, install a solid stud, cut that flush, and then re-drill and tap for the original size thread. In the case of a thin steel frame you may have to weld in a new blank slug of steel and drill and tap a new thread, or perhaps weld in captive nut. Now you know why the more time and care you exercise up front, the less work you are likely to have to do in the end.

But of course you will always succeed in the end. It's only a matter of how much time it takes, and how good you are at avoiding the pitfalls. There is a learning curve here, so don't be surprised if you get stuffed by the school of hard knocks on the first few tries.

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