Steel bolt broke off in an aluminum block - need ideas

I was installing a new fuel pump on a Kohler V twin small engine when a 1/4 inch bolt twisted off that holds the fuel pump to the top of the head. The fuel pump runs off of the valve rocker arm. There are 4 bolts total and the other three came loose easily. The bolts appear to have some hardness. Fortunately today I was able to drill entirely through the center of the broken bolt with a 9/64 drill bit. I tried to use an easy out to back out the broken bolt but was very careful not to break off the easy out. It didn't want to back out. Worst come to worst I can just keep drilling on the bolt and re-tap the hole and I might try some left hand drill bits. Here is my question .... what things can I do to help loosen the bond between the aluminum and the steel if I want it to back out with either the easy out or left hand drill bits? I have been spraying penetrating oil on it. Does heating and cooling help and should this be done with a hand held propane torch? Any ideas are appreciated. Thank you.

A little heat will break the corrosion between the steel and aluminum.

Can you hit is with welding rod and nut?

Chances are the bolt has seized in the hole, pulled threads out of the hole and embedded them in the bolt.

If you're sure it's a 1/4-20 bolt, try drilling the remains out with a #7 drill, then carefully tap the remaining threads out. Might check a good bolt, I'm thinking they are 6mm.

If there are good threads left, job done.

If there are no threads left, either step up to the next bigger bolt, or install a Helicoil thread repair coil.

Be very careful, keep everything straight and centered.

An old tractor mechanic always swore on a stuck bolt or nut....or in your case broken one....try to tighten it first.
Theory is it breaks the bolt loose so you can back it out.

Guy I know does all tritan broken exhaust studs for Ford dealers in our area mig weld a washer around stub of bolt then weld nut to washer let cool and they usually release

Bolt is broken off below the surface. Cannot weld on a nut of washer.

Well first, unless your Kohler is a K-series or a Magnum, its not a 1/4" bolt, its metric.

Second, that bolt is probably self-threading and somewhat harder than a normal bolt.

Third, you are not alone. That same thing happens to me when I work on those engines at least once a year. Those fasteners tend to seize in the head. I bet you backed it out about 1-1/2 turns, then it broke. What happened is a bit of aluminum froze to the threads of the bolt and wedged tight as it backed out.

While you can try heat-like from a propane torch, not acetyline, your best bet is to just drill it out, run a tap in the hole, and put a new bolt in.

I've had that happen plenty of times. Lots of times the 3 bolts are more than sufficient to hold the pump on. I've drilled a slightly smaller hole inside the broken off bolt, tapped it, and just used a smaller bolt to hold it in as well.

If you're hell bent on getting it out, drill 1/8", tap it, get a grade 8 1/8" bolt, shove some jb-weld in the hole, screw it in. Let it sit for a day or so, then try to back it out. The JB weld should be gummed up in the threads providing plenty of grabbing power. Heat up the aluminum before you try to back it out, the aluminum will heat up much faster than the steel.

Do you have any left hand drill bits?

If you have a wire feed welder, you can build up the broken off bolt until it protrudes and a nut welded on.

Just me...I have started giving things a little squirt of Kroil if you have time to let it work. Electrolysis is a B.....h.

Add a little heat to the equation and it should come right out. Don't try to heat the aluminum casting; it's just a big heat sink. Instead, if you have an acetylene torch, put a small brazing tip on it and apply heat directly to the broken bolt. Once you get the bolt to glow a bit, it should come out easily with the easy-out.

Bee's wax?

Can usually catch them with welder down to 2 or 3 threads below surface.

Left handed drill bit. Your job is what they are for.

I would think the safest way would be continue drilling it out so you can re thread it.

I would use heat and my left hand drill bits.

If all your efforts fail, how about using an electric fuel pump?


Paul in MN

I agree with the suggestion to use heat. Aluminum's coefficient of expansion is 2 to 2.5 times that of steel. A propane torch is the tool to use. If it was assembled with the high-temp loctite the heat will also break that loose. I have also had luck "shocking" a broken bolt out. Drill all the way through the bolt as you have done and fill the hole to the top with penetrating oil. Let it sit overnight--this will allow the oil to work its way up the threads from below. Then, drill a second small hole as close to the edge of the broken bolt as possible. This hole should be drilled to a depth about equal to its own diameter--just a dimple, really. Then, put a nail set in this hole/dimple, angling it to drive the broken bolt counter clockwise. Give it a good sharp "pop" with a hammer and the bolt should hopefully shift a little bit in its hole. Once it starts to go, the oil will seep in all around it and help things a lot, and you should be able to work it out by repeated sharp hammer blows. The shock trick may be hard here as this bolt sounds pretty small. I used it on a frozen sheared off radiator housing but that one was 3/8. I drilled through with a 1/8 drill bit, then made my dimple with the same drill bit as close to the edge as I could. I also used it on frozen bolts in aluminum magneto housings where I couldn't use heat for fear of ruining the magnets, and it worked great. As the others have said, you do have to let the oil work overnight or longer if you can, which is hard for me as I am impatient on these things. Good luck.

Thinking further, 9/64 is about 3.5 mm, so if this is a 6mm bolt there won't be much metal left for the dimple. Unless the bolt broke off with a jagged corner, which would give you something to drive the nail set against, then the heat trick is probably your best bet. I would try heat on both the bolt itself but also on the aluminum. You never know which one will work. Good luck again.

The thing you have to realize is that the only thing holding the remnant of the bolt is corrosion between the bolt and casting. If there was no corrosion, the bolt would have easily come out with the easy-out. Actually, it wouldn't have broke in the first place. So you have to break the bond between the threads of the bolt and the threads of the casting. And heat is the best way to do that.

Aluminum rate of expansion is approx. 8 times greater than steel.

Coefficients of linear expansion:
Aluminum 21 to 24 x 10(-6) in/in/F
Steel 10.8 to 12.5 x 10(-6) in/in/F

It is the linear, or diametral change in the hole vs the bolt that is of significance here.
list of expansion coefficients