How to remove stuck fasteners, Part 3: Smelt and pelt

I’ve been emphasizing patience when dealing with stuck fasteners, but when all else fails, there is a nuclear option: The oxy-acetylene torch.

First, let’s get down to basic academics and why heat loosens stuck fasteners. There’s actually a bit of a debate about this. An often-heard idea is that it’s the differential rate of expansion that is the mechanism for loosening. For example, the nut heats and cools at a different rate than the bolt it’s threaded on. There’s certainly some truth to this, but you need to be aware of which part of the fastener is doing the expansion and which is doing the contraction. (Pelican Parts owner Wayne Dempsey explains that when removing a steel stud that’s stuck in an aluminum head or block, you shouldn’t heat the stud but should instead heat the aluminum around the stud to get it to expand.)

Another school of thought is that differential expansion has little, or nothing, to do with it, and the heat works because it breaks the bond of corrosion between the threaded insert and the hole it’s screwed into.

A third idea is that the heat makes the metal more malleable, which helps it to release its death-grip on its companion fastener.

For any stuck fastener you’re removing, there may be some combination of the above three mechanisms, but in truth, most of the time you really don’t care what the physics of the situation is as long as heat helps you get the dang thing off.

But you do need the correct amount of heat. A propane torch is almost useless. MAPP gas is, Goldilocks-style, just right much of the time, as it can get a fastener practically glowing red without melting it. An oxy-acetylene torch can get the metal so malleable that it melts, which is usually the last thing you want, except in the very special circumstances we’ll talk about now.

There’s little doubt that heating a stuck nut or bolt near the point where the metal melts will facilitate removal. Whether that’s helpful is highly dependent on the situation. You may heat a nut to the point where it’s like putty, put a ratchet on it, apply torque, and find that the nut stays put but the bolt twists off like taffy. Maybe that’s helpful, but maybe it’s not. To take it to the point of ridiculousness, an oxy-acetylene torch, when used it as a cutting torch, can get things so hot you can remove not only the stuck nut and bolt but the piece of the engine they’re attached to. This is almost certainly not helpful.

The best application I ever had for oxy-acetylene—in fact, the reason I bought the set-up—was when I was changing a clutch on my 1999 BMW Z3 M Coupe (a.k.a. “The Clown Shoe”). To remove the transmission, the exhaust first had to come out. While attempting to remove the nuts holding the exhaust downpipes to the exhaust manifolds, I’d snapped four of the six studs. This was before my developing my Tantric-like approach of heat, wax, the right amount of torque, and patience. I was younger, less wise, and with more upper body strength. I simply put a big old pipe on the end of a breaker bar. It broke things.

I read up on the problem and learned that the studs are a hardened corrosion-resistant Inconel alloy that’s difficult to drill out. But I also learned that the Inconel studs aren’t threaded into the exhaust manifold flange like I thought. Instead, the studs insert from the back, have fine splines that seat in the holes when you tighten the nuts, and have a flange on the back that prevents them from pulling through.

Because they’re not threaded, you should be able to knock them out from the front. So I tried. I first used an air chisel with a rounded punch attachment. I swear, I could hear the studs snicker at it. I applied the MAPP gas torch to heat up the broken studs as hot as I could get them. Nothing. I stepped up my game to a small sledgehammer and a punch. The studs didn’t snicker this time, but even when I hit the four broken-off studs so hard that I was concerned I’d crack the car’s expensive M head, they still didn’t budge.

It was then that I read the thing to do was use an oxy-acetylene torch to get the snapped-off stud bases so hot that they’d become malleable. Then when they’re smacked, the impact simply pushes them out like soft Play Dough. And that’s exactly what happened. Out they popped with no drama (if you can call getting a flame like that that close to a high-dollar motor “no drama”).

You can find DIY oxy-acetylene setups with brazing tips suitable for heating stuck fasteners in the $100–$200 range on Amazon or Harbor Freight, but be careful, as typically they’re pictured with small 20-cubic-foot “type R” or 10-cubic-foot “type MC” oxygen and acetylene tanks that aren’t included in the price, or they’re sold with non-D.O.T-approved tanks that no legitimate welding shop will refill. Plus, even if you buy new D.O.T-approved tanks, in order to get them filled, many welding shops will treat them like propane cylinders and swap them rather than fill yours, so you may never even get to use your pretty new tank. For this reason, for the do-it-yourselfer, it often makes sense to buy a used pair of beat-up tanks. They show up all the time on Craigslist from welders and plumbers who are getting out.

Oxy-acetylene does have a learning curve. It isn’t like MAPP gas, where you just open the valve, light it, and go. With oxy-acetylene, you need to know where to set the pressure regulators on the gas bottles, how to light and adjust the acetylene, and how to dial in the oxygen using the knobs on the torch. You need goggles, as that flame gets wicked bright. And obviously, with that much heat, safety is paramount. A fire extinguisher should be at the ready; wear long pants and long sleeved shirt; and make sure there are no gas leaks in the car and nothing flammable on the floor. But it’s all learnable.

The torch attachments usually have the recommended pressure settings printed right on them. There are some good videos on flame adjustment on youtube. And, like any tool, once you own it, you can use it for more than just the thing you bought it for. Simply heating fasteners until they’re malleable is only the first level of capability. You can use braze pipe. You can buy a cutting attachment (what’s commonly called a “blow torch”) and cut through metal. Pretty soon you’ll be slicing off and brazing on your own exhausts.

When I have a stuck bolt, I don’t reach for the oxy-acetylene setup nearly as often as I go for the cheaper, smaller, lighter, and less intimidating MAPP gas torch. But once you learn to recognize and differentiate “need to get it hot to loosen it up” from “need to get it soft so I can punch it out”—or, as I call it, “Smelt and Pelt”—oxy-acetylene can make the difference between being stuck and possibly damaging something big and expensive, and having a way out.

Plus, admit it. You want to swagger a little bit and say, “It was stuck, but I used the blow torch.” Technically, unless you have the cutting attachment, you probably didn’t. But I won’t tell.


Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel Magazine for 30 years. His new book, Ran When Parked: How I Road-Tripped a Decade-Dead BMW 2002tii a Thousand Miles Back Home, and How You Can, Too, is available here on Amazon. In addition, he is the author of Memoirs of a Hack Mechanic and The Hack MechanicGuide to European Automotive Electrical Systems. Both are available from Bentley Publishers and Amazon. Or you can order personally inscribed copies through Rob’s website:

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