Yes, J-B Weld can repair a cracked cylinder head, but…

There are a few automotive aphorisms that people dispense like hot dogs at a barbecue when what you really want is a cheeseburger. One is, after seeing a photo of an accident, “That’ll buff out.” Another is “LS1 it” (drop a Corvette engine in a car in need of a motor, usually a woeful rat of a car worth less than the LS1). Both sayings are usually offered with a heavy dose of irony.  But a third, “Just J-B Weld it,” is often said seriously.

There are many two-part epoxies out there, but J-B Weld has a very good reputation in the automotive world, not only for bonding plastic to plastic, plastic to metal, and metal to metal, and not only for repairing broken bits in a pinch, but for the repair being so strong that it edges on being a semi-permanent solution. I keep tubes of J-B KwikWeld in my road kit just in case something weird happens like a coolant neck snaps off a radiator or a thermostat housing or an alternator mount breaks. At least it gives you a fighting chance of making it home.

But I never used J-B Weld to repair a cracked cylinder head before. And the idea that this would be a reasonable thing to do was something I regarded with the same skepticism, if not outright disdain, as when I was discussing the trouble I was having rebuilding the engine to my Lotus Europa and folks said, “Just LS1 it,” thinking they were being clever or funny.

Here’s the backdrop. I own a 1972 BMW 2002tii, the subject of my book Ran When Parked. I bought the car, a very original example with the patina of age and use, sight unseen in Louisville about three years ago. It hadn’t run in a decade. I traveled down there with tools and parts and resurrected it where it sat, finding housing through the kindness of folks I know through the BMW Car Club of America. I got it running, and six days later had it sufficiently well sorted that I road-tripped it the 1000 miles back to Boston.

1972 BMW 2002tii
The resurrected ’72 2002tii about to begin its trek home. Rob Siegel

But an oil leak onto the exhaust threatened to derail the trip. The leak was due to the valve cover stud in the lower left corner of the head being stripped in its hole and thus not tightening, which prevented that corner of the valve cover from sealing. This caused oil to drip directly onto the downpipe, which was a clear fire hazard. At the time, I repaired the stripped threads with a Time-Sert (helicoil). The car still appeared to be weeping a small amount of oil through the threads onto one of the exhaust studs, but this is a common problem, as the lower exhaust studs thread directly into oil runoff area on the low side of the head. I made it home without incident, and that spring I road-tripped the car 2000 more miles to other events.

Then, the 2002tii was accepted into an exhibit in the BMW Car Club Foundation’s museum in Greer, South Carolina, that commemorated the 50th anniversary of the BMW 2002. In February 2018, I prepared to take the car to the museum.

I pulled the valve cover to adjust the valves and was horrified when I found the root cause of the oil leak. It turned out that the stripped hole for the valve cover stud was just the tip of the iceberg. The cylindrical boss that the stud threaded into was cracked. Tightening down the nut on the stud only caused the crack to yawn open wider. The oil that I thought was weeping through the exhaust stud’s threads was likely seeping through the crack.

It was not a good situation. I hated the idea of having to spend the money to ship the car there and back. I considered telling the museum that it was unavailable due to mechanical issues.

Cylinder Head Crack
The source of the problem—a cracked boss inside the cylinder head. Rob Siegel

Further, I saw that someone had previously tried to seal the crack from the inside with blue RTV. This reinforced something I’ve long thought—that when you find a long-dormant car that supposedly “ran when parked,” there’s usually a reason why it was parked, and often that reason is that the owner was faced with an expensive repair. So when you buy a long-dead car, you still need to go through the necessary steps in sorting out something that’s been dormant for years (back-to-front fuel system cleanout, oil change, engine rotation, bleeding the brakes, etc.), but often you’re waiting for the other shoe to drop in terms of the real reason the thing was off the road. In this case, considering the blue RTV on the crack, the forensics seemed pretty clear.

But then I thought, well, I’ve driven this car 3000 miles this way since its resurrection, and the oil leak hasn’t gotten any worse. Maybe the crack is, you know, stable.

As they say, denial ain’t just a river in Egypt.

So in February 2018, I sealed up the valve cover tightly by using Permatex “The Right Stuff” (the idea being that this heavy-duty sealant would keep oil from leaking out the valve cover gasket without needing to tighten down on the nut on the stud into the cracked boss and force the crack open). I successfully drove the car the 1000 miles to South Carolina.

BMW CCA Foundation Museum
Me and the car at the “2002: ICON” exhibit at the BMW CCA Foundation Museum in Greer, South Carolina. Rob Siegel

The BMW stayed in the museum for nearly a year. As the exhibit was closing in January 2019, I went down to pick it up. I was prepared to drive it home when my wife called me and told me that a blizzard had moved into Boston, and said that in addition to the concerns of driving a valuable rust-prone vintage car with no snow tires through snow and salt, I wouldn’t even be able to get the car into the garage because there was two feet of snow in the driveway and the garage door was literally frozen shut. A friend of mine, who had several cars in the exhibit, overheard all this and said, “I have a trailer coming down for my cars. There’s room in it for one more. Do you want me to just truck your car up to my shop in Cincinnati? You can pick it up in the spring.” I jumped at the offer.

So the car sat in Cincinnati. Spring turned to summer. Things in my personal life took precedence. Before I knew it, it was October. I got a call from someone who works for my friend asking gently when I was going to pick up the car, and oh, by the way, it’s leaking a fair amount of oil.


I arranged to pick up the car as part of a road trip that had me dropping off another car. I brought general road trip tools, plus an upper engine gasket set and a variety of sealants. But if the car was gushing oil or wasn’t safe to drive, I didn’t really leave myself a convenient way out.

When I got to my friend’s shop, I learned that the car had been moved into a warehouse a short distance away. We fired it up, drove it over to the shop, gave it a quick check, cleaned old oil off the front and back of the head, then I took it for a drive of several miles.

2002tii BMW Stored
Retrieving my 2002tii from its penthouse storage. Rob Siegel

When I got back and opened the hood, oil smoke was clearly rising from the back of the head. We put the car up on the lift and could see oil dripping directly onto the downpipe, but I couldn’t see the source. I wiped the lower corner of the valve cover off with a paper towel, but it came up dry, indicating that it wasn’t leaking out the valve cover gasket near the cracked boss.

A guy at the shop used an inspection camera with a flexible wand to look at the lower corner of the head and caught it in the act: A non-trivial amount of oil was coming out of what was clearly the portion of the crack that had perforated through to the outside of the head.

Cylinder Head Crack
Inspection camera showing the crack (upper arrow) and oil path around #4 cylinder (lower arrow). Rob Siegel

Double damn.

I pulled off the valve cover, which wasn’t easy, as “The Right Stuff” sealant had a death grip on the gasket. I inspected the crack from the inside. It didn’t look any worse than I’d remembered. But that didn’t matter. The inspection camera was definitive. With oil actively dripping onto the exhaust downpipe, the car wasn’t safe to drive. What a lousy and expensive way for the trip to end. I began checking on my phone for one-way flights and looking up the contact information for a shipper I’d used recently.

Then, I backed off. I had a friend nearby who I could stay with overnight. I gave myself the day to come up with a solution. I was hoping The Automotive Powers That Be would drop a fully-assembled cylinder head in my lap, but I also wondered if I could simply divert the oil flow.

I was looking at the leak and the exhaust configuration, wondering if I could buy a disposable aluminum roasting pan and cut and bend it into a shield that kept oil off the exhaust, when two senior guys who worked in the shop came over. I explained the options I was considering, then said offhandedly, “I have some J-B Weld with me.” I didn’t really have any energy behind it; it was just FYI. I fully expected them to say, “I HATE when people say ‘Just J-B Weld it.’ They don’t know what they’re talking about. You can’t bloody J-B Weld a cracked cylinder head.”

However, to my surprise, one of them said, “That could work if you get the area dry and clean enough. I’d advise flushing both sides of the crack—inside and outside—with brake cleaner, then heating it with a torch to draw out and burn off any residual oil. The torch will also burn out that old RTV that’s in there.”

The other fellow added, “My advice would be to not just pack the J-B Weld into the crack, but to slather the entire surrounding area with it, both inside and out. And once you’re done, pack a bunch of this pipe dope into that threaded hole in the boss where you shot in the helicoil. It may help to seal the crack from the inside. And the repair doesn’t have to completely stop the oil leak; it just has to make it not drip onto the exhaust.”

Well, OK then. Suddenly I had a plan. I’d brought brake cleaner and J-B KwikWeld with me. The guys at the shop loaned me a torch. I cleaned and heated the area, then did it again for good measure. I mixed up the J-B KwikWeld and laid it on both inside and out. The inside was fairly straightforward, as I could see what I was doing and applied it with a flat piece of a wooden paint stirrer, but on the outside of the head, I had to do it blind, as there was no way to get a direct sight line on the area with the crack. I just took a big glop, put it on the fingers of my Nitrile gloves, and worked it into the general region to the right and above #4 spark plug hole where I knew the crack was.

Cylinder Head Outside Corner
J-B weld on the outside corner of the head. Rob Siegel

As the J-B was drying, I packed the pipe dope into the stud hole, screwed in the stud, and wiped off the excess. I then applied a fresh bead of Permatex “The Right Stuff” to the top and bottom of a fresh valve cover gasket and tightened the nuts on all the studs except the one screwed into the cracked boss; that one I just put on finger tight. The idea was that “The Right Stuff” would seal that corner even without pressure from the stud. At least, that was what I hoped.

Inside Cylinder Head
The repaired area inside the head. Rob Siegel

The J-B KwikWeld label says it sets in six minutes, fully cures in 4–6 hours, and is good to 300°F. I sped curing with a heat gun. Less than three hours later, it felt rock-hard to me. I ran the car in the shop, then took it for a test drive and saw absolutely no leakage.

I made it 230 miles to Harmony, Pennsylvania, by nightfall, and I pounded out the remaining 620 miles to Boston the following day without incident.

Even though it’s still not leaking, I don’t really consider this a permanent or semi-permanent repair. However, I am continuing to drive the car, maximizing its use until the snow moves into New England. At that time, I’ll pull the head and probably try to find a replacement.

So, yeah, I totally J-B Welded a cracked cylinder head. Women love me, men now want to be me. But before you give me the badge of honor, let’s be clear: This was not a crack through the combustion chamber, a water passage, or anything else under pressure. It was at the very top of the head, not the bottom. The purpose of the repair was simply to stop oil from seeping out onto the exhaust, not to hold compression.

Still pretty cool, right?

So, the next time someone shows me a photo of a piston that took a rapid exit through the side of the block, I’m going to be that guy who says, “Just J-B Weld it.”

And, regarding my BMW 2002tii, if the J-B Weld patch fails and I can’t find a replacement cylinder head, I can always, you know, just LS1 it.


Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel magazine for 33 years and is the author of five automotive books. His new book, Resurrecting Bertha: Buying back our wedding car after 26 years in storage, is available on Amazon, as are his other books, like Ran When Parked. You can order personally inscribed copies here.

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    i have a mk2 golf gtd cylinder head thathas three cracks one between each cylinder is there any point using j b weld on these because the craks are tight and i cant see how this will fill them ,Chas

    Prep the hairline cracks by *beveling* them with the appropriate grinder or hand file. Rough, then meticulously clean the area to be sealed. Any crack should be prepped this way b/f attempting a repair with adhesive/sealant.

    On tomorrow’s episode: can you kintsugi a cracked head? Let’s mix some gold in the epoxy and find out!

    I used JB Weld to repair a stripped spark plug and it worked. I wrapped the spark plug with wire, put on epoxy and tapped to proper depth, very important. You can always break it loose with a 1/2″ ratchet wrench and a lot of effort. Better than replacing head. Good luck! 🙂

    I used the standard J&B Weld to glue in a right angle press-fit water hose fitting on a Buick V-8 that the head refurb shop had yanked out, and was now unobtainable and loose. Ran at least two more years ‘jes fine, when I sold car due to a move from Orlando to Destin/Niceville (too many cars)…..

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