JB Weld Fixes Cracked Cylinder Heads

There are persistent rumors that the main ingredient in JB Weld is magic. This two-part epoxy that you would normally find on a shelf next to your basic 5-minute epoxy, Titebond, various cyanoacrylates, and Gorilla glue is somehow different. Stories of ‘some guy’ in the Yukon using JB Weld on a cracked engine block abound. These stories are of course met with skepticism.

Now, finally, we have evidence you can use JB Weld to fix an engine. [Project Farm] over on YouTube gave it the ultimate test: he took the cylinder head off a lawnmower, took a grinder to the head, and patched the hole with JB Weld. The head had good compression, and the engine actually ran for 20 minutes before the test was concluded.

If this were a test of a field repair, it would be a test of an extremely crappy field repair. [Project Farm] made no attempt to ensure the piston didn’t make contact with the blob of JB Weld, and in fact, there was some slight knocking from the piston tapping against a blob of epoxy. Still, this repair worked.

While this serves as proof of the feasibility of repairing an engine block with JB Weld, there is one ultimate test of JB Weld epoxy: build an engine out of it. For years, I’ve been casting my leftover JB Weld into a small square plastic container. In a few more years, I’ll have a block of JB Weld ‘stock’, large enough to machine the parts for a small (.049 cc) glow engine, like what you would find in ye olde tymie model planes and cars. Will it work? I have no idea, but now I can’t wait to find out.

78 thoughts on “JB Weld Fixes Cracked Cylinder Heads

      1. Craigslist is an excellent source for HAD articles.

        HAD should post the coke can exhaust pipe fix, Bondo newspaper trick for filling large holes and “rock guard”.

  1. This is ludicrous. The ultimate test?

    Was this written by a 14 year old just discovering 2 part epoxy for the first time?

    This is a get you out of trouble bodge and not a repair in any sense of the word.
    Nothing wrong with a bodge. But “Ultimate test” ?

    A crack is very different to a cut.
    Cracks can propagate and have to be specially treated.
    Cuts wont as they wernt caused by stress or fatigue.
    A bit of epoxy wont cure a crack and stop it cracking further.
    Yes epoxy can fill a cut hole. for a while.
    20 minutes running for an ultimate test?

    20 minutes to repair a motor to get you out of trouble till you can repair it permanently is more realistic and much more interesting.

    Im sure I wont be the only one looking at this and wondering why its been written like that.

    1. Exactly! Head cracks typically form between the chamber and an adjacent chamber, typically coolant. We have used it to get someone home quickly, but we sure did not blob in on in that picture. While it did get him home on a 6 hour drive, he had to be careful because of pre-detonation as a result of the small blob (turbo car). What is in the picture would wreak havic under load.

    2. There are many two-part epoxies. JB Weld is one of the good ones.

      It smells like some stuff I used to use in college, to repair the plastic housings on Teletypes, which took a beating from the fists of students whose programs didn’t run the way they wanted them to. (MS-907?) That stuff was great, and JB Weld is a commonly available product that equals it in toughness and robustness. I’m a big fan of JB Weld.

    3. Agree with this. I was going to chip in with experiences on race engines about belzona 111, 3m structural, devcon and others in comparison but I’ve decided not to support benchoff’s clickbait by responding in depth.

    4. A crack is very different to a cut
      They propagate so easily
      But cuts don’t
      You might want a bit of epoxy
      This might help with a cut hole
      But is that worth it?

      Sorry, that just sounds like a haiku or whatever :)

    5. To remove epoxy you heat it and/or use a hot x-acto and it is easy to dig out, done it lots of times in R/C. That the engine would run I do not doubt, but that it would hold up long enough to be worthwhile for the effort is highly dubious.

      1. Still, this would be a much more valuable test if the cut went all the way to the edge of the casting. That’s how cracks usually behave, and what makes them problematic.

        This was way too easy a test.

    1. I like your logic. A “one-way” repair. This has merit where the outcome isn’t critical. I guess if the plug blows out, you can still drill it out and put in a Helicoil thread. I don’t doubt the epoxy can help out of a bind. The bigger issue on an engine is how J-B Weld reacts to repeated extreme heat-cool cycles.

      1. If J-B holds through the first few cycles it seems to cure and do pretty well. Cleaning the surfaces is important. A friend had a 2.2L “Iron Duke” which had it’s oil filter attached with this stuff too.

        The belt popped off and ripped the oil filter and threading right out of the block. That 1984 Cavalier lasted another year like this.

        1. Follow up: That 1984 Cavalier “Type 10”? It was scrapped in 2010 after the drivers’ seat collapsed through the floor because of rust. It was still running fine.

          1. Handy tip for floor pan rustouts:
            Stop by your local commercial air conditioning contractor. Mine had DUMPSTERS full of galvanized steel cutoff pieces and, when I asked for a few to fix my fender, they laughed and said to help myself.


          2. @ Antron Argaiv
            I did that to a 1973 Chevy Nova when I was in high school there was a small rust hole in the driver side floor so I cut it out rusted part and used a piece of galvanized steel from an AC compressor that was being scrapped.
            Just happened to be the exact same thickness as the floor pan metal.

    2. A friend of mine did this kind of repair with high temperature silicone (red or blue stuff). I don’t know for how many 10k of kilometers, but some. Although he never tried to replace this sparkplug. :-)
      Probably the silicone cures/hardens somehow with the heat.

  2. Man, you needs some serious help! Why in the world you want to do this. By the way,will it works on 4, 6, or 8 cylinder engine? I supposed if they have a crack in engine and will JB weld works on it?

    1. Neighbour and his brothers like to fix and build things. Would try anything. Would figure out anything. One brother lived out in the woods and used to go into the bush to find and haul out the old army vehicles that were air-dropped for practice. One duce and a half truck needed a new piston. So he made one out of hard Maple. Drove it to meet the guy to get the cash…
      I’ve always wondered what the buyer said and the condition it was in when he found it.

  3. JB weld is great, but I doubt that would last real long in the heat of combustion like that. I have an ancient Bolens 1256 tractor with a worn out PTO shaft/pulley. It’s a double sheave pulley with a splined hole. The splines were worn down to almost nothing, and the corresponding ones on the shaft, similar. I carefully rotated the pulley so it was “against” in the torque-wise direction, and squirted the gaps full of JB. It lasted about 12 years before I had to do it again. But not much heat there.

  4. In racing circuits, JB Weld is frequently used to glue in soft plugs on engine blocks, and for other things that absolutely must not come undone during a race. So how do they get the soft plug out when they do a rebuilt? They hit it with the flame of an oxy-acetylene torch. Because heat is one thing but JB Weld doesn’t hold up to flame very well. The other commenter is correct, a crack will continue spreading if not properly repaired. The other other commenter is also correct, this was by no means an ultimate test. Taking refuge in a paucity of responses willing to criticize obvious BS is BS, you should do some research and either rewrite this article to correct the tone and overall message or perhaps better, just pull it because it is buncombe of the first order.

  5. This stuff really has steel in it! Do play with a magnet above a spearing of the completed mix and watch small whiskers slowly form.

    Don’t get the 5-minute set stuff, it is nowhere as strong as the 24H set stuff. The 24H stuff is good for decorating rough-cut and roughly folded steel/Alu/etc into a more naturally squared shape as it hardens into a serviceable coating, and, it can be polished. Even then I didn’t know it was as strong/hard as this article points out!!!

  6. I will say this:

    One application I used it on was exactly what was discussed above, a crack head on an engine. It held up nicely until the engine finally gave up the ghost and a new lawn mower was needed. I was easily into days (30 minutes to hours usage for cutting grass). This was a “patch” until I could get another mower. The intent was to simply get me to a point until such time I could buy a new mower. I pushed it as far as I could.

    Another application was an old aluminum boat that someone decided to use as a sled in the winter. Yes, they had an old aluminum boat (that leaked) they chained to a tractor and pulled several people around in the snow and ice. Problem being, this was a very thin layer of frozen water and they pretty much wore off the ribs on the bottom of the boat due to sand and rocks. This should have easily been a throw away, but, in this scenario, aluminum boats are no longer cheap (at least in my neck of the woods). As luck would have it, I had come across a large supply of JB Weld that was being disposed of by a company closing so, with nothing but time to loose, I set up rebuilding the ribs using JB Weld and some sheet rock mesh tape. This took me about an hour in the evenings over about three to four days. I will not lie, this thing was ugly and would hurt your hands, I could have easily spent triple the amount of time with a sander or grinder but, what the hell, the fish and the ducks wont mind and if it didn’t work, no need spending time making it pretty.
    This was several years ago, the boat is still used. I think there are a few spots that may need patching, but overall, it worked.
    Now, I know someone will easily take stabs at this, to each his/her own, but, keep in mind, this boat holds two people, on water, people are stepping around on the bottom, it runs into stumps in the water, and it’s still holding up.

    1. I keep it around like most for emergency repairs where other two-part epoxies don’t work. Had to fix a gas tank awhile back, and it stands up to most chemicals pretty well.

      1. It did not fix the cracked radiator (plastic portion) I had several years ago.
        The loss of coolant warped the heads, and I had to junk my favorite car. (I had it for 18 years.)

        1. I have used it to patch aluminium radiators with 100%sucess, the issue with tge plastic ones is often adhesion. If its roughed up glass reinforced plastic i have had it hold up there too.

        2. It also did not work long to fix a cracked flange on a Holley carburetor if that happens you either have to have it welded or just buy a new base for a permanent repair.

  7. Did anyone else notice the neat arrangement of other products behind the mower? Looking at the other videos in his channel, it’s obvious this guy is paid to test and advertise products. If he gave negative reviews, I’m betting the sponsors would stop paying him, so there’s incentive to make their products look good.

    That said, JB Weld is a good product, IMO, and the test is an impressive display of its strength and durability. It is far from the “ultimate” test, however, and an example of poor craftsmanship. My vote is to avoid similar articles in the future. I don’t come here for the commercials…

  8. a guy i knew threw a rod in his 4cyl that went thru the side of the block, after fixing the rod he jb-welded a plate over the hole, drilled and taped the 4 corners bolting the plate in place. then drove the car for years i saw this about a year after the repair no leaks no appreciable damage

  9. This looks very much like someone who’s trying to build a YouTube subscriber base to make money. 40K views isn’t quite to skookum level but more power to him.

    JB weld is wonderful stuff – I have a set of small kitchen knives that perpetually test their JB-Weld festooned handles by sliding into the disposal – so far so good.

    That said, this specific kind of repair is most fun when braized/brazed (sic) old school fashion – heat it in the coals of the barbecue so it doesn’t break when you hit it with a torch, then braze the stop-drilled crack shut.

    Glue is fine, but the blue wrench is fun.

  10. JB Weld is great stuff alright. I am told you can even drill and tap it, but I have yet to do that.

    My favorite repair is the aluminum pot in a rice cooker. It must have been 20 years ago, the pot developed a hole due to corrosion on the bottom edge of the pot. I fixed it with a blob of JB weld figuring that it wouldn’t last long due to the thermal cycling involved every time the pot is used. Well, it is still going strong 20 years later, and that pot gets used several times every week. Far better than I had expected (but no engine head mind you).

    Based on the comments though, I won’t bother to watch this video. Thanks.

    1. There are other products similar to JB Weld (Liquid Titanium, Liquid Aluminum) that I have personally seen machined.
      I worked for a company that tested new designs for flywheels. The flywheels had cast in fins for airflow. By adding, removing material, filling in certain areas, etc… one could measure various airflows in a controlled chamber to determine if, yes, airflow increased and if it was directed into the area of desire. I’ve seen this stuff added and shaped and ran into several thousands of RPMs.

      1. Its regularly used in cylidner heads to “raise the port floor” to increase the turn radius onto the valve alowing for better flow, a slightly more permenant method than the putty used on flowbenches but not as hard as welding.

  11. Bought an ’86 4 cylinder mustang (back in ’91), previous owner had thrown a rod.

    Could see the rod end poking through the side of the block, luckily not in the bore.

    Removed engine, disassembled, cleaned and JB welded the hole in the block.

    Honed the cylinders, new rings, new rods, back together, drove for 50k+ miles before selling.

    Admittedly a non critical area ( not like cylinder head ), but still was impressed.

  12. For those reading, look at the context. Yukon. Those guys dont, and cant play by the same rules we do. They dont have a farm store, or an autoparts store a block away that they can run to real quick. Often their equipment fails them while theyre out and working in some of the harshest conditions humans perpetually live in. A general, quick, temporary repair is the way things are done up there, until the snow plane comes around and brings parts they ordered the month before. The fix doesnt need to be pretty, doesnt need to be certified by anyone. It just needs to get the job done for another few weeks, until the real parts show up.
    JB Weld can take heat, I used it to attach some joints to an aluminum foundry I put together, fired by coal and forced air (later a propane/forced air burner), as I lacked a welder. It ran for an hour or 3 at a time, and I have easily 15+ burns in it, melting down a kg of aluminum at a time. Still sitting at my parent’s somewhere, waiting to be used again. Never needed patched up or redone. My only concern with the engine repair, would be the piston hitting it, as it /can/ be brittle when hot, so thatd be your real mode of failure, cracking/chipping, and having bits that swirl around in the chamber, and getting trapped in the exhaust valve. Other than that, the patch job seems like itd do alright, and should hold for a week or or three until proper replacement parts show up.
    Some people are forced to live by the way of the bodge, and you’d be surprised what people can do with ‘trash’ ;)

  13. Years ago my dad had a portable dishwasher that leaked through a plethora of rusted out spots in the tub and door. As I hated doing dishes I happily took it off his hands and patched all the holes using JB weld. It never leaked after that and worked great for a free dishwasher!

  14. Years ago my dad had a portable dishwasher that leaked through a plethora of rusted out spots in the tub and door. As I hated doing dishes I happily took it off his hands and patched all the holes using JB weld. It never leaked after that and worked great for a free dishwasher!

  15. I had a 5HP Briggs & Stratton engine that burned oil badly, tore it down and discovered at some point in its life it had swallowed a chunk of a bolt (Thread imprints in the top of the piston and underside of the head) along with a nice gouge in the cylinder wall. I filled the gouge with JB weld, after it hardened I used a piece of emery cloth to get it to about the same contour as the rest of the cylinder and put it back together. Didn’t burn oil anymore and had much more power, used it that way for another year or so until I came across another engine in the scrap pile that just needed the carb rebuilt.

    1. There is literally million of combinations to try out – as with anything that involves plastic.
      But 2k epoxy and PUR – even the consumer stuff – yield great results these days.

      1. For polymers, I currently use JB Weld plastic bonder with 15 minute set time. With a little surface prep, this stuff is permanent as far as I’m concerned. It’s not mechanically hardened like regular JB weld which is metalized…but it really refuses to come off the surface. Good advice I learned about plastic bonding: it’s inherently greasy stuff. De-grease it with plain old detergent and hot water before using alcohol and sand paper.

  16. I had a 1986 Cadillac Cimarron where the emergency catch on the hood latch had worn a pinhole in a tube on the air conditioning condenser.

    I set up my old Gast rotary vane vacuum pump, mixed up a bit of JB Kwik and dabbed it onto the hole. Turned on the pump and let it run about 30 minutes to pull a vacuum in the AC. Then I let it sit over night. Next day I ran the vacuum for an hour then recharged the AC. Worked perfectly for some years, still working when I sold the car.

    I was expecting the pressure in the AC system would blow the JB Weld patch off the hole.

  17. Properly mixed and cured J-B Weld is rated for nearly 4,000 PSI tensile strength and 550F continuous temperature eposure. It won’t hold against direct ignition flame forever but it will plug a crack or small hole pretty permanently if the tensile bond or a small drilled hole can keep the crack from spreading. J-B Kwik, the 5 minute version, isn’t as good; it’s only rated for 2500 PSI and 300F.

    The OP’s dream of carving an engine block out of it is probably doomed to failure, though, because of all the surfaces that would be exposed to direct ignition flame. Something it probably would work for quite well would be a modest Stirling engine or even a model steam engine that uses a small open-flame boiler instead of the much hotter industrial live steam.

  18. When I was a kid, we had a Toyota Tarago van to haul the family around in. The thing blew a couple of mufflers (in 2 months…), had a stuck starter, and eventually decided to crack the casing of the thermostat coolant outlet from the engine. Nice big, long crack too. Dad drilled out the end of the crack and then proceeded to JB weld the crack. He spent ages sanding it down and making it nice and smooth and clean and it almost looked like the original part. Drove the van around like that until they sold it a few years later. Amazing stuff!

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