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Thread: Need help on welding cylinder head

  1. #1

    Default Need help on welding cylinder head

    I recently picked up a small 50cc mini chopper. One of the exhaust studs was broken in the head. I removed the motor and tried to use a bolt extractor but the extractor snapped off in the bolt. I went a little extreme and cut the flange in the casting so half the bolt was exposed and knocked it out. Now I want to tig to build it back up so I can drill and tap a new hole. So now the questions...
    1. Should I leave the motor together or take it apart? I'm inclined to leave it together since it will most likely warp less.
    2. I'll need to manage heat. I've seen Jody do this in videos with a non contact thermometer. I have one. What should my target heats be? How hot should I let it get and how cool before I start welding again?
    Any other advice would be helpful. I've welded lots of cast aluminum but never on a head. Thanks in advance!

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  2. #2

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    I'd weld it as is. Put a layer on, wait 5 min and repeat.
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  3. #3
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    Screw extractors, DON'T!

    Next time weld a nut to the broken stud. Jody has a pretty good video on that.

    Another option is to cut a chunk of aluminum to fill in the missing material, that will reduce the amount of weld and heat you will put into the part.
    Either way from how it's broken I would probably leave it together, at least while you weld it. After you're done make sure that valve and rocker are free. I probably wouldn't do any preheating, the part will get hot enough from the welding. As Zoama said, give it some cooling off between passes. Once you can put your hand on it, do another pass. Make sure to clean out all the junk in whats' left of the threaded hole. I would open that part up with a rotary burr, so you are not trying to put new threads right where the weld meets the parent metal. Your biggest issue might be the quality or lack thereof in that casting. From your cut edges it looks like it's pretty good material. I've welded quite a few of those Honda knockoffs, and while some are great, others are really poor castings.
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  4. Default

    Rambozo is correct about the quality of the metal. They use a lot of zinc in the aluminum to make it flow into thin sections better. The trick is to use a lot of cleaning action in the weld and you might even have to weld on a layer then grind most of it off and weld over it again. This dilutes the zinc to the point where it does not bother the weld much. If there is a lot of zinc the metal will bubble and leave air pockets in the weld. When I had a propeller repair shop some of the props that came in were so bad that the welds were useless. I would also use 4043 filler because it will flow better and will still have enough strength to hold new threads .
    Last edited by TheGary; 07-14-2014 at 09:09 PM.
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  5. #5

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    Quote Originally Posted by TheGary View Post
    Rambozo is correct about the quality of the metal. They use a lot of zink in the aluminum to make it flow into thin sections better. The trick is to use a lot of cleaning action in the weld and you might even have to weld on a layer then grind most of it off and weld over it again. This delutes the zink to the point where it does not bother the weld much. If there is a lot of zink the metal will buble and leave air pockets in the weld. When I had a propeller repair shop some of the props that came in were so bad that the welds were useless. I would also use 4043 filler because it will flow better and will still have enough strength to hold new threads .
    Thanks! I've welded some of this material before but I like your idea of putting down a base then grinding it back a little.
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  6. #6

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    Quote Originally Posted by Rambozo View Post
    Screw extractors, DON'T!

    Next time weld a nut to the broken stud. Jody has a pretty good video on that.

    Another option is to cut a chunk of aluminum to fill in the missing material, that will reduce the amount of weld and heat you will put into the part.
    Either way from how it's broken I would probably leave it together, at least while you weld it. After you're done make sure that valve and rocker are free. I probably wouldn't do any preheating, the part will get hot enough from the welding. As Zoama said, give it some cooling off between passes. Once you can put your hand on it, do another pass. Make sure to clean out all the junk in whats' left of the threaded hole. I would open that part up with a rotary burr, so you are not trying to put new threads right where the weld meets the parent metal. Your biggest issue might be the quality or lack thereof in that casting. From your cut edges it looks like it's pretty good material. I've welded quite a few of those Honda knockoffs, and while some are great, others are really poor castings.
    I would have liked to do this but it was a 6mm stud and had broken about 1/4" below the surface. No way I could have controlled the arc in such a small hole even with my mig.
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  7. Default

    Quote Originally Posted by Ron66 View Post
    I would have liked to do this but it was a 6mm stud and had broken about 1/4" below the surface. No way I could have controlled the arc in such a small hole even with my mig.
    Saving threads in an aluminum part is the most difficult of any metal. ( in my opinion ) Contrary to popular opinion alum. oxidizes when in contact with a dissimilar metal, like a stud encased in it. This locks it in and makes removal very hard. On most metals if you have a bolt or stud that will not come out ( even with a nut welded onto it ) the best way is to drill out the center and keep going larger with a carbide burr ( single cut ) until you can actually see the threads around the hole. Then you can pull the threads out with a dental pick. On alum. the metal is so soft that one slip with the carbide burr and you have an oval hole. On these I usually go ahead and grind all the bolt out then drill it to size for a heili coil of the original size of the stud or bolt. This is a last resort option if the more traditional methods fail. Easyouts seldom work and if they do it is on a bolt or stud that was not stuck very hard. The best easy outs I have used ( with limited results ) are the cork screw ones like snap-on sells. That said they are not worth buying. For the record the carbide burr is also a good way to get a broken easyout or even a broken tap out of a hole.
    Last edited by TheGary; 07-15-2014 at 01:17 PM.
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  8. #8
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    Quote Originally Posted by Ron66 View Post
    I would have liked to do this but it was a 6mm stud and had broken about 1/4" below the surface. No way I could have controlled the arc in such a small hole even with my mig.
    That's what TIG is for. More precise.
    But as Gary said, often with exhaust studs in alum, the best thing is just to plan on a Heli-Coil right from the start. Because it's so easy for the drill to dive into the aluminum, it's often best to make up an alignment jig that you can bolt to the good stud, with a guide hole or drill bushing to keep you centered and aligned.
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  9. #9

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    Thanks for all of the great advice. Here is my progression of welding. Actually taking pictures helped me pace the welding so I wouldn't rush it.
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    So a little rough at first but I turned up the cleaning (AC set to 75%) and got enough heat in it to make it flow
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    I ground down the material I added a little to get rid of the crap that formed like a skin over the top. It was pretty hard, much harder than the material just under the skin.
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    Another layer and a another session of grinding.
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    Here's the almost finished product. I spent a little more time with a detail sander after this and it looked pretty good. I just need to make a drilling rig this week and I'm ready to drill and tap it!
    Lessons learned:
    Take your time. I used a IR thermometer and let it heat to 260F then cool to about 200 before I started again. I figured for an air cooled motor this is pretty close to normal operating temps.
    I started using the pulser on but quickly realized that it was counterproductive when a lot of heat is needed.
    I also noticed that I needed a lot more pre-flow time to keep the weld clean. I ended up at around 4 seconds.
    Stick out is good when your in a tight spot.
    Again, thanks for all of the advice!
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  10. #10

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    Nice work, Ron.
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  11. #11
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    Now the hard part drill it straight and tapping it.
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  12. #12

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    I made a jig to drill and tap the hole straight. I was only 5 thou off from bottom of studs to top of studs. Not bad at all! Anyway here is the bike finished. Next time I'm at the metal supply store I'll pick up some small diameter steel rod and fab up a hollow fender for the front tire to keep the brake and speedo cables off of the tire. I also have some stainless sheet that I'm going to use to make a cover for the electronics under the seat.
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