Thursday, March 21, 2013

How to Hack'n'Weld

This should be short but sweet, most of you who have been around mopeds for awhile already know this so you can skip this one and tune in to the next post. If you're a newbie or have limited fabrication skills, you may find yourself needing this. People on the forums and elsewhere will tell you that for your Sachs, Batavus, Kriedler, etc you need to 'hack and weld' a pipe, its not exactly rocket science but there is a right way and a wrong way to do it. The right way leaves you with a nice finished pipe that fits and flows well and the wrong way leaves you with broken cylinder studs, poopey welds that leak, grease and black smut all over your pipe, or a exhaust gasket air leak.

What we're gonna do here, basically, is take the header of a pipe from a stock bike and weld it to the chamber or muffler from another bike. In this case, I'm just modifying a stock M56 exhaust to fit on the M48/56 hybrid motor that you can put together with the MopedFactory batavus cylinder shim. (Treats/1977). For this example I'm only using a stock M56 exhaust, the stock m48 muffler would work, but the M56 muffler works too, and I dont need to chop up 2 exhausts for no reason. This would be the same application you might want to use if you are going to put a 2hp cylinder on a magnum and keep the stock pipe.

If you want to use an aftermarket pipe, you will have to buy one. It always helps to get a pipe that mounts on the same side, in roughly the same place. When you are doing this sort of thing, you want the header length and diameter to be close to the same as the un-molested performance exhaust you are using. You also want to try to use the stock mounting bracket as much as possible because it is probably a lot stronger than whatever crazy crap you're going to make out of zip ties and hose clamps. So figure that out first.

Once you have your donor pipe in hand, and you've figured out how you're going to mount it, you can cut the header off the donor pipe. You'll want to have a pretty good idea at this point of how the two pipes are going to come together. If you can get your header on there and let both pipes clear that's great, if you cant, just cut them off so they have as much extra as possible. If you are trying to save the header off the donor pipe or the muffler off the stock pipe, think about that too. Sometimes you'll get lucky and have one pipe that can fit into the other one in some place, like a A35 tecno estoril which as a big section in the header... say you want to put that on an A55, you can cut off at the big section and weld the smaller A55 header into it or something like that.

If you can cut one pipe off in such a way that you leave a ton of overlap between the two, that's the best. You can then line them up with both pipes in place, holding them side to side of each other, and look along the curves to see where the two line up fairly close.

Now this is where you want to get impatient and whack the thing off where you see it. You gotta take your time and go slow, first tape off where you want the cut to be. When you tape it, the tape will line up and make an even planar cut, as in, it will all be in the same plane instead of some wacky stuff. When you get the first pipe taped, cut it carefully along the tape line. I prefer an angle grinder with cutoff wheel because you can follow the tape more accurately. Using a saw to cut straight through is harder because you have to pay attention to two cutting zones at the same time.

Once you have one side cut, you can start working on the other side. This is where the tape is really nice, you can put it on and take it off a couple times to get the angle just perfecto to line up. Move the pipes back and forth, side to side and up and down from each other and look past the cut one to make sure the tape line is in the right place so the angles match perfect. When you cut the other one leave, maybe, 1/8-1/4 of an inch extra all around the tape line. Sometimes I will take and draw that gap out with marker and cut the marker line.

You will need that extra material in case the mate isn't perfect. If there is a gap or a high side you will want to mark the high side opposite the gap with 1/2 the thickness of the gap, so when you cut it off its perfect. At this point you should probably switch to a grinding disc, or flap wheel like this one.
If you have a big bench mounted belt sander, those are the best, but if you have one you probably dont need to be reading this. The key being, both sides need to be planar, as in, when you put something flat against them they have a flat surface all around. You cant have any gaps in the seam or your weld will push in and block the flow through your pipe.

It should look like this when you're done:

Once you have a perfect mate, you can clean up the material for a good weld. I don't care how good of a welder you are, especially with this thin stuff, you wont get a good weld if it isn't clean. You've put in this much work so far, so do it right and get a nice result ya dummy!

The flap wheel is your best friend for this kinda thing.

If your mate isn't this good or better, keep at it until you get it right.

At this point, if you have a welder, you can just weld it yourself right in place. If you don't have a welder you have a few options:

A) Mark the weld with lines across the weld, the lines will line it up when you take it to a shop to have them weld it, and you will know the orientation of the two pipes. Before you take off, make sure that when you hold them together and line the lines up, you can tell where it is supposed to be. If you are doing a slip fit where one tube goes inside the other, draw the mating line around the edge of the big tube on to the small tube, and some cross lines too.

B) Put a rubber clamp over the two and ride it to the shop and have them weld it in place.

C) Drive it to the shop in the back of your car or truck.

C is probably your best bet, as you are sure to get it right the first time. If you do A, you might want to have them tack weld it and take it back home to test fit before finish welding.

If you take it to a shop they will probably know to do this, but if they dont, ask them to.

If you weld it yourself:

First tack weld the pipe in place on the bike. Be careful not to light your bike on fire, i've seen it happen to a beautiful mint condition white Cimatti city bike and it was heartbreaking. The guy who did the welding was a friend, and the guy who owned the bike was a friend and it was like 'sorry i started your bike on fire dude' ... awkward.

So yeah, tack it, less chance of you starting your bike on fire. Use wet cardboard all over the place to keep sparks from going into the carb or whatever. Keep a fire extinguisher handy. Dont be stupid.

Yep. Tack welded.

Now you can take it off your bike and its a lot easier to hold and manuever so you can seam weld it.

Start of the seam welding, looking good so far. Its probably the easiest to do it in 3-4 small passes.

Not the prettiest weld ever, but its what's on the inside that counts. Much better to have your weld ugly on the outside and smooth on the inside at this point because once you're done you can grind the outside.

Like this
Dang, that looks nice.

Don't forget to spray paint that. If you have chrome and you want to keep it looking nice you can mask off the ground up area and just paint a band of black or silver hi-temp paint on it. The little band is hardly noticeable.

Looky that there! Batavus with an M56 cylinder on it. What'dya know. In this picture with stock encarwi and stock M56 pipe, she was going about 32-33 mph, with plenty of torque.


  1. Cool blog site friend I'm about to suggest this to all my listing contacts.
    welding shop

  2. How did that new CDI workout? Do you have a part#?

  3. The pictures helped a lot with this explanation.