Building the ultimate 2 stroke exhaust protection

(It ain’t pretty – but neither was a Sherman tank…)

I love 2 strokes. Their simplicity of maintenance, light weight and buzziness will keep me a fan till they’ve all gone. But one drawback (the only one in my opinion) they have over their 4 stroke cousins is the expansion chamber. It hangs out there, just waiting to be trashed by rocks, and where we ride there’s plenty of those. Back in the old days the expansion chamber was up under the tank, and was protected (OK, I know in the really old days of Bultacos and Montessas and early Elsinores it went under the frame, but manufacturers soon caught on to that not being very wise). Nowadays though, it takes these convoluted loops as it comes out of the barrel, gets really bulky right in front of and to the side of the frame, and often hangs down below the frame in front. Rocks see it coming from miles away, and line up for a piece of the action.

So, what do you do about it? You could stick to really smooth sandy riding. I’d rather ride a Harley. You could be a pro and loft the front wheel over all the rocks, or be a real pro and just get a new pipe sponsored every ride. You could spend half your life with a gas torch blowing out dings – and the other half your life wrestling your exhaust back towards the mounts because every time you heat it up it twists out of shape. You could just live with flattened sections of the pipe, but then the bike performs like the 4 stroke you could have bought anyway. Or you could protect the pipe in some way or other. There’s various options. In my 30 odd years of riding I’ve tried most of them.

The first kind wrap around and attach to the pipe. Good for small hits and stones being thrown up, but don’t work for hard hits. (As I’ve aged the mean speed of my riding has gone down, but the mean speed of hitting rocks has increased exponentially). Hit a rock hard with these protectors and the pipe still gets dinged, or worse still warped; usually where it bends as it comes out the cylinder.

The next kind are plastic, (and I know someone out there will point out they aren’t plastic but some high falutin space age substance, but that doesn’t change what I’m gonna say about them), wrap around the pipe fully and mount onto the pipe and frame. I’m not going to mention names, they’ve been featured quite a bit lately on the site. I think they’re a step up from the first type, I take my hat off to the guy producing them (and he’s a local lad to boot!), and they do protect a lot better than the first kind I mentioned. But they’re not perfect. They bolt to the pipe and the frame with clamps. Give them a good knock and they move – and the pipe bends; usually once again where it comes out the cylinder. They’re quite a mission to take off and put back on, not only when you do work on the bike but just for washing it. They collect mud, which is not too bad since it’s down low; but it has to be washed off afterwards. This is not meant to be a mud slinging session (‘scuse the pun), just my observations. I do however think that they’re the best ‘off the shelf’ system you can buy. Just don’t stand up to what I need from a pipe protector.

 

The third type of guard is pretty good at protection. I don’t see them much overseas, but here in SA they’re pretty popular. Basically a cage (we call them ‘ribcage’s over here) is made around the pipe using 8mm mild or preferably stainless steel, mounted rigidly to the frame, and totally independent from the exhaust.  Advantages, they protect pretty well, they’re fairly lightweight, and they let a lot of the mud and goo through so it doesn’t collect too easily. Disadvantages?  Well the there’s two I’ve found. Firstly the way they mount (drilling the frame for bolts, or welding tabs onto the frame, or modifying the motor mounts) make them a bit of a pain to get off and on when you want to work on the motor. A bigger problem is that they do bend from big hits. You don’t notice this until you take them off, (they go ‘sproing’) and then try to get them back on again and the mounting holes don’t line up. So it’s out with the torch and a bit of wrestling to get them back in shape. Personally I’m not a big fan of drilling the frame or welding tabs onto it as well. Also you’ll still need a plate of some sort to protect the frame rails underneath. And you have to be careful you don’t intrude on the movement of the front wheel with the steel bars as the suspension compresses. Trust me, I’ve seen this happen (guy couldn’t understand why his front tyre knobs kept being shredded!)

So what do we need from a good pipe protector? Here’s a list which I think are the most important:

·         It must mount rigidly to the frame. Where it mounts the frame must be able to take a good hit, and we shouldn’t need to drill/weld to achieve this.

·         It should not mount onto the pipe anywhere, and also not onto the pipe and frame. Two stroke pipes buzz, and even if you don’t hit any rocks they’re still prone to cracking because the guard makes them rigid. That’s the reason the pipe mounts are rubber insulated. The pipe must be free floating within the protector.

·         It must take hit after hit and still protect the pipe, and it must still line up when you mount it again.

·         It must be easy to take off and put on when you wash the bike.

·         It should be pretty lightweight, and not build up too much mud.

·         It should cost less than the other protectors, and you should be able to build it at home (most of it anyway).

That’s where this design comes from. No it’s not easy, and will take you lots of time, but it will save you money in the long run. How well do they work? Well so far I’ve built 5 of them. I can’t vouch for the first one I built 10 years ago, because the KDX I made it for has long been sold. But I can tell you the next 3 – on a 98 200, an 03 250, and my current 06 200, are all going strong. Have a look at this pic from the underside of my 200, the plate is gouged and trashed. The pipe is still perfect though, and the guard still fits fine. I expect the new one I take you through building on the 300 will also last as well.

 

What do you need to build it? Basic workshop tools – drill, angle grinder, jigsaw, welder, gas bottles. I’ve only got a cheap arc welder at home, but luckily I’ve got a mate who can weld ally. He welds the whole plate up for 2 bottles of Rum and a dozen frosties! But it is critical you can get the alluminium welded somehow.

Where to begin? Start off by having a good look at the bike and the pipe. Note how far it extends below the frame. On the 200’s this isn’t much, and the bottom plate can come almost exactly straight off the frame rails and under the pipe. On the 250’s and 300’s though the pipe is quite low, so the plate has to bend downwards from the frame rails to clear the pipe. Check out what sort of coverage you’ll need on the sides. Try and get a mental picture of where you want protection for clutch and magneto covers. Check out under the frame to see where it will protect frame rails, foot peg welds, etc. Check out pipe mounts, you’ll need to clear these with the guard. Once again the 200’s are pretty easy, the bigger bikes have a mount right under the pipe in front, and require a bit of fiddling to work around. And then, most importantly, see where the lowest point of the pipe is. This becomes your starting point. On most bikes it’s a small section near the right hand (sitting on the bike) frame rail. It may look like the whole front section of the pipe is parallel to the ground but it isn’t, it tilts slightly up towards the left. Mark this lowest section (on the 300 its only 4 cm’s or so long) with a felt tip pen on the exhaust, as a reference.

Next hang the bike up in your garage, by using tie downs through then front forks, as shown. Get it as vertical as possible, without the back wheel coming off the ground. You’re gonna be spending a fair amount of time underneath it, so this hanging up makes everything a tad easier.

Now take neoprene weather sealing tape and start laying it on the exhaust, as shown in the picture. The sealer should be about 8mm thick; this will be the gap between pipe and guard later. Lay the strips along the extremities of the pipe, with the first one going along the contour of the lowest/widest part of the pipe all the way around it. The lay a few more strips around extremities. Take the sealer further up the sides of the pipe than you want the guard to go.

 

Next get yourself an old cardboard box. The cardboard should be pretty rigid, and much the same thickness as the ally you will use eventually. Mark out a piece of card which sits along the frame rails at the bottom, then bends downwards (score the card on top to get this right) till it sits under the weather stripping at the lowest part of the pipe. The cardboard will be parallel along the frame rails, but then taper (towards the right) down to about 5cm long at the lowest point of the pipe. This front taper is parallel to the ground across the bike. You need to work accurately. Mark and ‘tweak’ constantly. Use a Stanley knife (I believe they’re also known as box cutters) and steel rule to cut the board. This first part is critical that it fits nicely, because it forms the basis for all the other parts to attach to. Cut the front of the piece about a cm beyond the weather stripping. This will be where the join will be for the first part which curves upwards. The back of the cardboard ends up behind the underneath cross member of the frame. Don’t try and make it fit accurately here yet, leave it too long, you can trim it properly when the plate has been welded up.

Now zip tie this piece of card onto the frame rails. You may need a slight spacer under the front of the frame rails to make it fit nicely – I used a short piece of tile edging turned upside down to get the spacing/downward bend. Also put a piece of masking tape on the frame rails and mark a reference line from tape to cardboard. This will help you later.

Now work your way around the sides of the pipe. The next piece to the left will be a big broad piece, because the pipe is fairly flat along there. The pieces towards the right will be shorter. All the pieces extend back towards the downward bend of the original bottom piece. The front of each of them ends flush with the front of the previous piece. If your first weather stripping is laid accurately along the pipe extremity, then these pieces of cardboard all end up about a cm in front of that. As you go along, working from the centre outwards, tape the sections to the pipe (use long thin pieces of duct tape), before you move on to the next one. When you work around to the sides don’t go too high – the pieces of cardboard shouldn’t be ‘coming together’ around the pipe on either side, the last pieces should end pretty parallel to each other. Otherwise the guard won’t come off eventually, it will snag on the pipe.

When you’ve finished your first row of cardboard around the pipe it should look something like this:

 

Now go back to the first section of cardboard. Pull up the piece of tape holding it to the pipe, the other pieces of tape hold the rest of the cardboard in place. Cut a section of card the width of the original one where it tapers in the front. Place this new piece so that it folds up around the front of the pipe, not too far, it ends up about 4 – 5cm wide.  Place a piece of masking tale half over the join, hold the cardboard in place, and stick the tape to the first piece. I know I’m sounding pedantic, but it’s vital you work carefully and accurately, especially in these beginning stages. The more care you take now to get it right, the easier it becomes in the long run. Repeat this taking the tape off the section you want to work on, cutting the board to shape, sticking it in place, and move on; all the way round the pipe on both sides. Try and predict what the next piece will always look like, so that the ‘sides’ of the adjacent cardboard sections end up the same length (like the way the corners of picture frames work).

 

When this entire second row of cardboard pieces is done start with the third row. Once again go back to your original section in the middle, pull the tape back, cut a piece to size, tape it to the lower cardboard; and move on around both sides of the pipe. By now you’ve got off cuts and shreds of cardboard all over your garage, the only thing which outnumbers these off cuts is the beer bottle tops – because you’ve been going for quite a while. It takes time – I cannot stress this enough. But it does eventually come together.

 

When it s finished it should look something like this. Don’t take it off yet though, there are still things to do. Firstly go and put a piece of card on either side of the frame rail section, to protect the clutch and magneto covers. Don’t make them too wide; make sure you have clearance for the brake and gear lever.
Next take a pen and mark the inside edge of the exhaust pipe on the cardboard; this will be used as a reference later. Also mark places behind the exhaust on either side where you can create a mounting tab without it (or the bolt) touching the pipe. Mark another one under the bike towards the rear, but not too close to the rear frame member – it should be about 5 – 6cm in front of this.
Once that is done you can take it off. Remove all the pieces of tape holding it to the pipe. Snip the zip ties underneath. It should come off and form a cardboard replica of what the final guard will look like. Now you can start cutting sections of the tape so that it ends up as a flat template of cardboard, and will look something like this. You’ll find that where there were ‘concave’ bends (like the big one across the frame under the motor) the cardboard sections will overlap slightly. There’s nothing you can do about this, we’ll just weld in a small fillet later. Be careful with it, its pretty fragile. I usually put lots more tape over it on both sides to make it rigid before I carry on.

 

Now it’s off to your friendly aluminium corner store for a piece of ally the right size. Get something between 4 and 5mm thick – I use 4.5mm but I believe you can get a 4.2 and a 4.8 as well. Get them to guillotine off the right size piece. When you stick it under your arm you’ll think ‘Hey this things gonna be heavy!’ but in all honesty it ends up about a third of what you’re holding by the time you’ve finished. Head on back to your garage.

 

You need to transfer the cardboard cutout shape to the ally. Tape it down onto the ally. Now, don’t try and trace around it with a pen. What I do is use a centre punch and punch marks at all the joins, bends and cuts. Its much easier this way, more accurate, and won’t rub off as you work.  Also punch where the mounting holes will be, and put a series of punch marks through the cardboard along the inner line of the exhaust which you marked earlier. Take the template off, and join up the punch marks into your various cuts, bends, corners and joins.  With a pen mark where the inner mild steel will sit alongside the inside of the exhaust, and also a section going back towards the cross member of the frame. I use 20 X 3 flat bar across the front, and 25 X 3 towards the back. Also mark where your reference lines from the frame rails will be. You will adjust these slightly later, but it’s good to have a starting point.

 

Now arm yourself with jigsaw blades. Get those designed specifically for cutting non ferrous metals. I’ve yet to find the best way to cut the ally. I’ve tried high and low speeds on the jigsaw. I’ve tried different approach angles for the blade. None of them really make much difference. What I do know is that you should clamp the ally down close to where you’re cutting, and you should change the blade often. But it takes time still, and by the end of it your whole body (especially your eyeballs) will be vibrating at the same frequency as the jig saw… The most important cuts to get accurate are the small wedges in the end sections of the ally, where it wraps around the pipe. The other stuff round the extremities of the plate can be close, but you can always sand them down later. Don’t forget to cut some semi circle cuts (you’ll have to drill a few small holes close together first to get the blade through) around the mounting holes, to bend up as tabs later. These tabs bend inwards, so that the head of the bolt is protected away from the rocks.

 

Now you start ‘scoring’ where all the bends will be. Score on the inside of the bend, using a thin cutting disc in your angle grinder. You need to score the ally about half way through, deeper for long bends, shallower for short ones. Don’t forget to score on the reverse side where the plate bends downwards.

 

Then its time to drill all the holes. I do this while the plate is still flat, and use a combination of drill press and hole saws. Several things to bear in mind when you drill. Keep the holes clear of where the inner steel bracing will go. Don’t drill too close to edges you may need to trim later. You’re drilling for 2 reasons – to let the mud through and to lesson the amount of ally. Both of these come after strength though, so have that in the back of your mind as you decide where to make holes. One place you will have to make a series of holes is around the 2nd row of upward bends around the exhausts – not the outer row but the ‘middle’ row. Each section must have a hole in it for bending later. KTM’s drain oil from the side, but on my old KDX I also made a hole underneath the drain plug.

 

Now, the bending starts. First though it will help to make a small jig out of a section of trimmed down angle iron/ally, which fits above the frame rails but under the motor. Put a bolt through this, and tighten it with a nut all the way so that the bolt can’t turn. Then get a wing nut and big fender washer. When you start putting the plate up under the bike you can feed this bolt through one of the holes in the plate, and put the wing nut and washer on. This frees you up to eyeball the plate from all sides as you go through the bending process. You’ll find as you bend your plate will end up a tad different to your cardboard master, and you’ll need to adjust the reference marks. Typically (and don’t ask me why, I’ve never figured it out) the plate will end up slightly rearwards compared to the cardboard. That’s why I said don’t make the bottom mounting tab too far back, it could end up hitting the cross member. Trust me, I’ve learnt the hard way….

 

When you bend the plate start in the same process as when you made the cardboard master. Bottom bend down first (if your bike needs it) (actually to do this you have to bend the 2 sections which cover the clutch/magneto up slightly first for clearance), then to the middle working outwards on the initial row of bends, etc. Don’t rush it, it may seem like you’re halfway home but you’re not even close yet! Rather bend too little, check on bike with jig, then bend some more; than have to bend back because you bent a section too much initially.

 

To bend, clamp down the section adjacent to the one you want to work with. Use adjustable wrenches set to the thickness of the ally, or a metal bar through one of the holes you drilled. Apply pressure slowly and evenly – the ally suddenly ‘gives’ and then goes. Do each section till its right (ie up against the weather stripping), then move on to the next. If you have a section with a mounting tab in it, bend the tab first, then the section. The tabs should be bent enough to have the head of the bolt (I use allen caps) recessed, but not so much that they will end up beyond parallel to each other. The 2 side mounting tabs should forma slight ‘V’, so the plate can come off afterwards. Once again, I’ve got the t shirt… Then do the second row, then the third. Then you’ve got to do the correct bend needed for the clutch/magneto cover sides, and the 300 also needed a bend upwards at the back towards the footpegs. Don’t worry too much about this section right now; you’re going to need to tweak it at final fitment.

 

By this stage you’ll be glad you hung the bike up, because you’ve been under and around it so often. But when you’ve finished the bending process you get a pretty good idea of what it will look like eventually. Pat yourself on the back, you deserve another frostie.

Now you need to cut some small ally fillets to fill in any areas which are a tad wide to weld. You also need to make some curved ‘half moon’ shaped fillets to go around the mounting tabs. Then head down to your local to pick up the bribe for your alluminium welder guy, and take the plate round to him. Ask him to tack a temporary bar across the whole plate before he starts welding all the joints, to prevent it from warping.  He must also weld along all the score marks where the bends are. Believe you me, he’ll earn that bribe!

While you wait for the plate to come back you can start the next part – creating the section which attaches to the frame and the plate bolts on to. Don’t ask me what to call this – I’m gonna call it a ‘spider’ for no other reason than it looks like one, or at least the web of one.

 

 

I use mild steel for this, mainly because that’s what I can weld at home – it would be nice to be able to make it in ally though. Start by fashioning some ‘U’ shaped sections 2 which fit around the front frame downtubes where they curve. The 08’s use a funny square shaped tubing with the edges champhered, and they’re a tad difficult to mould the sections carefully. Make sure it fits nicely; this part of the guard bears the brunt of the force. You need 2 pieces which go on the outside, and 2 more for the inside. The outside ones on the 300 you have to fit around the exhaust mount on the RHS. The inner piece on the left hand side needs to fit around the magneto cover mounting bolt extrusion. It’s a bit of a pain, but it can be done.  Once this is done you join the pairs with some cross members which have 8mm bolts through the outer section, screwing into nuts tacked onto the inner cross member. Just lightly tack everything together, and then make sure you can actually get the pieces in and out before you weld them thoroughly. Also obviously disconnect all the wiring under the tank before you weld anything attached to the frame. At this stage leave the ‘ends’ of the outer sections long, you can cut them off to the correct length when the entire ‘spider’ is made. It’s a bit fiddly to get these clamping sections done, but it’s worth doing carefully because they are the heart of the entire guard.

 

By now you hopefully have the plate back from the ally welder. Admire it for a while before you temporarily mount it on the bike. Then mark the inner curve of the exhaust again. It should correlate closely with the previous one you made, but you might find after the ally welding you need to change it slightly.  Now you can start fashioning the bracing part inside the plate. Start by grinding down all the welds in the corners of the bends where the brace will go. Also grind down the weld on the mounting tabs, so that these are a smooth surface. Then make some plates up with 8mm nuts tacked on, bolt these through the mounting tabs. Now start making the bracing on the inside of the plate. I use 20 X 3 flat bar across the front, following the inner line of the exhaust. Make a small section, clamp it down, and then tack on the next small section. Do the same for one middle piece extending to the rear mount, except here I use a bit bulkier 25 X 3 flat bar. Always keep everything clamped down while you weld, otherwise the heat tends to distort it.

 

The next thing is quite important. Drill 2 X 8mm holes though the rear bracing and the ally plate (with them attached together) and weld 2 short 8 mm dowels in these holes (still with the plate attached). These dowels position the plate when you take in on and off the bike, and also keep the load off the mounting bolts. Make sure the dowels are in the section of the frame/plate which is flat – don’t have them on adjacent angular parts because then the plate won’t be able to come off. Grind them down till they’re level with the bottom of the plate. Now loosen all the mounting bolts, and make sure the bracing can come away from the plate. Don’t weld the underside of the brace yet. Leave that till last or otherwise it distorts and doesn’t fit. You want the brace to be flush with the plate along all its lengths. Now bolt it all together again. Then cut a section of angle iron (doesn’t have to be too bulky – 1.5mm thick is plenty) shorter than the lower back cross member of the frame and temporarily zip tie it in place on the bike.

 

Now you can mount it back on the bike using your temporary wing nut jig. The curved sections should already be bolted up to the frame. Make sure you position the plate accurately; this is going to be the final ‘fit’. Then – and once again this is a bit fiddly – fashion 3 pieces of flat bar to go from the bracing back to the parts attached to the frame. Make sure you fashion them in such a way that you can get a welding rod to them (remove the gear lever to make the back one accessible). Hold your breath, and tack them in place. Check the tacks are good, then unbolt the ally plate (remove the wing nut jig too) from the brace, which is now attached to the frame.

 

Before you go any further have a good look at the setup. Once again note where exhaust mounts are, and also make a mental note of how you’ll be getting tools to the curved section bolts to take them on and off. Also think about getting tools to the exhaust mount bolts. You don’t want to go and weld anything that’ll intrude on these. Then take the brace off the frame – you may need to remove the exhaust first.

 

 

Now just start adding ‘legs’ to the spider, from the curved frame bits back to the bracing. Think of where the guard will be taking punches – not only while riding but when you fall over in my case as well! – and brace back to these sections. I also add 2 short sections from the curved parts straight back down to the bracing which goes to the rear, so that if you loft the front wheel over something and the bike comes down hard on it the bracing takes the knock.  Put the spider back on with the exhaust and check for clearance between the two; you don’t need much – 4 to 5mm is fine. In fact the nearer you can get the bracing to the exhaust the better. You’ll see from my pics that I try and tuck the bracing in under the exhaust, and then grind away any parts which touch.

 

 

 

 

 

 

 

 

Now you’re close. Just a bit of tidying up with the sander, getting it to fit nicely round the rear and below the foot pegs, a coat of paint – I use an ally colour for that ‘works’ look, Hah!  Then I glue some thin rubber on the curved frame mounts to protect the frame paint; mount the spider, mount the exhaust, lastly the plate (and anything else you took off/disconnected) and go looking for rocks. Remember how they used to line up for a piece of the action before you had a guard? Well now you’ll see them slinking away as you come round the corner…

 

 

 

 

 

Incidentally I weighed the whole contraption on my bathroom scale, and it comes in at a tad over 3.5 kg’s. Now, my wife swears blind that this scale over reads by 5 kg’s, so it seems you’ll actually make the bike lighter than stock by adding an exhaust guard!

 

 

 

 

 

 

Oh, and by the way, any remarks about my welding will be treated with the same disdain my welding deserves! I told you up front it wasn’t pretty! Enjoy the project.

Pete

(Founder Member - 'The Rock Bottom Club')