The Laziest Possible Way To Install A Big Bore Kit In A Motorcycle

Big Bore Installation For The Lazy

There seems to be no write-up of how to install a big bore kit in a DRZ400, so I decided to fill the void. All I really wanted to know was

A) Do I have to remove the engine, and
B) How lazy can I be?

The answers, for those of you who can’t wait til the end, are:

A) No, and
B) Like, super lazy.

Fortunately the DRZ400 is a total tractor, so you can really softball this one in. You might be a bit nervous about performing surgery on your dear Suzi, so start by drinking at least one beer to take the edge off.

The first step is to remove the fuel tank. To do that, you’ll have to remove the front radiator shrouds, and also, you’ll have to remove the seat.

So the first step is to remove the seat, then the other stuff. Really what you want to do is remove everything between you and the piston.

At some point in removing everything that looks like it might get in the way, you’ll remember that you should probably drain the coolant. You want to do this so that when you take cylinder off, it won’t dump coolant into your crank case. That way you can just reuse the same oil, which is awesome given how lazy you’ve decided to be in this endeavor. To drain the coolant, just undo the lowest coolant hose you can find. If you’re married, you’ll want to get some sort of drain tub to catch all the coolant. If you’re single, feel free to get that towel that your neighbor left in the laundry room and toss it on the ground.


Now that you’ve drained the coolant, and removed everything that is between you and the valve cover, you’ll want to take off the valve cover. While doing this, you will notice that you haven’t yet removed everything between you and the valve cover. So remove the Horn. Eventually you will also have to take off the radiator fan, but you haven’t figured that out yet, so don’t bother.


Once you get the valve cover off, it would be a good time to check your valve clearance. To do this, you Google “how to check your valve clearance Suzuki DRZ400” or something similar. I suspect there is a write-up on that.


If you don’t have a feeler gauge, now is a good time to buy one. It is a super cheap, easy to use tool, and if you own one you can totally feel like some sort of master engine technician. You’ll want to check your clearance against the recommended clearance, unless you are an overconfident automotive engineer and you think maybe the numbers you measured probably seem about right. Exhaust is supposed to be the bigger gap, right? It’s hotter, so yea, probably, what with all that thermal expansion.

Next, remove the caps that hold down the camshafts a little at a time in a crisscross pattern. Remove the camshafts from underneath the cam chain and check out that cool decompression throw out weight thing that probably comes in really handy on the kick start models.


Before you remove the last camshaft (you haven’t already removed the last camshaft, have you?) you’ll want to put some safety wire around the cam chain to keep it from falling into the black abyss of death that is the crank case. If this happens you will have to walk all the way to the toolbox, search tirelessly for the telescoping magnet, stick it into the case, and pull up the cam chain.

But not yet.

Now you need to remove the head bolts. There are four below the camshafts, 12mm maybe, 14 possibly, who can keep track of these things.


And don’t forget the two on the outside of the engine. Also there are two more holding the cylinder down to the case; this is probably a good time to remove those as well.


Once all the important bolts are removed, and all the random horns and fans are out of the way, it is time to take off the cylinder head. It seems like I might have taken the cam chain tensioner off before this last part? Maybe. I’d go ahead and take the cam chain tensioner off about now if you haven’t already done that.

Once you have the head off, you’ll want to inspect it. For what? I don’t know, like massive head damage or something? Does it look like this:


If so, it’s probably fine. Set the head down on the extra rag you found which happens to be a Formula SAE judge shirt from four years ago.

Now is probably a good time for a beer. You disassembled it all; you can probably reassemble it all while slightly intoxicated. You didn’t build a beer dispenser using a car door handle so you could not drink all the time, did you? No, you didn’t.

Once you’re nice and lit, flip the head over and pull off the buckets with that telescoping magnet you worked so hard to get a few minutes ago. Under the bucket you will see a shim. This is what you would replace if your valve clearance was outside the specifications that you didn’t bother to look up.


When you put the bucket back on, the shim will likely be slightly out of its correct position. You won’t notice this until after the engine is together and it is not working, but you don’t know that now, because you’re too lazy to check for that sort of thing. And besides, you’re a professional automotive engineer, being careful is for people who don’t know what they’re doing.

Now, to get the piston off the connecting rod, you’ll need to remove the wrist pin. To get that out, you’ll first have to pull out the little ring clip holding it in. Since you’re replacing the pin, the clip, and the piston, feel free to just jam a flat head screwdriver in there and pry it out, quickly ducking out of the way as it shoots off in the exact direction of your eyeball.


Next, lay out all the shiny new pieces from the big bore kit you bought on eBay. Feel pride in the fact that you bought the kit with the forged JE piston, and not the cheap Chinese piston that will totally grenade your engine in the middle of the desert in two weeks.


I mean, you got the cheap Chinese cylinder, but that’s probably fine. Before you get too far ahead of yourself, check the ring gaps. It will probably be smaller than the instructions say, but only by a couple thousandths. That’s probably fine. You’d have thrown in a safety factor of a couple thousandths if you designed this thing, right? Yea, so it’s fine.


Before reassembling everything, be sure to clean off all the gasket surfaces. I’m sure there’s a great cleaner for this, but there is also some aerosol shit you have within arm’s reach. Use that.


Once that is done, go ahead and install the piston onto the connecting rod, using the wrist pin and the new clips. The clips are a total bitch to install, so have fun with that.


Check out that sweet looking piston, machining marks all on it still. Aww yea.

Now you’ll want to put the rings on the piston. The instructions say something about the top one going on top, and the gaps facing different directions. That is probably useful info to follow, mostly because it’s easy.

Once your rings are on, you can slide the cylinder down over the piston, carefully making sure the rings are compressing nicely into their grooves. You could probably use some sort of ring compression tool for this, but that is probably overkill, even if you weren’t being totally lazy.


Oh yea, before you do all that last stuff, you need to be sure and put the new base gasket on. If you haven’t done that, then just leave it off and see if the piston slams into the valves when you fire it up for the first time. It probably won’t, and you should get a little more power because of the higher compression ratio. Let me know how it turns out.

Next, install the head gasket (you probably need that one) and the head. If you never bothered to put safety wire around your cam chain, now would be the time to fish it out of the case with your telescoping magnet. Bolt down the head in a crisscross pattern to whatever torque the internet tells you is correct. Or just whatever torque feels right to you.

Now, before you install the camshafts, you’ll want to put the piston in its “top dead center” position. Or just its “top” position, which is the same thing with two less words.

To do this, you need to take a cover off the left side of the engine that will allow you to turn the crank with a socket and see where “top” is. Again, if you’re married, get a bucket. If you’re single, just assume that this is above the oil level, and just take the cover off.


With the piston at “top”, install the camshafts in the correct orientation.


Remember that the cam chain tensioner will apply pressure, so shove your finger in there and give it a little push to make sure the arrows are still lined up with the top of the head. Double check that the engine is still at top, and bolt the camshafts down in a crisscross pattern to some torque value that seems like it is probably enough, but not too much.

Right now, you will be thinking, “Ahh! I know what is next! I need to install the cam chain tensioner.”


Slow your roll. You can’t install the tensioner without first removing this little black plastic box thing. So remove that.


To install the cam chain tensioner, you need a special tool to retract the spring loaded tensioner mechanism. If you don’t have this tool, even though you have like 8000 other tools, just use a small screwdriver. You have to turn the screwdriver to unload the tensioner while you bolt it in. You need some dexterity, but not too much, so if you’re too drunk to continue, you are probably too drunk to be rebuilding your motorcycle engine.

By now, your bike has mostly emptied its bowels and your neighbor’s towel looks like Optimus Prime’s tampon.


Try to clean up as best as you can. Your garage floor is filthy, so you can probably make it look like this never happened with minimal effort, and your roommates will never know, until they read this on the internet.

Go ahead and bolt everything on in roughly the same order you disassembled it all. Don’t forget to fill your coolant back up, and add some oil.

Once your bike is all back together, you’ll notice that it doesn’t start. This is normal, and by normal, I mean not normal at all. After cranking it over for a solid minute, you’ll notice the battery starting to die. This is a good time to hook it up to the charger, grab another beer, and ponder some of the enigmas of life, like how you can be a professional automotive engineer and yet be so colossally bad at rebuilding engines. Once your battery has a bit of charge, but before you’ve finished your 7th beer, you’ll want to check one by one for the four things you need for an engine to run: spark, compression, fuel, and timing. Let’s start with compression since that is what that giant piston you just installed is supposed to be doing.


0 PSI. Check the manual to make sure 0 is within spec.

It is not.


Go back to the beginning of this guide, and repeat everything up until you remove the valve cover. After that is done, you’ll notice that one of the tiny little shims that go under the bucket has popped out of its little hole and is holding the valve open all the time. Now put your hand over your eyes, squeeze your temples, and say “Jesus Christ I hope I didn’t bend a valve.”

Sure, you’re bummed out because you rebuilt the engine without making sure everything went back where it was supposed to go, but hey, at least your last job wasn’t at an engine component supplier as a professional valvetrain engineer. Oh wait, that was EXACTLY YOUR LAST JOB.

But don’t get too bummed out, you did decide to embark on this whole endeavor super lazy and half drunk, so you’re actually doing pretty well considering.

Go back to the part of this guide where you put everything back together, but go ahead and check the compression before you get too far, just in case.

Once you get everything all assembled, just double check to make sure everything is really assembled, just in case.

Good? Good. Now, some people will tell you that the best way to break in a new engine is to keep it under some thousand RPM for some hundred miles. And, on certain engines with certain uses, that may be true. But, I’m here to tell you that wheelies are way fun. So get on your bike, and go do a wheelie. Once that is done, do another wheelie. After 6 or 7 wheelies, stop dong wheelies and go inside before the cops get show up.

Crack open another cold one, and sit on your couch with a smug sense of self satisfaction. You’re not such a bad professional engineer after all. Maybe next time you can add even more power. Maybe next time you can install nitrous! But don’t start yet, because eventually you will sober up and realize that is a terrible idea.

Big Bore Installation For The Lazy


  1. Dear Matt Brown,

    So my eight year old daughter and I love everything you do. I am a good handy father that lets his kids use powertools but I really know how to do kitchen and bath remodeling because that was my living for a long time. I don't know shit about how to work on engines. Any ideas where to start. My daughter wants to rebuild old cars with kick ass motorcycle engines – just like you – and, you know, I want to be that supportive parent but don't really know how.

  2. Very entertaining writing style .I was involved with Formula SAE at Waikato University in Hamilton New Zealand in the School of Science and Engineering as The School Research workshop Manager. The guy’s involved In the SAE and Solar car research were all real characters. Thanks for the Book thoroughly enjoyed it.

  3. Reading this in anticipation of working on my SR500 tomorrow. Coincidentally it has no compression and was built by yours truly. However where your reasoning was “I’m an engineer and don’t need the manual” mine was “I’m an artist and the rules don’t apply to me.”

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