STL SmallBore Blog post: MiniMoto Series


Honda Grom/Monkey Upgrades overview

This is intended as a beginner overview, one that someone new to the Honda mini-moto (and similar) bikes might be able to use to help them understand and decide on what upgrades they want to do, if any. Personally, I’m not on team “keep it stock,” and I’m guessing that if you’re reading this you aren’t either. While you're reading through this please note that any underlined words you see are links to related product listings and instructional content, any links to products you see may not be current or appropriate for your year/model of bike. They're intended as examples only.

The 21' Honda Monkey is my main minibike, and so it's important that you keep in mind that this overview is written primarily with that bike in mind, although I'm going to try to point out important notes for the re-design that was released in 2022. As a cautionary tale, one thing that I learned after swapping out a couple parts is that Honda’s design team did a really great job balancing out the visual weight of this bike. Once you start swapping parts, either for looks or performance, you’re going to realize that you’ve begun replacing links in a very long chain. As you make changes to parts that can be seen, you’ll may soon find yourself thinking that without that big ugly airbox, the seat looks disproportionally large, and once you change the seat to a sleeker one you notice how comically large the tail lights are, and on and on… This isn't quite as bad when making changes to the Grom. Most things are hidden by the plastic body panels, so you can pretty much just do a rear fender delete, change the tail light and blinkers to some low-profile integrated lights and call it a day.

Get familiar with what’s available

Spend some time going through the full parts catalog for your bike from some of the most popular vendors. DrowSports, SteadyGarage, Mnnthbx, HardRacing, Japan.Webike.net, beezdeals, TST Industries, T-Rex Racing, CJR, DCR, DHM.

While it's possible to get parts from AliExpress, Amazon, Etc. I don't personally recommend it. Some things are fine, some things are poorly engineered with questionable materials, and some are straight up counterfeits. On a personal note: lets support the people who research and develop awesome parts, please don’t support counterfeiters just because it’s cheaper.

Consider Build trajectory: Street Vs DualSport Vs Offroad Vs Stunt

The Monkey is pretty dual-sport in its stock form, and the grom is pretty street. While you might say that any bike can be used just about anywhere, keeping your specific use case in mind will help when choosing fit-related parts like handlebars and rearsets, and things that affect the overall performance of your bike like street tires Vs. knobbies. Consider how far you want to go with your build. Do you want to change a couple things to personalize it or add a little power, or do you want to build a statement piece that you might not ride as much? That may sound silly, but I've seen plenty of builds that aren't ridden simply because they were built with style in mind more than functionality. Some very cool looking modifications can make a bike quite impractical to ride. For those concerned with practicality, consider the cost of replacement parts for your use-case. You probably don't want to deck your stunt bike out with a bunch of expensive lightweight carbon body panels since, although they look good, they're also much more fragile. Similarly, OEM parts and pieces can be more expensive on the monkey than other mini-motos simply because they're made with more metal than plastic, which is why i decided to make the monkey my street build, and the grom my off-road one. Once you decide on your direction, the research stage begins, this is where it's particularly important that you get familar with what's available. There are so many options with aftermarket parts It can be overwhelming to spend weeks browsing through website after website as you make notes of what parts you like, and which vendors carry them. Sometimes it's easiest to consider which manufacturers & stores naturally fall in line with your goals. Examples of this include: If Off-road or ADV is your thing, start at T-Rex Racing. For stunting, start at https://www.tacticalmindz.com/. For drag-strip oriented engine mods, Koso is your brand. For everything else, any vendor listed in the previous section is great.

Aesthetics

Look for inspiration from the owners groups. Go through the popular parts vendor/manufacturer’s websites and just see what’s available. (Drow, Steady, Mnnthbx, HardRacing, Webike, beezdeals, TST Industries, T-Rex Racing.) Get on instagram and take a look at some of the wild builds that people do on these bikes. Don’t know where to start? Follow STLSmallBore on instagram and then check out some of the people we’re following.

Look into small bike cultures like the Japanese 4 mini style, check out special edition bikes like the Thai Special editions (Gundam, Hotwheels, etc.) Also, remember that aesthetics aren’t all just about what flashy parts you put on your bike. Practicality Is an aesthetic too. The most important thing is that you have a cohesive style. Don’t just throw a bunch of random parts at the bike that look good on their own but don't pair well with your bike or the other parts you’re using. Do what makes you happy, but to me aesthetic mods are more about looking good than just buying a pile of expensive parts; money can’t buy taste.

Suspension

The factory suspension on these little bikes is generally fine, but it's far from amazing and definitely not meant for heavier riders. Thankfully many aftermarket suspension options are available. There's a suspension upgrade for just about any budget, but you should spend as much as you can comfortably afford in this area because suspension is something that can be a huge safety issue if it's not functioning right.

Forks: This may be over-simplifying, but there are basically 3 components to forks, the springs, the valves, and the oil. All three of these components work together to determine the amount of resistance the forks put up when hitting a bump and how quickly they rebound. While the cheapest option is to simply change out your fork oil to a heaver weight, It's advised that you consider changing your fork springs to match rider weight. Ideally, you'll want to install a complete Fork insert kit, available from companies like Ohlins, Racing Bros, Progressive Suspensions, etc. These complete fork kits include springs and valves to ensure that compression and rebound rates are matched appropriately for your exact weight and suspension needs. If you're the DIY type watch this install video and then Buy this tool. Otherwise Steady Garage offers an add-on for installation if you ship your shocks to them, so check that out.

Rear shock: Rear shock options are even more plentiful and nowhere near as difficult to replace as the guts of your fork. The price can vary considerably more, as a great fork insert kit can be around $300, but a great rear shock can vary from $200-$1,000+. Since the design and production of these rear suspension options are made without the need to re-use Honda parts like the forks do, it's important to consider your specific application. Racing Bros suspension is great for the road, but there's been some mixed results with part failure when using them in off-road applications. Ohlins seems to be the top-tier suspension of choice for both on and off-road use. I'm personally using a Progressive Suspensions shock for my off-road grom since they offer a "heavy duty" option; I'll report back on the performance of that at some point. Just make sure you're buying from a reputable distributor, since there is a shocking amount of counterfeiting that goes on in the world of suspension. YSS shocks are commonly counterfeited.

Performance modifications

Again, this section is primarily aimed at the 2018-2021 bikes, so take it with a grain of salt if you have a 2022 or newer. Those newer bikes already have many of the common modifications built into the redesign that people did to the older bikes. For example, the newer engine has replaced the oil spinner with a replaceable oil filter, It's got a 5-speed transmission, and has a slightly higher compression piston than the previous gen.

No matter what year your bike is, there’s always something you can do to increase its performance. Since everyone uses a rating system that’s just a little bit different, we’ve put together a chart that will help explain the stages of engine upgrades shown here.

If you're a manufacturer or parts distributor, and you're reading this, please consider using our rating system

as we need some uniformity with this sort of thing in the community to help reduce confusion.

Engine mods can be very rewarding; However, it’s very important that you consider your skill level before attempting anything too complicated. Don’t just consider the cost of the parts, but the amount of time, specialty tools, and skill required to complete the project. Also, since this has came up a few times in the owners groups, When you're looking for torque specs in service manual, for any spec that's not specifically indicated in a given section there's a "general torque specs" listing in the front of the service manual (Not the owners manual.) please note that the generic torque specs are based on the shaft size of the bolt being used, not the head size which is the size of whatever socket you're using. So, if your bolt has a 6mm shaft, but requires a 10mm socket to put it on, use the general torque spec for an M6 bolt and not an M10.

Basic engine upgrades

Some of the best bang for buck can be had by just upgrading your intake, exhaust, and having your ECU tuned for the new fuel map (S1T1 on our chart.) This is something that anyone who has some basic tools and can change their own oil should be able to do without trouble. The general rule with these basic engine upgrades is that while we always advise you have your ECU re-flashed for any modification, you don't have to get your ECU flashed if you either only change your exhaust pipe or only add an intake. One or the other doesn't require re-flashing but both (and any modifications beyond that) do each time you make a change that affects engine performance. Questions like "should i wait until XXX miles to do this upgrade" are simple. If the part you're changing is inside the engine I'd wait until after the 300 mile break-in period. Anything outside of the engine is fine to do whenever. Change your exhaust on day 1, it's perfectly fine.


ECU Tuning: Any time you do performance upgrades you’ll need to have your ECU (fuel controller) re-flashed by a reputable tuner. There are more advanced fuel controller options, but at this stage it’s not necessary or advised.

Exhausts: There are US spec and Japanese spec exhausts. Different government regulations determine the design of each pipe. Japanese pipes still make great power, but not quite as much as the US spec pipes, and in turn US pipes are considerably louder than Japanese pipes, bordering on obnoxious. The Yoshimura RS-3 is one of the most popular exhausts right now for the Honda monkey, but it's very loud. This can be toned down by adding a spark arrestor but you're still going to wake the neighbors with it. The OverRacing titanium exhaust pipes are also very popular, these can be found on WeBike. For people wanting some performance, to keep the same general look of the bike, and to keep it somewhat quiet, check out the Takegawa stock style exhaust on WeBike. For our Grom friends it seems like this generic "Kemimoto Ebay Exhaust" is one of the most popular options.

Intakes: If you spend enough time on the owners groups, you’ll see some creative intake options. Intake modifications can be as simple as dropping in a high-flow air filter, making a modification to the lid on your airbox, or swapping to a brand new cold-air intake system. Intake designers focus on controlling the volume of air that enters your engine, the temperature of that air (due to distance from any heat-producing components,) and how turbulent the air is that's passing into your engine. Each manufacturer carefully considers the space and attachment points they have to work with and the performance characteristics they want the intake to provide. Design can affect how smooth your power curve is and it can also shift power toward the lower or higher RPM ranges. A longer intake will help provide more bottom-end power while a shorter one will help provide more top-end power.

Some of the most popular producers of intakes are Chimera, Dingerbuilt, Mnnthbx, and DrowSports. How can you know which intake or exhaust to go with? As with most upgrades, I recommend going onto the website for your ECU tuner (listed above) and looking at what the dyno graphs look like for your chosen modifications.

Clutch upgrades: Stiffer clutch springs are a very common upgrade, and a necessary upgrade once you move into the intermediate engine upgrades. When you pull the clutch lever on a Honda MiniMoto you'll notice that you could pretty much pull the lever back by blowing on it hard enough, this is because the clutch springs are very soft. This is fine for a really light rider, someone who doesn't try to power up steep hills, or isn't doing engine modifications. Most people who are doing engine modifications or who want to do wheelies should upgrade their clutch. The stiffness of your clutch springs affect how firmly the clutch plates squeeze together. That firmness translates into less clutch slippage. Many different manufacturers make clutch springs for these bikes, but everyone measures stiffness by rating them according to what percentage stiffer they are than the stock clutch springs.

If you're going to go for anything more than 15% stiffer, you should upgrade to a billet clutch lifter plate, ideally one of the lifter plates with 6 holes. You should also consider a modified backplate that has all 6 backplate posts threaded. Some things that I'd like you to keep in mind, which may not make sense until you're swapping out clutch parts, is that on the replacement clutch lifter plates the bearing doesn't drop in quite like the old lifter plate. You'll need to ensure that your bearing is fully seated, which may require a light press-fit with a bearing press or some light tapping with a rubber mallet or dead-blow hammer, depending on who makes your lifter plate. Use good judgement... Important installation note: if your clutch lifter plate is one that requires the bearing be press fit, DO NOT try to tap that bearing into place while the parts are installed into the engine. If you've removed the clutch basket to replace the back plate, You might be tempted to knock the bearing into place while the clutch is installed since you'll need to access the nut that holds the clutch basket on, which can't be done with the bearing in place. My recommendation is to re-install the clutch basket using your old lifter plate and springs, torque the clutch basket nut, and then replace the lifter plate and springs with your new ones.


Supporting Modifications

Supporting modifications aren't completely necessary when you're only doing basic engine modifications, but they're included in this section since they're very easy to install.

Clutch covers: Clutch covers are available for the 2021 and older bikes which delete the oil spinner in favor of a replacable oil filter, and it also gives you access to oil cooler lines if you want to add an oil cooler. By deleting the spinner you remove a couple lbs. of rotating mass from the flywheel, which results in a noticeable performance increase. These can be especially important If you're stunting, we've seen at least 1 instance where someone's oil spinner sheared off the crankshaft due to repeated hard wheelie drops.

High Volume (& Specialty) Oil pumps: In the 2021 and older engines, the oil pump had to be small enough to fit behind the spinner, when you start modifying these engines to the point that you're deleting the spinner, upgrading the clutch and adding coolers it's advised that you upgrade to a high flow oil pump. Kitaco, Takegawa, and Koso make the most popular oil pumps. Another thing to consider is that the oil pickups in these engines are designed to operate most effectively when both wheels are on the ground. If you're stunting your bike or just enjoy doing some long wheelies, you'll want to get a modified oil pump that's got a pickup tube extending to the back of the crankcase. Grombeardo and StraightUp Pumps are who you should be looking into for those specialty pumps.

Oil coolers: These little air cooled engines can run pretty hot without adding a cooling option. Please note that if you're getting your oil temperatures from the sensor in the cylinder of the engine, that's basically just giving you head temperature readouts, and while that's an important thing to be aware of it's not an accurate representation of oil temperature. You'll want to use a temperature sensor that's in contact with your engine oil, like this one from Trail Tech. Typically cooling solutions really aren't necessary until you get to or beyond Stage1Tier3. Anything beyond that should have cooling added. There are more ways to add an oil cooler than to change out your clutch cover like we mentioned above, some big bore kits have oil ports on the cylinder, while for the 2022 there's a kit sold at HardRacing which uses the intake valve tappet cover to add oil ports. Kitaco and takegawa makes some of the most popular oil coolers, but you can really use anything. Our only recommendation is that you avoid the Morin oil cooler that taps off the head bolts.

Side note on oil coolers: Make sure they're mounted correctly. Some people mount them upside down in order make uncommon mounting options work, but when you mount these coolers upside down gravity becomes your enemy. All of the oil will run out of an upside-down cooler when the engine is off. This makes it so that you'll really never be able to accurately gauge how much oil you should have in your engine, and it'll starve your engine of oil for a few seconds each time you start your bike while the pump is working to pump oil back through the cooler. It's not a great situation.

4th bearing mod: This refers to the Takegawa kit which adds a bearing into the flywheel cover to support the crankshaft. In it's stock form, the honda 125 engines used in the MiniMoto's only have 3 bearings to support the crankshaft (two on eitherside of the crank, and one in the clutch cover.) This upgrade isn't so much of a necessity as it is just a good idea. As mentioned in the clutch cover section, hard drops have sheared off oil spinners, and your flywheel is even heavier. These mods are reported to reduce engine vibration and increase reliability when adding power.

Watch this: https://www.youtube.com/watch?v=XHr1TOeCBEM

Cam Chain Tensioner: The stock cam chain tensioner doesn't do so well on engines making power. They wear out rather quickly. The stock tensioner is also pretty thin, which can cause some issues with the rubber button at the end of the spring tensioner. Personally, I like the Takegawa tensioner, but there's also quite a following for the Koso tensioner. In the 2022+ Models, Honda changed the design of the cam chain tensioner to a hydraulic tensioner, so it's not yet certain if the tensioners from the old model will fit in the new one.

Watch this: https://www.youtube.com/watch?v=M4baf0z6JOk (however, I'd recommend you drain your oil and lay your bike down on its side.)

Tools & parts

Don't forget to buy extra side-cover gaskets from your local dealership. They're typically cheaper than buying them from parts websites.

I'm not here to tell you how to fill up your toolbox, but some things make life easier. Since many of the fasteners on this bike are allen-key bolts, you'll want to get a set of metric allen-key sockets. Standard T-Bar or L shaped allen wrenches aren't going to cut it. Since there are so many small bolts that need torqued to relatively low values I recommend getting a 1/4 drive torque wrench that measures newton meters, like this one (which I have and absolutely love.)

The three most common specialty tools that you shouldn't go without can be found here . The flywheel puller, gear holder, and specialty socket in that kit are pretty much the only specialty tools you'll need aside from a valve adjustment tool and a quality set of feeler gauges.

Standard rear-wheel bike stands will work with these bikes. these cheap harbor freight ones are pretty popular. If you need to lift the front of your bike without using a standard front wheel stand, the most popular method is to use a ratchet strap to hoist the bike up by the handlebars using an overhead support. (overhead beams such as floor joists or ceiling rafters, a squat rack if you have a home gym, etc.)


Intermediate engine upgrades

Make sure you’re confident in your abilities to complete these tasks

Camshaft: Upgrading your camshaft is the next thing to do after you’ve upgraded your intake and exhaust. Simply put, camshafts control the intake and exhaust valves in the head of your engine. The shape of the camshaft lobes determines how far the valve opens, and how long it stays open. By selecting different camshafts, you can shift the power curve of your engine to the high or low end depending on how it’s designed. Check with your tuner to see what cams they recommend, and which ones they support with their flashes, etc. Popular cams include the ones offered by TB Parts, Takegawa, Yoshimura, DCR, DHM, & CJR. You’ll need to make sure to adjust your valve clearances after installing a cam. There are loads of videos on this, watch several before completing the job. There are special valve-adjustment tools, which I recommend, but not everyone uses those. Motion pro makes a great one


High compression piston: If you have a 2022 or newer bike, your piston is already a higher compression than the 2021 and older bikes. Keep that in mind when if you're considering a high compression piston. Going high compression means your ability to choose lower-grade gasoline goes out the window. It’s premium or nothing. Since these bikes have such a small tank volume, there have been concerns about topping off your tank at the pump since whoever got gas before you probably filled up with regular, and there’s going to be some amount of regular fuel remaining in the pump hose. It's not all doom and gloom though, these pistons are a great power adder if you want to keep the stock bore size.

When you’re going with a high compression piston, there’s a few options. WeBike has some, DrowSports has one, but the champion of high compression pistons for the stock bore Honda MiniMoto’s is DHM. There are plenty of videos to help explain install, but I want to stress the importance of making sure that your piston-rings are gapped appropriately. You'll need a set of feeler gauges and one of these filing tools. People have detonated their engines with poor fueling practice and by not bothering to check and set their rings appropriately. Additionally, make sure that the ends of the wavy looking oil scraper ring are touching and not overlapped when you install the bottom rings. There's also some debate about the direction you want the gaps to be on your wrist-pin circlips, so just watch this and do what Wiseco tells you to.



Advanced engine upgrades

These are a bit more complicated than the previous upgrades and far more expensive. They require several additional parts/tools/skills. I really don’t plan to discuss these in great detail, the point is really to make you aware of the options rather than give a detailed rundown of how to do them.

This is also the point most people stop since most things beyond this point can’t be managed by a simple ECU reflash. Because of the nature of custom work and the way these advanced modifications work when paired together, you’ll need to either take your bike to a tuner for dyno tuning, or use a fuel module like an Aracer with a wide-band 02 module to map your air-fuel ratio correctly.

Engine head upgrades: When you see people claiming that they make over 14HP on a 125cc engine, this is how they’re doing it. Some people see 17-18hp on their 125cc engines after doing all of the previously mentioned upgrades (intake/exhaust/cam/high comp piston/etc) and then adding a race head. Sometimes head-work is necessary to prevent valve float in high RPM applications. Valve float happens when your RPM's are so high that the valves aren't able to close quick enough, and they remain open to some degree when the next rotation of the camshaft opens it all the way. At best, this hurts performance, at worst your valves might collide with the piston and do extensive damage. There are two ways to deal with valve float, Lighter valve components, or stronger valve springs. Titanium valves, like the ones in the finbro heads, are lighter but the ends of the valve can mushroom out as the rocker arm slaps it thousands of times per minute. DHM Makes titanium valve spring retainers to reduce some weight, and DCR makes some dual-spring valve kits for super-high revving engines. Just keep in mind that super high RPM's (11k+) will shorten the overall lifespan of your engine no matter what sort of head work you do.

Ported / Big valve heads: Loads of people offer ported heads. DHM & JC Racing are who I'd go with. The general idea is to make the inside of the head smoother, remove casting imperfections, increase flow/volume for air and exhaust and reduce turbulence. Heads can also have bigger than stock valves to allow more flow. With big valves, you’ll need to make sure that your piston is able to accommodate the size of your valves without making contact. As you can see with the DHM high compression piston mentioned previously, reliefs have been cut into the piston dome to keep it from contacting the valves.

Big Bore Kits: BBK's are the reason the stage 3 classifications on our rating system are a little more vague. There are just too many combinations to effectively classify these things.

At the moment the largest BBK available for the 2022 and newer is a 149cc; this is due to the design of the engine case and spacing of the engine head studs. As of the time of this article, the general thinking is that the 2022+ bikes are best served by an intake/exhaust/cam/tune, since the new engine platform hasn't been around long enough for all of the bugs to be worked out with bbk designs. Hopefully that will change in the near future.

As for the 2021 and older bikes; big bore kits can be expensive. The cylinder and piston is relatively cheap, but all of the supporting mods you’ll need can easily top $4k. For fuel mapping, It's advised that you to run an Aracer with autotune, since off-the-shelf ECU flashes are limited for these things. You may also need to upgrade your fuel injector. Basically, every supporting mod you've read about so far comes into play in one of these builds. You’ll need a cooling solution like an oil cooler and a higher flow oil pump. Added power means stress to other internal systems, so while you may have already upgraded your clutch springs, you'll need to split the case and put in a forged crankshaft ($300ish), and while you’re in there you may as well upgrade the transmission ($400ish) to a 5 speed on the older models. Even more power can be had if you do work to the head on your bike, which isn't just a matter of getting a ported head, it's a matter of choosing between a 2-valve, 2-valve with big valves, 4-Valve, and DOHC (dual overhead cam) head setup. You'll need to choose an appropriately size intake, since oversized throttle bodies are common at this stage. Setups can vary widely due to the massive amount of available parts combinations...

High power big-bore builds come at a massive cost of time and money, but once you've gotten to try out a bike with a big bore kit you'll understand what the hype is about. They can be absolutely mind blowing. Before riding one I assumed the power increase would be decent but nothing to write home about. The first time I rode a 4-Valve 181 it blew my mind so hard It had me questioning everything I've ever known about small motorcycles. Performance aside, you need to really think about what you want out of a minibike before you do a big bore kit. For me, part of the allure of these small bikes is that they make you feel like a Street Rossi at 40mph. I want to be able to pin the throttle everywhere I go, and you just cant do that when your BBK has the kind of power a stock 300cc would have. Hopping onto your 25+HP track Grom might not be as great of an experience on those laid-back afternoons when all you want to do take a quick ride down to the corner store for a soda.

So remember, Personalizing/modifying your bike can be as simple and cheap or as complicated and expensive as you want it to be, and the satisfaction of doing a project yourself more than makes up for the time and cost of the parts. Make sure to get a copy of the service manual for reference while you work on your bike, and most importantly have fun with it. Keep your vision in mind for what you want out of your bike during the process, and if you don't know what you want yet, that's okay too. Just keep watching those videos save links to the things that catch your interest so you can find them later. Working on your bike can be alot of fun, but what really matters is what you do on it.


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- TJ (STL Smallbore) Feb 22, 2022