Wondering if a CRF150F big bore kit is actually worth your time and money? A good quality big bore piston kit (Amazon) is a great choice if you’re rebuilding the top-end, but there are quite a few factors involved that may affect your decision.
In this article, you’ll learn:
- What exactly a big bore kit is
- Why you should or shouldn’t get one for your Honda CRF150F
- How to choose the right kit and size based on your specific bike year and your budget
CRF150F vs CRF150R – are you looking at the right bike?
I just need to make sure that you’re looking for parts for the right Honda dirt bike because it’s easy to get some bikes confused. This is for the Honda CRF150F trail bike and NOT the CRF150R motocross bike.
What is a big bore kit?
A CRF150F big bore kit is simply a cylinder and/or piston kit that is larger than OEM standard size to increase the engine size and potentially the compression ratio. For example, a 2003 CRF150F piston size is 63.5mm, and a big bore piston would be 65.5mm in diameter.
Why get one?
The simple reason is to get more power. The more displacement, the more power potential you have.
The keyword there is “potential”. While a CRF150F big bore kit will instantly give you more torque across the RPM range, there are other factors that limit “how much more power” you’ll get.
Besides, if you need to get your engine rebuilt, you might as well buy a big bore kit because it probably won’t cost much more, especially if you need a new cylinder anyway.
Are big bore kits reliable?
When I buy a used dirt bike, I prefer one that’s stock without a bunch of mods because they’re generally not as reliable. But that doesn’t mean every mod or aftermarket part is unreliable.
In fact, a CRF150F big bore kit can be just as reliable as stock. The two most important factors are the quality of the parts/kit you’re buying, and how well the installation goes.
Common problems that lead to a major engine failure are using cheap aftermarket parts that use lower quality metals or they have poor tolerances, as well as poorly installing the components.
Timing chain issues
Another issue is when people use the original timing chain and tensioner when they rebuild a high-hour engine with a big bore. Even though the stock cam chain and tensioner are good when everything is still original, you should replace them if your engine has a lot of hours for a couple of reasons.
A cam timing chain slowly stretches out over time (faster if the engine was run low on oil), so the longer you run it, the more the timing of the cam and valves will be off because it’s acting like a longer chain.
When you rev the engine at high RPM, a worn chain is more likely to have a valve hit the piston because the timing is so far off – this will cause a bent valve and instant engine seizure, which is not only an expensive repair but dangerous if you’re riding it.
Can you install a big bore kit yourself and still be reliable?
If you can rebuild the top end of your CRF150F 4-stroke engine, then you can install a big bore kit with the same reliability. A complete kit will come with everything you need to bolt it back on like a top-end stock.
However, if you just buy a big bore piston kit without a cylinder, then you will need to get your OEM stock cylinder bored out. This is a simple machining operation that requires a mill or a lathe to bore the cylinder to make it perfectly round again, and then out it to the correct size.
Pro-tip for big bore pistons: If you’re sending your cylinder to get bored out, make sure you send the new piston as well to have them match it to the correct size. Every piston has a specific clearance needed, so if you just have your machinist “guess” the proper size, it might be too big or too small and cause problems right away when you install or start the engine.
Do you need to re-jet with a CRF150F big bore kit?
In most cases, yes, you’ll need to adjust the carburetor jetting if you actually want a better running dirt bike. If you don’t adjust the jetting, then it may run too lean or too rich, causing it to be:
- Hard to start
- Poor throttle response
- Low power improvement
- Poor reliability
- Poor fuel mileage
When you make the engine bigger, it’s going to suck more air-fuel through, generally making it a little richer (too much fuel). If your jetting was good before installing the big bore kit, then you’ll likely have to go slightly leaner on the jet circuits.
You can get started jetting the fuel screw in just a few minutes with my Free guide here so that it starts easier and runs better with more reliability.
Which year CRF150F do you have?
There are two different engine generations/styles of the Honda CRF150F – this means that not all CRF150F big bore kits will fit any year 150F model you might have.
This is how you tell the difference between CRF150F engine years:
- 1st Gen engine – ‘03-’05, kick-start only, 63.5mmx49.5mm bore & stroke (157cc)
- 2nd Gen engine – ‘06-’17, electric start only, 57.3mmx57.8mm bore & stroke (149cc)
The 2003-2005 engine is based on the CRF230F engine but has a much shorter stroke (cylinder height) and a slightly smaller bore (piston diameter size). The 2006+ is an updated engine that’s completely different – parts will not match.
1st gen vs 2nd gen CRF150F – which is better?
The older-style CRF 150F engine from 2003-2005 starts out with a 5% bigger engine (157cc vs 149cc), thanks to its extra large bore size, but that’s about where the advantage stops when you modify it.
Both are great dirt bikes if you’re a beginner or trail rider, but the newer generation 150F engine has more potential in power and reliability if you build it out big.
Kick-start engine issues?
The problem with the older engine when you add more compression and power is the kick-start gears. There’s no electric start, so you have to use the kick starter, whereas the newer engine has a reliable electric starter and an updated top and bottom end.
The gears are fine for a stock engine, but a high-compression engine requires more force to start it. If you don’t use proper technique when starting, you could crack the gears and/or the engine case, which is a huge problem.
Should I not use a big bore on my 03-05 engine then?
A big bore kit can still be reliable, you just have to keep two things in mind when modifying the older kick-start engine: don’t add a lot of compression, and learn how to efficient kickstart the engine. I wouldn’t go over 11:1 on the compression ratio unless you’re going to be careful when starting it each time.
What is the max size you can bore out a CRF150F?
On the 03-05 engine, you can bore it out to 175cc and still be perfectly reliable. Or, if you want to add a stroker kit, such as one from Enginesonly, you can bore and stroke it to 248cc!
For the 06-17 engine, you can bore it out to 225cc and it will still be reliable.
What size CRF150F big bore kit?
What are your goals and what’s your budget? If you just want to replace the top-end for cheap while getting a little more power, then a 175cc kit for your 03-05 engine or a 195cc kit for your 2006+ engine is a great choice for around $300 or less.
For the ultimate CRF150F build, you can get the 03-05 250cc big bore and stroker kit from EnginesOnly or if you source it yourself. The 06+ 150F can be bored out to 225cc for a reasonable price – that’s 50% more displacement for well under $1000!
How to make the most out of your big bore kit
While you can get a nice boost of torque and horsepower with a big bore kit by itself, you’d be shocked at how much better it can be if you put it all together.
What do I mean?
Well, the stock intake, engine, and exhaust all work together to pump and fuel in and out efficiently, more or less. So, when you make the engine bigger, it’s going to pump more air and fuel than the intake and exhaust are meant to handle – especially at a higher RPM.
This means you’ll need to adjust the jetting at a minimum, as well as use a better-flowing aftermarket exhaust (Motosport). That’s just the beginning – the stock cylinder head ports and camshaft will also be restricting the power potential.
What to expect with just a big bore kit
In most cases, just adding a big bore kit will give you better low-end and midrange torque. The is great for trail riding, but it will feel “flat” on top-end HP unless you open up the intake and exhaust.
For more overall power, you’ll want to modify other parts so that everything works better. Below compares the different set ups needed for different types of power curve characteristics that you may or may not want.
How to make the most low-end to midrange torque for trail riding
To make more torque in the low to mid RPM range, you want to have more air velocity at low engine speeds. This generally requires a smaller intake/carb, smaller intake and exhaust ports, less cam duration, and a smaller exhaust.
But when you make the engine bigger, stock is just too small for making the best power – especially the stock CRF150F exhaust.
For the best low-to-mid power curve on your CRF150F:
- Big bore kit with higher compression
- Stock carb re-jetted (or one size bigger at the most)
- Ported cylinder head for better velocity (keeping the ports as small as possible)
- Hotter camshaft with more lift and slightly more duration
- Slightly bigger exhaust pipe and better flowing muffler
How to make the most peak horsepower for racing
While keeping the velocity of air high from the intake is important, you also need more air flow to make more top-end horsepower (Remember, more air = more potential power). When you make one thing bigger, then you also need to adjust the whole set up to work well together.
For the best low-to-mid power curve on your CRF150F:
- Big bore kit with higher compression
- Bigger carb – Flat-side or PWK 28mm (Amazon)
- Ported cylinder head for better flow while keeping velocity high
- Hotter camshaft with more lift and more duration
- Bigger exhaust pipe and better flowing muffler – stepped pipe with megaphone for flat track racing
The best mod to make you faster & safer
Interested in riding faster or just safer with more confidence? While a CRF150F big bore kit will certainly increase the power, you’ll need better suspension and one more mod to actually ride it with control.
To safely handle your dirt bike off-road, whether it’s low speed or high speed, you need to be able to perform the basic techniques well, which most people fail to do. I want to give you a free guide that shows you these essential techniques – click or tap here to download it.