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CRF150R Big Bore Kit – Is It Worth It For Your Money?

Looking to buy a Honda CRF150R big bore kit but not sure if it’s worth the effort and your hard-earned cash? While you can easily get more power by increasing your dirt bike’s engine size, there are some things you need to know beforehand.

In this article, I’ll show you what exactly a big bore kit includes, why it may or may not be worth your investment, and the common mistakes that make it unreliable and how to prevent it.

CRF150R Athena 164cc Big Bore Kit CRF150R Big Bore Kit - Is It Worth It For Your Money?
Athena big bore kit for your CRF150R

CRF150R vs CRF150F – are you looking at the right bike?

First, are you looking at the right Honda 150cc dirt bike? There’s a big difference between the CRF150F vs CRF150R – if you have the CRF150F trail bike version then you need to read this article on 150F big bore kits.

But, if you have the CRF150R motocross bike, then you’re in the right place!

2009 CRF150RB 6 CRF150R Big Bore Kit - Is It Worth It For Your Money?

CRF150R vs CRF150RB – is there an engine difference?

The 150R and 150R expert (big wheel) models have the same exact engine. The only change to the engine over the years is in 2012 when Honda updated the piston, cylinder head, and camshaft.

When you’re looking at a CRF150R piston or big bore kit, you might notice that there are model year differences (2007-2009 and 2012-current). The 2007-2011 engines are all the same, but the 2012 and newer engine has different piston and cylinder head combustion chamber. After much research, it’s unclear if you can interchange the model year piston/big bore kits.

My best suggestion – buy the kit that is made for your generation of CRF150R engine. 1st gen is ’07-’11 and 2nd gen is ’12-current.

What is a big bore kit?

A big bore kit is simply a cylinder and/or piston kit that is larger than OEM standard size to increase the engine size. It’s a very common dirt bike mod whether it’s for a CRF 150R or another bike.

For example, a stock CRF150R piston size is 66mm in diameter, while a 164cc big bore kit from Athena (Amazon) has a 69mm piston diameter.

Are big bore kits reliable?

Many people think that a big bore kit automatically makes any dirt bike unreliable, but that’s not always the truth. While this does make the cylinder lining thinner, a quality aftermarket kit will only go slightly bigger so that there’s plenty of material to make it reliable.

When you produce more horsepower, you get more heat, which is an engine’s biggest enemy. However, the biggest problem is usually the result of poor installation and/or setup and tuning.

The 3 main issues with big bore kits

The first problem could be a low-quality big bore kit, whether it’s an off-brand kit for your CRF150R, or you just aren’t using the right parts. I like to be cheap and save money when I can, but the engine is not one of them – using name-brand aftermarket parts is worth the extra cost when it comes to a big bore kit.

The next problem is poor installation. Whether you’re paying a mechanic or not, you need to use all the proper gaskets and follow the instructions, and it’s a good idea to use a new timing chain when you have the top end off.

The third issue is not tuning the carburetor after installing a CRF150R big bore kit. A bigger piston will create a bigger vacuum and pull in more fuel. If you don’t adjust the jetting, your Honda 150R will run worse and not be as reliable as it could be.

How a big bore CRF150R can INCREASE reliability

The higher you rev an engine, the more heat it will create and this causes it to wear out faster. So, when you increase the low-end and midrange torque, you don’t have to rev the engine as high to get the same amount of power (acceleration).

Simply put, when you install a big bore kit, it can be just as reliable as stock, if not more so because you don’t have to rev it as much to go ride at the same speed.

CRF150R with an aftermarket head pipe
CRF150R with an FMF Megabomb head pipe

Why get one?

So, if there are so many issues with big bore kits, then why get one? Well, if you follow the proper steps to installing one (which is basically the same as rebuilding the top-end with OEM parts), then you shouldn’t have issues.

The biggest advantage to installing a big bore kit on your Honda CRF150R is getting an instant power increase – it’s almost a no-brainer if you have to replace the piston and cylinder anyway.

What to expect with a CRF150R big bore kit – horsepower?

If you do nothing else, besides properly adjusting the jetting, a bolt-on big bore kit will give your CRF 150R a boost in torque and horsepower through the RPM range. The biggest gains will be felt in the low-end and midrange torque – especially if you increase the compression ratio.

The top-end horsepower may feel a little bit flat, but that’s not because there’s less power. It’s just because there’s more power in the lower RPM range, so it’s not a big surge in power once you reach high RPM.

Horsepower increase

A stock CRF150R has about 23 horsepower, so a big bore kit alone would make about 24 horsepower, which is about a 5% increase, but you would most likely get a 5-10% increase in low-end and midrange torque. If you want more power, adding compression and a good cam would give you even better results.

Do you need to modify anything else when installing a big bore kit?

Typically, no, you don’t need to modify any other parts. However, you may feel a much bigger increase in overall power when you combine upgrades.

For example, when you increase the engine size, then the intake and exhaust aren’t properly matched for the engine size, so it won’t run as efficiently as you could.

Combining to get an efficient running engine

You see, when the engine is sucking more air and fuel but the intake port and valves can’t keep up that extra demand, they start restricting power potential. The same goes for the exhaust.

So, when you modify the ports and upgrade the camshaft and exhaust to match the size of the big bore kit, you will have a combination that makes much better power at every RPM because they’re all working together for the same result.

The only question is, how much money do you have? A big bore kit is one of the best and easiest ways to feel a real increase in power, but you could get a lot more if you spend more money on other areas.

What is the max size you can bore out a CRF150R?

The biggest bore kit you can install on your CRF150R is a 175cc kit from L.A. Sleeve. There are no machining or engine case mods required, but it requires a custom sleeve from them to work. It’s definitely not cheap, but it’s a complete kit that gives you a big increase in power when properly tuned.

What size big bore kit is best for you?

A stock CRF150R is 149cc, while the most common big bore kits increase the displacement to 159cc, 164cc, or 170cc. How do you know which one to choose?

Here’s what I would do depending on your needs and future plans:

  • 159cc: If you want the most reliability or want the ability to bore it out bigger when it needs to be rebuilt in the future.
  • 164cc: You want the best bang for your buck – these kits are fairly affordable and easy to find with a decent power increase (10% more displacement)
  • 170cc: You want a bigger power increase and don’t mind slightly more maintenance that might be necessary
  • 175cc: Money is not a factor and you want the biggest race engine possible

CRF150R Big bore kit vs high compression piston

The stock compression ratio on a Honda CRF150R is 11.7:1 – a high compression piston ranges from 12.0:1 to 13.0:1. 

The biggest difference between adding more compression and bolting on a big bore kit is that high compression generally adds more low-end torque and a big bore gives a little more power increase overall.

You should get a high-compression piston kit if you:

  • Are on a budget
  • Just need to replace the piston (the stock cylinder is still usable
  • Want more low-end torque

You should get a big bore kit if you:

  • Have a bigger budget
  • Are rebuilding the top end and need to replace the piston and cylinder
  • Want a bigger power increase overall

Where to get a CRF150R big bore kit or HC Piston

The best mod to make you ride faster (and safer!)

Increasing the power on your Honda CRF150R can definitely help you ride faster, but it’s not the most efficient way to do it, especially if you want to increase your safety as well. 

I struggled with a lot of basic riding techniques, such as balance because I’m short and not very strong, but I learned that focusing on the fundamentals is the easiest and quickest way to become a better and more confident rider, and now I want to share with you how I did that – click here to learn more.