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Athena Big Bore Kit
Athena Big Bore Kit

$252.99 - $1123.99
15% Off - Save up to $197.96

Item #ATH0004

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10 Reviews
4.2 out of 5 stars
5 Star   8 4 Star   0 3 Star   0 2 Star   0 1 Star   2
All Reviews
TODD Verified Purchaser
1 month 4 days ago Slidell, La
Durability5/5Ease of Installation5/5Performance5/5Power5/5Style5/5
Improved power

I installed the 290cc big bore kit on my YZ250F along with the hot cams stage1 cams. Nice improvement in the low to mid range power with no loss on top end. Allows me to be even lazier in the trails with plenty of roll on torque, LOL. Some tuning advise after installing this kit you will need to go down in jet size or it will be too rich. Yeah I know just the opposite of what you would think.

Markh Verified Purchaser
3 months 25 days ago
Durability1/5Ease of Installation3/5Performance2/5Power3/5Style3/5
Sheared piston pin clip

Installed this after 5 hours the pin found it's way out the side of the piston destroying the piston and cylinder. Faulty hardware

Andy C Verified Purchaser
Over 5 months ago Norcal
Durability5/5Ease of Installation5/5Performance5/5Power5/5Style5/5
Perfect improvement!

My son needed just a little extra out of his yz85, so 3 years ago I upgraded his top end to the 105cc big bore, and just like that he was back in the game, keeping up with the bigger bikes. Didn’t have a single problem with installation and even picked up extra set of rings and it lasted almost 3 seasons. Also no machining required.

Roger21 Verified Purchaser
1 year 1 month ago Tri cities Washington
Durability5/5Ease of Installation5/5Performance5/5Power5/5Style5/5
Awesome kit

Great kit really easy to install, I put it on my 07 CRF250R along wth a complete lower end rebuild, and stage 2 hotcam and it rips! Took some of the top end speed out so you’ll need a rear sprocket with less teeth good kit so far and seems high quality

Jerbo Verified Purchaser
1 year 2 months ago Santa Rosa, Ca.
Durability5/5Ease of Installation5/5Performance5/5Power5/5Style5/5

Fun to install and what an incredible change in performance!!!! The sound of the engine is a much deeper tone. Great for someone who doesn't necessarily know alot about engines. Follow the instructions and feel like you own your engine and its capability. It's very enpowering.

Ler2112 Verified Purchaser
1 year 3 months ago Murphysboro, IL
Ease of Installation5/5Performance5/5Power5/5
More power

I have recently installed the BBK and the stage 2 intake and exhaust cams. The power gain is amazing. My ‘08 DRZ 400sm had 33,000 miles on it. Instead of reringing it I dropped some coin and went with BBK and cams. I’m so happy I did. Installation was easy, I have almost 500 miles on the kit so far with no problems. I just recieved my fcr 39 and will be installing it tomorrow. This thing has really come alive with BBK and cams, can’t wait to see what the fcr does

2 years 5 months ago South Carolina, USA
Durability5/5Ease of Installation5/5Performance5/5Power5/5Style5/5

This kit absolutely made my bike stomp all competition track and trail once paired with a ported racing cylinder head and the correct jets. This is money well spent. The last test the bike was clocked at 128mph on stock sprockets. Kudos to Athens and a list of other upgrades

Twistedthrottlegarage Verified Purchaser
Over 3 years ago
Durability1/5Ease of Installation1/5Performance3/5Power3/5Style3/5
Save your money

Get your cylinder machined rather than buy this mass produced one. I dont think the quality is there. I would probably just take a stock 125 and do other mods to it. Not enough gain for the money imo.

Over 5 years ago
Great Bark

I have 2 2004 CRFs, one stock and one big bore. Big difference in power and really aggressive bark. Much more torque on hand. You have to have intake valves on spec to start the extra displacement. I may go for kibblewhite valves to keep the compression over time.

Over 11 years ago

Awesome part. Gave me alot more horsepower and a definately noticable difference in the corners. Overall worth every penny.

113 Questions
Answered:   106
Not Yet:   7
Q: Got a 2003 yz250 2 stroke   Blew my top end up looking to replace. With the Athena big bore for this bike and if so is there any mods I need to do ? 
2 months 18 days ago
Top 25
2 months 18 days ago
A: Some kits require case boring and it will specify in the instructions. Otherwise this kit should bolt right up without any other parts.
Q: I was wondering something about the kit. It will fit my bike, yes. But will I need to do anything to crank,etc. will I only need to adjust things like values, and carb? Is there anything else Ill need to do to the bike when installed or installing
2 months 24 days ago
Q: Will I need to buy anything else to install this on a 2017 Yamaha WR250R like a fuel pack or something? 
3 months 28 days ago
Top 10
3 months 28 days ago
A: The bike will need to be tuned to the bigbore kit, a fuel injection tuner like the one from jd jetting may be a good option to look in to. This will insure the bike is running at peak conditions.
Q: Will I have to bore the cases rmz450 with athena big bore kit 100 mm
4 months 4 days ago
Trevor Gearhead
Top 10
4 months 4 days ago
A: It doesn't look like that's needed on an RMZ450.
Q: With this kit on a 2019 YZ125, will I need to bore the case or is this a bolt-n-go kit?
4 months 18 days ago
Top 1
4 months 18 days ago
A: You will need to bore the cases for that bike. Here are the notes from Athena's site:

ATTENTION! To assemble the cylinder kit P400485100030 Ø 58 you need to make the boring of the engine crankcase.All ATHENA products, which are manufactured with higher displacement and power than those permitted by law of the country where the end user lives, are intended solely for competition-sports usage. Use on public roads as well as in aeronautics and marine is prohibited. ATHENA is not responsible for any different usage.The customer takes full responsibility that the distribution of the articles purchased from Athena is in line with the current regulations of his country and therefore frees Athena from whatever responsability in this matter.
Q: What else would I need to buy in addition to this kit to rebuild the top end of my 2014 yz125?
4 months 19 days ago
jolly bob
Q: I have a 2002 yz250f I want to install a big bore kit. what should I start with for a main jet?
Over 5 months ago
Top 25
Over 5 months ago
A: I am sorry we cannot give jetting recommendations.
Q: Will this fit your stoker crank?  2007 trx450er
Over 5 months ago
Top 25
Over 5 months ago
A: I checked and the stroker kit will only work with a Cylinder Works Big Bore kit.
Q: Does this bolt up to the stock head for an 04 yz250f
Over 5 months ago
Top 25
Over 5 months ago
A: Yes
Q: What size jets wuold I need for a 2007 klx110
Over 6 months ago
Top 10
Over 6 months ago
A: Athena does not offer the Big Bore Kit for the 2007 Kawasaki KLX110.
Jet kits for the KLX110 come with a variety of jets so that you may fine-tune your jetting to your particular needs and conditions.
1 - 10 of 113

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See Fitments
# Part Numbers
Product Variant Manufacturer Part #
164cc P400210100023
280cc P400210100033
280cc P400210100009
490cc P400210100030
490cc P400210100001
490cc P400210100021
130cc P400250100005
280cc P400270100004
280cc P400270100007
80cc P400270100002
144cc P400250100011
80cc P400250100007
290cc P400250100013
290cc P400250100018
490cc P400250100003
490cc P400250100015
490cc P400510100008
435cc P400510100002
290cc P400510100004
290cc P400510100010
290cc P400510100020
490cc P400510100006
490cc P400510100012
490cc P400510100016
480cc P400210100007
490cc P400210100017
290cc P400485100032
478cc P400485100016
144cc P400485100030
144cc P400485100029
293cc P400485100042
105cc P400485100038
290cc P400485100026
290cc P400485100012
478cc P400485100021
478cc P400485100014
300cc P400220100002
300cc P400220100004
490cc P400250100010
290cc P400250100019
490cc P400250100023
365cc P400270100005
276cc P400270100015
276cc P400270100017
276cc P400270100021
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