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Athena Big Bore Kit
Athena Big Bore Kit

$250.99 - $1108.99
14% Off - Save up to $187.00

Description See Also: 5 Reviews 81 Questions Fitment Part Numbers
Item #ATH0004

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5 Reviews
4.2 out of 5 stars
5 Star   4 4 Star   0 3 Star   0 2 Star   0 1 Star   1
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All Reviews
Verified Purchaser
Murphysboro, IL
Ease of Installation 5/5 Performance 5/5 Power 5/5
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

South Carolina, USA
Durability 5/5 Ease of Installation 5/5 Performance 5/5 Power 5/5 Style 5/5

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

Verified Purchaser
Durability 1/5 Ease of Installation 1/5 Performance 3/5 Power 3/5 Style 3/5
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.

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.


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

81 Questions
Answered:   75
Not Yet:   6
Q: On a 2006 YZ125, will there need to be any case boring, or is this kit going to be a bolt up and go?
8 days 19 hours ago
Top 250
8 days 19 hours ago
A: Some kits will require case boring it depends on your machine
Smoke me mx
Q: Do you have a 2003 RM 144 kit?
22 days ago
Top 10
22 days ago
A: At this time I am currently not showing that I have anything of that sort for you.
Q: Do you offer a high compression piston with your kit, and do I have to modify my cases for this kit?
3 months 8 days ago
I have a 2005 Yamaha YFZ450. I already have a Wiseco 13.5:1 compression piston. So, can I get a high compression piston with your big bore kit? I want at least a 12.5:1 or higher compression ratio piston? Plus, I want to make sure that I don't have to modify or machine my cases, for this big bore kit to fit on?
Top 10
3 months 8 days ago
A: The manufacturer is not listing any info as to whether your specific make/model of bike will need the cases bored, nor are they giving specs on the piston that is in this kit. Prior to your purchase I would suggest contact Athena via email (they are outside of the U.S.) with your questions.
3 months 23 days ago
I have a rm 125 stock 54.05 piston change to a what bore is that considered to be ???
Top 10
3 months 23 days ago
A: If you had it bored to a 56.00 mm piston it would be 134cc displacement. Big bore kits are 58.00mm pistons with a 144cc displacement for your rm 125.
Q: Tuning on 490 kit
4 months ago
Hi I have an unfinished 08 YZ450F with the 490 kit, and looking at a hot cams stage 2 cams. Can you please suggest what jet sizes or carby modifications that would be needed to suit this setup Thanks
Top 10 Contributor
4 months ago
A: We don not have the ability to suggest jets or tuning for custom applications due to multiple variations.
Q: Will timing chain and timing chain guide fit?
Over 6 months ago
Will the original timing chain and timing chain guide fit with this kit?
Top 10 Contributor
Over 6 months ago
A: Yes.
Q: Would case boring and bottom end work need to be done on a 2014 yz 250?
Over 6 months ago
Top 10 Contributor
Over 6 months ago
A: This is a direct bolt up to your motor.
Q: Will the athena 290cc big bore kit make a noticeable difference and is there any thing you have to change to make it run correctly
1 year 3 months ago
Q: Is case boring required on a 2014 crf450r?
1 year 4 months ago
Top 250 Contributor
1 year 4 months ago
A: case boring should not be required
Q: Would case boring be required for installation in a 2004 KX65?
1 year 4 months ago
1 - 10 of 81
See all bikes that this part fits...
See Fitments
# Part Numbers
Product Variant Manufacturer Part #
164cc P400210100023
280cc P400210100033
280cc P400210100009
490cc P400210100030
490cc P400210100001
490cc P400210100021
130cc N/A
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
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