Free Shipping Over $39.99

Orders $39.99 or more ship free within the contiguous U.S.

Free Shipping Policy

No Restock Fees

Doesn't fit? Don't love it?

Return any unused item within 30 days for a full refund.

No-Hassle Returns

  1. Obtain an RA number
  2. Package your product
  3. Send it back to us!
Return Policy

Price Match Guarantee

Found it for less?

RevZilla will match any advertised price on new merchandise available through another authorized U.S. dealer.

Price Match Policy

Elite Service Rating

Our goal is to provide the best possible shopping experience to every enthusiast who visits RevZilla.

See what our customers are saying about us:

Customer Reviews

ZillaCash Rewards

The ZillaCash rewards program is very straightforward - if you have a RevZilla account, you may already be taking advantage of ZillaCash rewards. Review the following guidelines to see how easy it is to maximize your rewards and put your ZillaCash to work for you.

  • Earn and redeem ZillaCash credit automatically with each order - no fine print, no strings attached, and no hoops to jump through to cash in.
  • All you need is a RevZilla account to start earning - Simply log in each time you shop with RevZilla and we’ll take care of the rest, keeping more money in your pocket just for shopping with us.
  • Earn $5 for Each $100 You Spend on eligible products - up to a maximum of $30 ZillaCash per order for orders totaling $600 or more. Any ZillaCash you earn is automatically applied to the next purchase, so you can sit back, enjoy the ride and feel the savings blow through your hair.
Learn More About ZillaCash

Koso 170cc Big Bore Kit With 4V Head Honda Grom 2014-2018

Item: P389264

This kit is for Grom owners who are dead serious about getting the most from their engine and who aren't afraid of serious modification. Read More

Save $94.99 (10%)
Rebate Available! View Details

This product is vehicle specific. Please to ensure fitment.

Learn More

Similar Categories

Product Details

Koso 170cc Big Bore Kit With 4V Head Honda Grom

This kit from Koso is for riders looking to push their Grom to the absolute limit, far past what Honda's engineers ever imagined. 

To make this possible, Koso has paired their 170cc Big Bore Kit with a 4 valve head conversion.

Installation is no walk in the park; massive changes are required to turn the little Grom into a fire breathing beast. Be sure to check the fitment notes and review the additional requirements document below to make sure you are ready for the challenge. 


  • Greatly improved engine performance
  • Increases cylinder displacement to 170cc
  • Upgrades engine from 2 to 4 valves
  • Stainless steel 24mm intake valves
  • Stainless steel 20mm exhaust valves
  • Performance cam shaft

Note: Additional parts are required, see the Additional Requirements/Information (pdf) below for additional information.

Product Style RevZilla Item # MFR. Product # Availability
Product Style Koso 170cc Big Bore Kit With 4V Head Honda Grom 2014-2018 RevZilla Item #1246457 MFR. Product #MB623003 AvailabilityAvailable: Requires 1-3 business days of lead time prior to shipment
Please review this product's fitment notes & exceptions prior to purchase: Add to Cart

This product is vehicle specific. Please to ensure fitment.

Fitment Notes & Exceptions
  • This kit should be installed by a skilled or experienced engine mechanic, catastrophic engine failure could occur if this kit is installed or tuned incorrectly
  • Additional upgrades are required when running this kit
  • Fuel tuning is required
This Product Fits
Grom MSX125 2014, 2015, 2017, 2018


Orders $39.99 and over qualify for our fast free shipping offer. Click Here for FREE Shipping Terms and Conditions.

Please see our International Orders page for orders shipping outside the US.

We show real-time availability on this product page when you select the size/color item you want.  Most items will ship the same business day an order is placed,however if an item requires additional processing time a message will be shown indicating such.

See our full Shipping Policy for further details.


If your order arrives and it is not right, we will fix it, NO NONSENSE, we promise.

Doesn't fit or just not happy with it? You can return any new, unused and unaltered item within 30 business days of delivery receipt of your item. We will issue a full refund to your original payment method.

See our full Return Policy for all of the pertinent details.

Product Description
Part Numbers
Shipping & Returns

Reviews & Questions

There are no reviews for this item. Write Review
Start typing your question and we'll check if it was already asked and answered. Learn More
No questions have been asked about this item. Be the first!
0 Customer Reviews
Read All Reviews
0 Questions & 0 Answers
Ask A Question