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Engines and Components› Pistons and Piston Rings

TB Parts

138cc Big Bore Kit

$199.99 $199.00 Save $0.99

SKU: TBW9374

In Stock
In Stock
Ships by Sep 13
Free Shipping over $200
$100 Minimum Order
30 Days Return
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Overview

The TB Parts Big Bore Kit is a 138cc engine upgrade for the Honda CRF125. This kit aims to increase power and fun for your trail bike by offering more power and higher compression. It is designed as a bolt-on upgrade that does not require boring of the engine case.
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The TB Parts 138cc Big Bore Kit is designed to enhance the performance of your Honda CRF125, transforming it into a more powerful and enjoyable trail bike. This kit offers a significant increase in power and compression over the stock engine. It boasts an 11.0:1 compression ratio, up from the stock 9.0:1, and is compatible with 91+ octane pump gas. The cylinder is constructed from aluminum with a steel sleeve, and the kit includes a head gasket kit with an exhaust gasket. This is presented as the largest bore upgrade achievable without needing to bore the engine cases. Installation is described as a straightforward bolt-on process, similar to replacing OEM parts, making it accessible for those with piston replacement experience. However, professional installation is recommended for individuals without prior engine work experience, or the purchase of a shop manual. For Fuel Injected CRF125 models, tuning is a mandatory post-installation requirement. The ECU will need a flash/tune to prevent a lean condition and potential engine damage. CJR Performance is mentioned as a service provider for this tuning, capable of adjusting tunes based on other modifications. They note that to maintain affordability, packaging is basic and installation instructions are not included.

Product descriptions are provided by our supplier partners and may not always be perfectly accurate. Not sure this fits? or call (760) 282-4645 and we’ll verify before you order.

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Specifications

BrandTB Parts
Part NumberTBW9374
CategoryEngines and Components

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Returns & Fitment

Built for enthusiasts. Every order is backed by our 30 day return window on eligible items. Unused and uninstalled products may be returned with approved RMA and no restocking fees when received in new condition. Orders placed through the MOTORMIA app with a complete build qualify for our Fitment Guarantee — free return shipping if the part doesn’t fit your exact setup. The guarantee covers fit only; installation must follow manufacturer instructions, and we recommend a qualified shop for parts marked Difficulty 4–5.

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Shipping

Orders are processed in 1–3 business days. In stock parts ship promptly from our partner network. Special-order items either aren’t currently stocked or are built to your spec — expect 4–6 weeks. If a part goes on backorder while your order is in the queue, we’ll reach out and work through discounted alternatives together.

Free shipping over $200 in the continental US. Alaska, Hawaii, U.S. territories (Puerto Rico, U.S. Virgin Islands, Guam, American Samoa, and the Northern Mariana Islands), and international orders carry surcharges — most appear at checkout. Some exceptions (LTL freight, or suppliers who quote freight only after we place the order) won’t. In those cases we’ll reach out with the exact charge before your order moves forward.

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