Honda CR125 Top End Rebuild Guide

One of the more rewarding experiences in motocross - off the track - is the maintenance and upkeep of your bike. Wrenching around on your bike is a great way to understand the mechanics of the engine parts as well as introduce you to more of the Do-It-Yourself aspect of the sport.

A popular bike many riders like to upgrade or rebuild is the Honda CR125. This is a peppy little bike from the box but in time, you can add solid aftermarket parts that really crank on the track or trail. There are two ways to go about rebuilding your Honda CR125 - a top end re-build or a bottom end rebuild.

For this guide, it is assumed you already know the set-up and the ins-and-outs of the mechanical side to replacing parts so the focus centers on what's needed to rebuild the top end of your bike - consider this basic maintenance parts to keep your CR125 on the track and you first at the finish line. For a helpful "How-to guide" see How To Do A Top End Rebuild On A 2 Stroke Dirt Bike.

The top end involves the pistons/rings and all of the replaceable parts above the base gasket. It's time to rebuild the top of your Honda CR125 once the parts reach their service life or are simply worn out. Service life depends on a variety of factors including how hard you ride, the bike's age, and the quality of parts, among other things. The parts listed below work exceptionally well on the Honda CR125. Typically, you'll need to rebuild the top end of your ride about every 10-30 riding hours.

The easiest and most convenient method to a rebuild is to get a Complete Top End set which includes everything you need to replace the top end of your bike in one convenient package. The other route is to purchase your pistons and rings individually which some riders and mechanics prefer. This guide covers both applications.

One bit of information you should know is cast vs. forged pistons. OEM manufacturers all spec cast pistons for their 2-stroke bikes because they perform better in most situations. A forged piston generally provides a longer life span.

Complete Top End Kits for the Honda CR125

Vertex End Kit

Pro-X Top End Kit

Wiseco Top End Kit

Vertex 2-Stroke Piston Kit - Stock Bore

Pro-X Piston Kit - 2-Stroke

Wiseco Pro-Lite Piston Kit - 2-Stroke

Price: $45.99 - $161.99

Price: $49.99 - $129.99

Price: $79.99 - $219.99

Features:

  • Gravity cast piston created with premium aluminum alloys utilizing an 12-21 percent silicon content
  • Molybdenum Disulphide (MOS2) coating
  • Wiseco Needle Bearing
  • Cometic Top End Gasket Kit

Features:

  • New skirt coating assists the wear-in process and reduces friction
  • Cast piston puts engine back to standard specs without boring or replacing cylinder
  • Wiseco Needle Bearing
  • Cometic Top End Gasket Kit

Features:

  • Stronger than stock
  • Features a race-ready dome finish with smooth edges for improved combustion flow
  • Wiseco Needle Bearing
  • Cometic Top End Gasket Kit
 

Pistons

Buying parts individually of course gives you more flexibility especially if you prefer a specific brand or running OEM parts. Just remember, when buying a piston don't forget the gasket and you'll also need a wrist pin bearing. The following pistons are the individual pistons sold separately from the Complete Top End Kits.

Vertex Piston

Pro-X Piston

Wiseco Piston

Vertex 2-Stroke Piston - Stock Bore

Pro-X 2-Stroke Piston - Stock Boret

Wiseco Pro-Lite 2-Stroke Piston - Stock Bore

Price: $75.99 - $211.99

Price: $80.99 - $188.99

Price: $109.99 - $274.99

Features:

  • Bore diameter: 53.9 mm (Standard)
  • Gravity-cast piston created with premium aluminum alloys utilizing an 12-21 percent silicon content
  • Contains a complete set of nitride-coated or chrome-plated piston rings, a fully case-hardened wrist pin and circlips

Features:

  • Bore diameter: 53.95 mm
  • Piston kit comes complete with piston, rings, pin and end clips
  • O.E.M. style tin-coated pistons assist the wear-in process
  • Interchanges directly with O.E.M. part

Features:

  • Bore size: 56 mm (Standard)
  • Features a race-ready dome finish with smooth edges for improved combustion flow
  • All Pro-Lite pistons include rings, wrist pin and circlips
  • Tight tolerances for quiet running
 

Gasket Kits

Like we said - don't forget the gaskets! The Cometic Gasket Kit is the only one we carry for the Honda CR125 and the aftermarket brand we recommend.

Top End Gasket Kit

Cometic Top End Gasket Kit

Price: $17.99 - $82.99

Features:

  • Head gasket is constructed from non-asbestos, flexible graphite that is laminated to a stainless steel core
  • Manifold and base gaskets are constructed from non-asbestos nitride rubber-bound fiber
  • Kit contains 1 each: Head gasket, base gasket, exhaust gasket, intake reed gasket to carburetor boot, right power valve cover gasket, left power valve cover gasket, upper power valve cover gasket
 

Written By: AndrewT

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