Honda GX25, Part 1: Stripping

Introduction: Honda GX25, Part 1: Stripping

This is a part 1 of 3 instructables: Stripping, Porting and Modifying, Rebuilding. To much info for just one instructible.

The ultra-lightweight GX25 Mini 4-Stroke efficiently delivers the kind of efficient cleaner, powerful, quiet performance that you’d expect from a larger engine – but in a very compact package.

COMMON APPLICATIONS Commercial lawn and garden equipment Tillers / cultivators Construction / industrial equipment Agricultural equipment Water pumps.

Engine Type: Air-cooled 4-stroke OHC

Bore x Stroke:35 mm x 26 mm

Displacement:25 cm3

Net Power Output*1.0 HP (.72 kW) @ 7,000 rpm

Net Torque0.74 lb-ft (1.0 Nm) @ 5,000 rpm

PTO Shaft Rotation Counterclockwise (from PTO shaft side)

Compression Ratio8.0:1

Carburetor Diaphragm-type (overflow return)

Ignition SystemTransistorized magneto

Starting System Recoil

Lubrication System Crankcase pressure–driven

Governor System NA

Air cleaner Semi-dry

Oil Capacity2.7 US oz (80cc)

Fuel Unleaded 86 octane or higher

Fuel Tank Capacity.61 U.S. qt (0.58 liter)

Dry Weight6.8 lb (2.7 kg)

DimensionsLength (min)7.6" (192mm)Width (min)8.7" (221mm)Height (min)9.1" (230mm)

Step 1: Whats Needed

This engine is easy to strip with basic tools. You will need:


-3mm and 4mm Allen key

-8mm, 10mm, 14mm and spark plug sockets and wrench

-flat and Phillips head screw driver.

-flywheel puller

-hook tool

-2x8mm spanners (can just use a spammer and socket to remove exhaust studs.See step 9)

- spark plug socket or spanner

When stripping keep components grouped together to assist with assembly.

Step 2: Stripping

1. Remove any attachments. Clean the outside of your engine. Degrease if necessary.

2. Drain fuel and oil. Replace the dip stick and fuel cap (this will stop oil going everywhere).

3. Remove 4mm Allen screw on top red cover and remove. Remove the air filter cover. Remove air filter.

4. Remove 2x 8mm nuts, disconnect breather hose and remove remaining part of air filter housing.

5. Remove fuel lines from carby. Remove carby and gasket.

6. Remove the 3x4mm Allen screws holding the intake manifold. The intake manifold has a tight Oring requiring it to be popped out of the head. A bit of force will lever it out.

7. Unplug the spark plug lead. Remove 2x3mm Allen screws holding the CDI in place. Unscrew Allen screw holding ground wires and remove CDI.

8. Remove the 4mm Allen screw underneathe exhaust manifold. Remove the two 8mm nuts holding the exhaust manifold in place. Remove the exhaust and shroud.

Step 3:

9. Remove exhaust studs. Place an 8mm nut upside down enough to screw on second nut. Tighten against each other. Undue stud CCW applying force to bottom nut. If the nut slips either re tighten, or try tapping spinner with hammer or similar to jolt stud loose. Be gentle!

10. Remove 4x4mm Allen screws holding the attachment housing on.

11. Remove 3x4mm Allen screws retaining the pull start and the 2x4mm screws under the clutch. Remove the pull start,guard and fuel tank.

12. Remove 3x4mm Allen screws on the flywheel cover and remove.

WARNING: While removing clutch, ensure you DO NOT TOUCH the friction material. Grease on this part will damage it.

13. Place a long bar/screw driver between clutch mounting posts. Remove 2x14mm clutch bolts using the bar to stop the assembly from spinning. Remove clutch and keep the bolts and washers in place. If removing the bolts and washers pay attention to arrangement.

Note:If you want to remove the gear for the pull start, do so now using screw driver to stop the assembly spinning. It’s not necessary though and easier to just leave attached.

Step 4: Fly Wheel Removal Crankcase Split

14. Place 10mm socket on flywheel nut. Place a flat screw driver between socket and clutch mounting post. Undue nut CCW. An impact wrench or jarring the socket will help undue the nut.

WARNING: Avoid jamming the flywheel cooling fins as they can snap. This will cause engine to vibrate horribly! 15. Remove flywheel using appropriate puller. Any other tool or technique will result in damaged parts or frustration. Just get a cheap one.

Step 5: Head Disassembly

16. Remove 2x4mm tappet cover Allen screws on top of engine and remove.

17. Using 90’ angle hook or similar, push out the timing gear pin and remove. Slide the timing belt off the gear and remove gear from the head.

NOTE: Unless there is a need, leave tappets in the head. They are a press fit and do not need to be removed. 18. Remove oil hose from side of engine.

19. Remove 2x4mm Allen screws on bottom of crankcase near pull start then remove 4x4mm screws in a criss cross pattern. Use a screwdriver to split the case by inserting between the too halves.

20. Remove valves by pushing down on the spring retainers with your thumbs and pushing down and across to unseat them. If valves drop when you push down, you snuff some clean rag into the cylinder.

21. Remove all gasket materials and clean parts and inspect (this piston is in good shape with no wear spots on the piston skirt).

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    3 Discussions


    Question 2 months ago

    I removed my Mantis tiller from a couple years of storage, filled it with fresh gas and, when I tried to pull the start cord on the Honda GX25 4-stroke motor, it would not turn. Eventually I found that the combustion chamber had become filled with oil. I drained the gas tank and the crankcase, removed the spark plug and propped the unit upside down overnight to clear the combustion chamber then installed a new spark plug and refilled the crankcase and gas tank. After two or three pulls of the starter cord, the motor would not budge - the combustion chamber was again filled with oil. I guessed that somehow (perhaps by being frozen) a ring or maybe the piston had been broken while in the storage shed so I tore the motor down only to discover no broken rings and an undamaged piston. There was more carbonization on the valves than I would have expected for a motor that has probably less than 10 hours of run time on it but, other than that, I see no signs of wear or damage anywhere. If anyone has any suggestions as to how to fix my problem, I would very much appreciate them.


    4 months ago on Step 4

    Looking for detailed instructions on how to replace oil seals on Honda GX25. This insructable is extremely well written, just wondering if all these steps have to be taken to replace and reassemble the engine for oil seal replacement? Thank You for your time.