Stihl MS 250 Rebuild Kit (Installation Guide)
A successful MS 250 rebuild starts by identifying the exact failed part and matching the kit to the saw’s serial number and version. Clean and inspect before disassembly, follow the correct STIHL service information for assembly values, then verify crankcase sealing before running an engine rebuild. Finish with fresh 50:1 fuel and safe function checks.
Repairing the failed part can keep a serviceable saw in use and avoid replacing parts that are still sound. It also helps limit waste, but only if the repair addresses the cause of failure. A piston kit will not fix a leaking crankshaft seal, and a carburetor kit will not correct low compression.
I use the same rule at the bench: diagnose first, order parts second. The MS 250 is a 45.4 cm³, single-cylinder, two-stroke saw, but “rebuild kit” can mean several different things. The right procedure depends on which parts you are replacing and on the saw’s exact version.
Identify the kit and the saw
A rebuild kit is a seller’s general term, not one standard STIHL assembly. Before ordering, record the model designation, serial number, and market version, then compare those details with the parts listing and the applicable service information. The kit must match the component and the saw, not just the name MS 250.
| Kit type | Typical parts | What it does not automatically repair |
|---|---|---|
| Piston and cylinder kit | Cylinder, piston, rings, wrist pin, clips; contents vary | Crankshaft seals, bearings, intake leaks, carburetor faults |
| Carburetor repair kit | Diaphragms, gaskets, needle or related parts; contents vary | Worn carburetor body, blocked fuel line, ignition or compression faults |
| Engine or gasket kit | Contents vary by supplier | Any damaged parts not included in the kit |
Check the listing’s contents and part compatibility against a STIHL parts diagram or dealer parts information for the serial range. Confirm whether the kit includes every small part you need. A similar-looking kit may still have the wrong parts for your saw.
The MS 250 commonly uses a .325-inch chain pitch, but approved setups can differ. Read the bar markings and match chain pitch, gauge, and drive-link count to the bar and drive sprocket. A 16-inch bar is common, not universal; use only an approved combination for your saw. Next step: identify the failure and verify the exact kit before taking the saw apart.
Diagnose before disassembly
Diagnosis means checking the likely failure systems before replacing parts. A no-start can come from spark, fuel, or compression, and a fault in one system can look like another. These quick checks narrow the cause; they do not replace proper testing or prove an internal engine is sound.
First, work outdoors or in a well-ventilated area. Let the saw cool, engage the chain brake, disconnect the spark-plug lead, and keep fuel away from flames, sparks, and hot surfaces. Wear eye protection and suitable gloves. Clean dirt from around the cover and engine before opening anything.
Check the basics in order:
- Fuel: Use fresh fuel mixed at 50:1 with STIHL two-stroke engine oil, or follow the fuel and oil makers’ instructions. That ratio is 100 mL of oil per 5 L of gasoline. Do not use straight gasoline. Confirm the tank has fuel and look for damaged fuel lines or a blocked filter.
- Spark: Inspect the plug for damage, heavy deposits, or wet fuel. A proper spark tester is safer and more reliable than holding a plug against the cylinder while cranking. If there is no spark, inspect the plug, lead, stop-switch circuit, and ignition system using the correct service procedure.
- Compression: A compression gauge can help compare the engine’s condition, but do not use a guessed universal pass number. Readings depend on the gauge and test method. Scoring visible through the exhaust port, abnormal cranking feel, or poor measured compression calls for more inspection.
- Air and exhaust: Check that the air filter and exhaust path are not badly blocked. A blocked path can affect running, but cleaning it will not repair a scored piston or a crankcase leak.
A wet plug after repeated pulls may mean the engine is flooded; it does not prove the carburetor needs a kit. Follow the manual’s clearing procedure rather than continuing to pull. If checks point to more than one system, resolve the evidence before ordering parts. Next step: document what you found and choose the repair that matches the fault.
Install a piston and cylinder kit
A piston and cylinder kit replaces damaged top-end parts, but it does not make the lower engine airtight by itself. The exact disassembly order, fastener torque, clearances, and ring details depend on the saw version and service information. Do not borrow values from a generic kit sheet or another model.
Clean the exterior before opening the engine. Photograph each stage, label parts, and keep dirt out of the crankcase. Remove covers and components in the sequence given in the correct manual. If you find metal debris, deep scoring, damaged bearings, or evidence of overheating, stop and assess the full engine rather than fitting new top-end parts over the damage.
During assembly:
- Compare the new parts with the removed parts, including dimensions and details that affect fit. Stop if a part does not match.
- Inspect the cylinder, piston, rings, wrist pin, circlips, crankshaft, bearings, seals, and intake parts. Replace damaged parts; do not hide the cause of failure with a new piston.
- Install the piston in the orientation specified for the saw. Fit rings according to the manual’s markings and instructions. Ring end gaps and piston direction are not details to guess.
- Keep circlips seated correctly and prevent them or other debris from falling into the crankcase. Use the specified method and tools.
- Refit the cylinder, intake, exhaust, and related parts in the stated order. Use only the specified fastener torque and seal materials.
Before any first start after internal engine work, pressure- and vacuum-test the assembled crankcase using the procedure and limits in the applicable service information. A leaking seal, joint, or intake can let in excess air, create a lean mixture, and quickly damage the new parts. A brief start test does not rule out that leak. Next step: do not tune or operate the saw until the seal test passes.
Install a carburetor repair kit
A carburetor kit restores specific wear or sealing parts inside the carburetor. It cannot fix a blocked tank filter, damaged fuel line, ignition fault, low compression, or a worn carburetor body. Identify the carburetor model fitted to your saw and use its correct parts diagram and instructions before removing covers or diaphragms.
Clean the carburetor’s exterior first. Work on a clean surface so small parts do not get lost or contaminated. Photograph the original arrangement before removal, then compare it with the carburetor maker’s diagram. Diaphragm and gasket order can vary; do not rely on memory or assume all kits use the same layout.
Inspect the inlet needle, lever, screen, and body for wear, dirt, or damage. Use only cleaning methods approved for the carburetor and its materials. Replace only parts specified by the kit procedure, and do not enlarge passages with wire or drill bits. If the body is damaged or a passage remains blocked, a kit alone may not restore proper operation.
Refit the carburetor and fuel connections as directed. Do not guess the adjustment positions. STIHL engine speed references commonly list about 2,800 rpm at idle and 13,000 rpm maximum, but the correct version’s manual governs. Carburetion must be checked with a tachometer, not tuned by ear. Next step: confirm the carburetor and engine settings using the correct service information before cutting.
Reassemble, test, and return the saw to service
A safe test checks more than whether the engine starts. The saw must idle without driving the chain, respond to the throttle, keep the chain brake working, and show no fuel or oil leaks. Use fresh fuel, suitable bar-and-chain oil, and any break-in instructions supplied for the kit or stated by STIHL.
Refit the bar and chain only after matching pitch, gauge, and drive-link count to the bar and sprocket. Tension the chain as the manual directs. Start outdoors with the saw secured, the bar clear of people and objects, and all guards in place. Follow the approved starting procedure; do not start with the chain contacting the ground.
| Check | What to verify | Stop if… |
|---|---|---|
| First start | No fuel leaks; engine starts and idles steadily | Fuel leaks, abnormal high idle, or racing occurs |
| Idle | Chain remains stationary; engine does not stall | Chain moves or idle cannot be controlled |
| Controls | Throttle returns; chain brake works as intended | Any control sticks or the brake fails |
| Oil system | Suitable bar oil is present and lubrication is verified by the manual’s method | The chain appears dry or oil is leaking abnormally |
| Engine speed | Set and verify with a tachometer and correct manual | Speed is being judged by ear alone |
Let the saw cool before rechecking for leaks or inspecting the repair. Stop the engine before cleaning, adjustment, or refueling. If the engine races, hangs at high speed, or will not idle after an internal rebuild, shut it off. Recheck for an air leak and verify the assembly; do not try to mask a sealing problem with a carburetor adjustment.
A short successful run is not proof that a rebuilt crankcase is sealed. Complete the specified pressure and vacuum checks first, then follow the kit or STIHL break-in directions. Next step: return the saw to cutting only after the safety and sealing checks pass.
Common rebuild decisions and lessons
The useful lesson from a rebuild is often knowing when not to install the kit yet. In a common bench scenario, a saw has a wet plug after many pulls. Replacing the carburetor may seem logical, but checking fresh fuel, clearing a flooded engine as directed, and testing spark first can prevent an unnecessary kit purchase.
Another common scenario is a new piston and cylinder that start but race or fail to idle. That symptom does not prove the carburetor is misadjusted. A crankcase seal, intake joint, or other air leak must be ruled out with the specified pressure and vacuum tests before tuning. Running the saw to “see if it settles” risks damaging the new parts.
| Finding | Sensible next step | Avoid |
|---|---|---|
| No spark on a tester | Follow ignition checks for the correct version | Buying a piston kit without evidence |
| Wet plug, spark present | Use the approved clearing procedure and check fuel condition | Assuming the carburetor must be rebuilt |
| Scored piston or cylinder | Inspect for the cause and assess seals, bearings, and intake | Installing a top-end kit without checking why it failed |
| High or hanging idle after rebuild | Stop and test crankcase sealing | Adjusting the carburetor to cover an air leak |
I treat the kit as a repair part, not a diagnosis. Key takeaway: match the work to measured or visible evidence, and stop when a test points beyond the kit.
Frequently asked questions
What does an MS 250 rebuild kit include?
There is no single standard kit. It may contain piston and cylinder parts, carburetor parts, or a different selection. Check the listing contents and compatibility before ordering.
Can I install a piston kit without replacing the crankshaft seals?
Only if inspection and the required leak tests show the seals and crankcase are sound. A top-end kit does not repair a leaking seal.
What fuel mix does the MS 250 use?
The specified mix is 50:1 gasoline to STIHL two-stroke engine oil, equal to 100 mL of oil per 5 L of gasoline. Follow the fuel and oil manufacturers’ instructions and use fresh mix.
Can I use a 16-inch bar on every MS 250?
No. It is a common setup, not a universal fit. Confirm the bar is approved and match chain pitch, gauge, and drive-link count to the bar and sprocket.
How do I know whether the carburetor needs a kit?
Check fuel condition and delivery, spark, compression, and intake or exhaust restrictions first. Rebuild the carburetor only when findings point to it and the kit matches the fitted carburetor.
Can I set the carburetor by ear?
No. Use the correct manual and a tachometer for engine-speed checks. Do not guess adjustment settings.
What torque should I use on cylinder fasteners?
Use the value for the exact saw version in the applicable STIHL service information. Do not use a generic value from an unrelated kit or model.
Is a brief start enough to check a rebuilt engine for leaks?
No. After internal engine work, use the specified crankcase pressure- and vacuum-test procedure before tuning or operating the saw.
What should I do if the chain moves at idle?
Stop the saw and correct the cause using the service procedure. The chain must remain stationary at idle.
How can I avoid wasting money on the wrong kit?
Record the serial number and version, identify the failed part through testing, and confirm the kit contents against the correct parts information before disassembly.
The safest rebuild is the one that fixes the confirmed fault and passes the required sealing and function checks. Match the parts, follow the correct service information, and stop if the evidence does not fit the repair.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
