What Is Stihl MS 661 3/8-Inch Chain Pitch? (Drive Specs)
The STIHL MS 661 uses a standard 3/8-inch chain pitch, not a 3/8-inch low-profile chain. The chain, guide bar, and drive sprocket must also match in gauge and fit. A .063-inch (1.6 mm) gauge is common, but check the bar marking and STIHL fitment information before buying a replacement chain.
Finding an unfamiliar chain term in a manual can make a routine replacement feel risky. The key is that “3/8 inch” describes only one part of the fit. I’ve seen owners focus on pitch and overlook gauge or drive-link count, which can leave them with a chain that does not fit the bar.
Here’s how to read the MS 661 drive specs, check a replacement chain, and prepare the saw for work without guessing.
MS 661 Bar, Chain Pitch & Safety Setup
The MS 661 has a 91.1 cm³ two-stroke engine and uses a standard 3/8-inch chain pitch. Pitch must match the drive sprocket, while the chain’s gauge and drive-link count must match the installed bar. Check all three details before fitting a chain; the engine size alone does not identify a complete cutting setup.
What 3/8-inch chain pitch means
Chain pitch is the spacing of the chain’s drive links, measured as the distance between three rivet centers divided by two. The MS 661’s standard 3/8-inch pitch is not the same as 3/8-inch low-profile pitch. The names look similar, but the chain types are not interchangeable.
The chain pitch, bar, and drive sprocket must form a compatible setup. If you are replacing a chain, read the pitch marking on the bar and confirm that the sprocket is set up for the same pitch. If markings are missing or unclear, use the saw and bar model details to check STIHL’s fitment information or ask a qualified dealer.
Gauge and drive links are separate fit checks
Gauge is the thickness of the chain’s drive links where they sit in the bar groove. It must match the groove width. A .063-inch (1.6 mm) gauge is common on MS 661 setups, but the correct gauge depends on the fitted bar. Do not assume every bar for this saw uses that size.
Drive links are the small projections on the underside of the chain that travel through the bar groove and engage with the saw’s drive system. Their count is specific to the bar and chain combination. Do not work it out from pitch or bar length alone. Look for the bar’s markings, count the links on the old chain if it is the correct one, or check the exact bar in STIHL fitment information.
| Fit detail | What it tells you | How to verify it |
|---|---|---|
| Pitch | Chain spacing and sprocket match | Check the bar and sprocket setup |
| Gauge | Drive-link thickness for the bar groove | Read the bar marking or fitment chart |
| Drive-link count | Chain length for that specific bar | Read the marking, count the old chain, or confirm the bar model |
| Bar length | Usable cutting length | Read the bar marking and choose for the work |
Match the chain before you install it
Before purchase or installation, compare the replacement chain with the bar and sprocket setup. A chain that shares the 3/8-inch pitch can still be wrong if its gauge or drive-link count does not match. Do not install a 3/8-inch low-profile chain, or a chain with an unverified fit.
Use this sequence:
- Stop the engine and let the saw cool. Wear chainsaw protective equipment when handling the chain.
- Read the bar’s markings for pitch, gauge, and drive-link information. If a marking is worn or incomplete, check the bar’s exact model.
- Confirm that the chain is standard 3/8-inch pitch and that its gauge and link count match the bar.
- Confirm that the drive sprocket matches the chain pitch. If you cannot identify the sprocket setup, consult the manual or a STIHL dealer.
- Install and tension the chain as the owner’s manual directs. A chain that is too loose or too tight is not ready for cutting.
Fitment scenarios: two common replacement questions
These examples are practical scenarios, not reports of specific customer jobs. They show why checking every marking matters. In the first, an owner finds a chain labeled 3/8 inch but does not notice it is low-profile. That label alone is not enough: the MS 661 calls for standard 3/8-inch pitch.
In the second, an owner finds a standard 3/8-inch chain with .063-inch gauge, but its drive-link count does not match the fitted bar. It still is not the right chain. The correct next step is to confirm the bar’s exact fitment, not to force the chain onto the bar or remove links without proper equipment and guidance.
Field Cutting: Matching the Saw to the Work
A compatible chain is only one part of a safe cutting setup. Choose a STIHL-approved bar and chain combination, then select bar length for the tree, cutting method, and operator’s skill. A larger engine does not, by itself, mean a longer bar is the right choice.
Choose bar length for the job
The bar is the guide that supports and directs the chain. A shorter or longer bar changes the reach and handling of the saw, so consider the work and your ability to control the tool. Use a bar and chain combination approved for the MS 661, and confirm the chain’s gauge and drive-link count for that bar.
| Situation | What to check |
|---|---|
| Replacing a chain on the existing bar | Match standard 3/8-inch pitch, bar gauge, and drive-link count |
| Changing bar length | Confirm the new bar is approved for the saw and find its exact chain fitment |
| Unsure whether a chain is low-profile | Do not install it until the chain type and sprocket compatibility are confirmed |
| Markings are unreadable | Identify the bar model or seek fitment help from STIHL or a qualified dealer |
Prepare and start the saw carefully
Before cutting, wear chainsaw PPE suited to the task and follow the saw’s manual. Inspect the chain brake, throttle lockout, chain catcher, handles, chain tension, bar rails, and chain lubrication system. Check the work area for bystanders, overhead hazards, tree lean, and a clear escape route. Follow applicable OSHA logging requirements and ANSI Z133 practices where they apply.
Start the saw on stable ground using the manual’s cold- or warm-start procedure. Engage the chain brake for starting, and release it only when you are ready to cut. Keep a secure grip and footing, and maintain control of the saw throughout the cut.
Avoid the bar tip and stop when the cut binds
Rotational kickback can happen when the upper part of the bar nose contacts wood or another object. The saw can move rapidly toward the operator. Keep that area clear, maintain two-handed control, and make sure the chain brake works. Do not rely on the brake as a substitute for safe handling.
Use a sharp, correctly tensioned chain and a suitable bar. If the chain is dull, loose, or damaged, stop and correct the problem before cutting. If the saw or bar becomes pinched in a cut, do not force the saw through the bind. Stop and reassess the cut using safe procedures from the manual or qualified instruction.
Chain, Bar & Engine Maintenance
Regular checks help keep the MS 661’s drive setup working as intended. Chain condition, tension, bar condition, lubrication, and fuel all matter. Follow the manual for inspection and service, and do not run the saw if a safety part is damaged or a fitment problem remains unresolved.
Check tension, wear, and lubrication
A chain can loosen during use, so check tension as directed in the manual and after the chain cools. The correct adjustment depends on the saw’s design and procedure. Do not judge fit by pitch alone: also check that the drive links sit correctly in the bar groove and that the chain moves as the manual specifies.
Inspect the bar rails and groove for damage or debris, and confirm that the chain lubrication passages are clear. The chain must receive oil during operation. If lubrication is in doubt, stop and check the manual’s troubleshooting steps rather than continuing to cut.
Use the specified fuel mix
The MS 661 is a two-stroke engine. STIHL specifies a 50:1 gasoline-to-two-stroke-oil mix for its equipment when using STIHL two-stroke engine oil. An equivalent oil should be explicitly rated for air-cooled two-stroke engines. Use fresh fuel and follow the owner’s manual for fuel requirements and handling.
Do not confuse chain oil with engine oil. The engine’s fuel mix serves the two-stroke engine; bar-and-chain oil lubricates the chain and guide bar. Use the correct product for each system and check the oil supply before cutting.
Shutdown, Cleaning & Seasonal Storage
Correct shutdown protects the operator and makes inspection easier. Stop the engine before adjustments, transport, or cleaning, and let the saw cool before handling hot parts. Cleaning away debris also helps you spot wear or damage that might affect the bar, chain, or oil flow.
After work, clean the air intake, sprocket cover, bar groove, and oil passages as described in the manual. Check chain tension and wear before the next use. For storage, follow STIHL’s instructions for the saw and fuel system; do not leave a saw in service when its chain, bar, or safety parts need attention.
A short pre-cut check
Use this quick review before each cutting session:
- Confirm the chain is standard 3/8-inch pitch and matches the sprocket.
- Confirm gauge and drive-link count match the fitted bar.
- Check chain sharpness, tension, bar condition, and lubrication.
- Inspect the chain brake and other safety parts.
- Check footing, hazards, bystanders, and escape routes.
- Start and operate the saw according to its manual.
Conclusion
The MS 661’s 3/8-inch specification refers to standard chain pitch, not the full chain fit. To choose a compatible setup, verify pitch at the sprocket, gauge at the bar groove, and drive-link count for the exact bar. When a marking is unclear, use the bar model and STIHL fitment information rather than guessing.
Frequently Asked Questions
These quick answers cover the fitment details that most often cause confusion. Use them as a starting point, then confirm the exact bar and chain combination for your saw. If the markings conflict or a part cannot be identified, pause and check the owner’s manual or ask a qualified STIHL dealer.
What chain pitch does the STIHL MS 661 use?
It uses standard 3/8-inch chain pitch. Confirm that the drive sprocket and chain match.
Is 3/8-inch low-profile chain suitable for the MS 661?
Do not use it as a substitute for the standard 3/8-inch chain. The profiles are different.
Does every MS 661 chain use .063-inch gauge?
No. That gauge is common, but gauge depends on the installed bar. Match the chain to the bar groove.
How many drive links does an MS 661 chain need?
The count depends on the specific bar. Read its marking or check the exact bar in STIHL fitment information.
Can I choose a chain using bar length alone?
No. Bar length does not give a reliable drive-link count. Check the bar marking or fitment chart.
Does the chain pitch need to match the sprocket?
Yes. The chain pitch and drive sprocket must be compatible, as must the chain and bar.
What fuel mix does the MS 661 use?
STIHL specifies a 50:1 gasoline-to-two-stroke-oil mix when using STIHL two-stroke engine oil. Follow the manual’s fuel requirements.
What should I do if I cannot read the bar marking?
Identify the exact bar model or ask a STIHL dealer to confirm its approved chain fitment.
(This article was written by one of our staff writers, Evan Bradley. Visit our Meet the Team page.)
