How to build a hot saw: parts, rules and safety
In this guide
A hot saw is the dragster of the saw world. It's built for one job: cutting three discs off a log in a few seconds, at a timber sports event. This page explains what goes into one and what the rules say. We don't give engine tuning or wiring steps. Those belong with an experienced builder, for safety reasons. For everyday saws, start at the cutting hub or our guide to types of saws.
What is a hot saw?
STIHL TIMBERSPORTS calls the hot saw an "individually tuned and customized chainsaw". Here are its own numbers for the pro event:
- Power: 60 to 80 hp.
- Chain speed: about 240 km/h (about 150 mph).
- Weight: close to 30 kg (about 66 lb).
- The cut: three full discs ("cookies") from a 46 cm (about 18 in) log. The cuts go down, up, then down, inside a 15 cm (about 6 in) zone at the end of the log.
- Fastest times: under six seconds.
Intermediate athletes don't build their own. They race a stock STIHL MS 881 that the organizers provide.
Parts of a hot saw
| Part | What it does | Rule or watch-out |
|---|---|---|
| One-cylinder engine | Makes the power | Rules allow one cylinder only. A rotary engine may have one disc only |
| Tuned exhaust (expansion chamber) | Boosts power at high rpm | Required by the rules |
| Pull-start | Starts the engine | Saw must start by hand with a pull cord |
| Clutch and drive sprocket | Connects the engine to the chain | Sprocket must match the chain pitch |
| Frame and handles | Holds it all together | The athlete must be able to carry the saw alone |
| Bar and race chain | Does the cutting | STIHL advises against changing the chain's side plates, tie straps or rivets |
| Top chain catcher | Stops a broken chain from flying back | Required on all hot saws |
| Bottom chain catcher | Same, below the bar | Strongly recommended by STIHL |
| Chain cover and rotor cover | Guard the moving parts | Required on all saws |
| Stop switch | Kills the engine | Events run a technical check on the running saw |
Ways to get a hot saw, by situation
You want to race: buy or commission a saw
This is the route most people should take. Experienced builders make saws to the rules and know which engines hold up. The Drive profiled one California builder who had made more than 20 saws in five years. He put a custom saw at around $5,000.
- Go to a local timber sports or lumberjack event. Ask the athletes who built their saws.
- Ask which rule book you'll compete under. STIHL TIMBERSPORTS rules differ from other events.
- Have the builder walk you through starting, stopping and the safety parts before you ever pull the cord.
You already build race engines: plan your own build
If you have real two-stroke engine, welding and machining skills, a build follows the same order as the parts table:
- Pick the rule book first. It decides the engine type, guards and weight you need.
- Choose the engine. Builders often start from a one-cylinder two-stroke out of a dirt bike, snowmobile or go-kart. The Drive's example used a go-kart engine bored out to 144 cc.
- Design the frame around the engine. It must hold the clutch, sprocket, bar mount and handles in line, and take heavy vibration.
- Fit the drive. Clutch, sprocket, bar and race chain, all matched in pitch and gauge. Our chainsaw chain size guide explains pitch and gauge.
- Fit every guard. Top chain catcher, bottom catcher, chain cover, rotor cover and a stop switch.
- Have it inspected by an experienced builder before the first start. Events do their own technical check too.
If your engine runs on premix, mix it at the ratio the engine builder gives you. Our two-stroke mix calculator does the math.
You just want a faster work saw
Don't build a hot saw. A sharp chain and the right bar do more for daily cutting than any mod. See how to sharpen a chainsaw and how to measure chainsaw bar length.
If it won't run right
Hot saws are hard to start and touchy to tune. These are the usual suspects. Have a builder diagnose them; don't guess.
- Won't start with the pull cord. Usually fuel, spark or compression. Check the plug and fuel first.
- Bogs in the cut. Often carb tuning, a worn chain, or the wrong sprocket for the chain.
- Throws or loosens the chain. Stop at once. Check bar wear, chain tension and the sprocket. Never run it without the chain catcher.
- Shakes so hard it's hard to hold. Frame or mount problem. Stop and fix it before the frame cracks.
When to call a pro
Always, for the first start, for any engine work and for every safety part. Join a timber sports club or event. You'll get coaching, a safe place to run the saw and a technical check from people who have done it many times.
Safety gear and rules at events
STIHL TIMBERSPORTS rules set the minimum. Hot saw athletes must be at least 18. Hearing protection, eye protection and chaps are required from the moment you start the saw, and everyday glasses don't count as eye protection. Shields must stand in front of and behind the athlete. OSHA's chain saw fact sheet lists leg protection and eye protection for any chainsaw work.
Curious about other saws with odd names? See what is a buzz saw.
What you'll need
- One-cylinder two-stroke engine: Builders often use dirt bike, snowmobile or go-kart engines
- Tuned exhaust (expansion chamber): Required by the STIHL TIMBERSPORTS rules
- Centrifugal clutch and drive sprocket: Matched to the chain pitch
- Frame, handles and a pull-start: Usually custom welded or machined
- Long bar and race chain: Matched pitch and gauge
- Chain catchers, chain cover and rotor cover: Top catcher is required; bottom is strongly recommended
- Stop switch: Have a qualified builder fit it
- PPE: chaps, eye and ear protection, boots, gloves: Required at events
Key takeaways
- A hot saw is a one-cylinder racing chainsaw. Pro saws in STIHL TIMBERSPORTS weigh close to 30 kg (about 66 lb) and make 60 to 80 hp.
- The STIHL TIMBERSPORTS rules require one cylinder, a tuned exhaust, a chain catcher above the bar, and a chain cover and rotor cover.
- The saw must start with a pull cord, and the athlete must be able to carry it alone.
- Most first-timers are safer buying or commissioning a saw from an experienced builder, then learning to run it at sanctioned events.
- Hot saws are race tools. They don't belong in the woods, on a job site or near bystanders.
Questions people ask
What is a hot saw?
A hot saw is a custom racing chainsaw with a large one-cylinder engine and a tuned exhaust. In STIHL TIMBERSPORTS, athletes use one to cut three discs off a log as fast as they can. The best times are under six seconds.
How much horsepower does a hot saw have?
STIHL TIMBERSPORTS says its pro hot saws make 60 to 80 hp. A stock chainsaw makes a small fraction of that.
How much does a hot saw weigh?
Pro hot saws weigh close to 30 kg, about 66 lb, according to STIHL TIMBERSPORTS. The rules only say the athlete must be able to carry the saw alone.
How much does it cost to build a hot saw?
It varies a lot with the engine and parts. One builder interviewed by The Drive in 2022 put a custom saw at around $5,000. That doesn't count PPE or event costs.
Is there an engine size limit for hot saws?
In STIHL TIMBERSPORTS, the saw must have one cylinder and a tuned exhaust. In a STIHL TIMBERSPORTS interview, Jörg Bläsi said there's no displacement or carburetor limit. The saw still has to start with a pull cord.
Can I use a hot saw to cut firewood?
No. Hot saws are built for a few seconds of racing. They are heavy and very loud, and they aren't built with the safety features of a work saw.
Sources
- STIHL TIMBERSPORTS discipline: Hot Saw (block size, cut zone, weight, power, chain speed, PPE)
- STIHL TIMBERSPORTS Rules and Regulations 2024, sections 1.3.3 and 1.4 (Hot Saw technical rules)
- Interview with Jörg Bläsi: hot saw engine rules (STIHL TIMBERSPORTS)
- Homebuilt hot rod chainsaws (The Drive, Dec 2022)
- Chain saw safety fact sheet (OSHA)
Change log: Nov 6, 2026: Rewritten from the archived post at /power-tools/saw/how-to-build-a-hot-saw/. Removed the wrong 'blade guard', 'wiring' and '80 cc minimum' claims; added the STIHL TIMBERSPORTS rules, parts table, safety framing and safer routes in.
