Animated service guide
Dirt Bike Bottom End Rebuild Signs and Checks
The bottom end is the crankshaft, connecting rod, big end bearing, main bearings and crank seals. Rebuilding it means pulling the engine and splitting the cases with special tools, so most riders send it to a shop. This guide covers the signs, the hours your manual sets, what you can check yourself and what to ask the shop.
The animated guide loads here. Every step is also written out below.
Before You Start
Tools
- Your owner’s or service manual
- Clean rag
- Drain pan (to look at the oil)
Parts And Supplies
- A shop you trust, for the rebuild itself
Safety
- Stop riding if a new knock or heavy vibration shows up. A failed rod can wreck the cases and the cylinder.
- Case splitting and crank pressing need special pullers, presses and a truing setup. Don’t improvise them.
Step By Step
Know what’s down there
The piston pushes the connecting rod, which turns the crankshaft on its big end bearing. Main bearings carry the crank in the cases, and seals keep the crankcase tight. On a two-stroke that crankcase also pumps the fresh charge, so it must be airtight.
Rod and crank bearings Bike Spec Source KTM / Husqvarna / GASGAS 250-300 two-stroke Needle big end, 1 ball + 1 roller main KTM 250/300 SX-XC 2024 OM, PDF p. 132 KTM / Husqvarna / GASGAS 250 / 450 four-stroke Slide (plain) big end, 2 roller mains KTM 250 SX-F 2024 OM, PDF p. 149; 450 SX-F 2024 OM, PDF p. 154 Your manual’s technical data lists the bearing types.
Learn the warning signs
Riders and shops list a knock at idle or under load, new vibration you feel in the pegs and bars, and on a two-stroke, leaking crank seals. Yamaha’s manual names a worn big end bearing, worn crank journals and their bearings as causes of noise from around the crank.
Careful. A knock that gets louder fast means stop riding. A rod that lets go usually takes the cases and cylinder with it.
Look in the drained oil
On a four-stroke the crank shares its oil, so check what comes out at every oil change. Yamaha says if you find metal chips or other foreign material in the drain, open the crankcase and check it. Glitter on the drain plug magnet is a reason to look further, not to keep riding.
- Drain pan
- Rag
Know your hours
Manufacturers set the bottom end by hours, and race and enduro manuals use different ones. KTM’s major engine service replaces the rod, big end bearing and crank pin, all engine bearings and seals. Yamaha has its dealers inspect the crank far more often. Log your hours so you see it coming.
Bottom end service interval Bike Spec Source KTM / Husqvarna / GASGAS 250 / 300 SX, XC, TC, MC, EX two-stroke (race) Major engine service every 45 h KTM 250/300 SX-XC 2024 OM, PDF p. 36 (same in the 2026 manual and the GASGAS MC 250 / EX 300 2026 manual, PDF p. 36; 2022 carb 250 SX: every 40 h, PDF p. 34) KTM / Husqvarna / GASGAS 250 / 450 SX-F, XC-F, FC, MC (race) Major engine service every 90 h KTM 250 SX-F 2024 OM, PDF p. 42; 450 SX-F 2024 OM, PDF p. 41 Yamaha YZ250F / YZ450F (race) Inspect and clean crank every 5th race (12.5 h) Yamaha YZ250F / YZ450F 2026 OSM maintenance intervals, PDF p. 93 (dealer item) KTM XC-W / EXC, Husqvarna TE 250 / 300 (enduro two-stroke) Major engine service every 90 h KTM 250 XC-W / 300 EXC / 300 XC-W 2026 OM, PDF p. 55; Husqvarna TE 250/300 2026 OM, PDF p. 55 KTM 450 / 500 EXC-F, XCF-W (enduro four-stroke) Engine service every 90 h KTM 450 EXC-F / XCF-W / 500 EXC-F 2025 OM, PDF p. 54 (con-rod, conrod bearing, crank pin and piston) Honda’s CRF owner’s manuals don’t list a bottom end interval; that work is in the service manual. Check yours, and follow it when it gives one.
Check rod play (top end off)
When the cylinder is off for a top end, stuff a clean rag in the case opening, bring the rod to the top and pull it straight up and push it down. You should feel no up-and-down play. Some side-to-side slide along the crank pin is normal. A rod that clunks up and down, or looks discolored or scorched, needs the bottom end done.
Careful. Keep the rag in the case mouth the whole time. A dropped clip or nut in there means splitting the cases to get it out.
Check the crank seals
Crank seals sit outboard of the main bearings. On a two-stroke a leaking seal lets air into the crankcase, which makes the bike hard to tune and can lean it out. A leak-down test seals off the intake and exhaust and pressurizes the engine to find it. KTM replaces these seals at every major engine service.
Use the bottom end service interval from the table above.
What the shop does
The engine comes out and the cases are split with a separating tool. The shop fits a new rod kit (rod, pin, big end bearing) or a new crank, new main bearings and seals, then checks crank width and runout on a stand. Yamaha installs the crank with a dedicated installer, not a hammer.
Crank limits (shop measures these) Bike Spec Source Yamaha YZ250F Runout 0.030 mm max, width 55.93 to 56.00 mm Yamaha YZ250F 2026 OSM, PDF p. 56 and 290 Yamaha YZ450F Runout 0.030 mm max Yamaha YZ450F 2026 OSM, PDF p. 54 Crank width and runout limits are in the service manual. A shop measures them with the crank out of the engine.
Ask the right questions
Ask what was replaced: rod kit or whole crank, main bearings, seals, and whether gearbox bearings were checked. Ask for the runout reading. Afterward, break the engine in again; Honda says to repeat break-in whenever the crankshaft or crank bearings are replaced.
Sources
Log It And Know When It Is Due Next
MotoMind logs engine hours and every service job with notes, parts and cost, and shows each task as upcoming, due or overdue. Available to download on iOS.
