Phase 4: Leak-Down & Flywheel

TOOL Husqvarna set for this saw: cover plate 502 71 38-01, spacer 502 71 40-01, pressure gauge 502 50 38-01, vacuum gauge 502 50 37-01. A hand vacuum/pressure pump with a 0–15 psi gauge plus shop-made blanking plates does the same job — record the exact rig you used. An improvised test is valid; an undocumented one is not.

1. ACTION Blank the intake and exhaust ports with plates and gaskets. Fit an adapter at the spark plug hole.
2. CRITICAL Plug the crankshaft through-hole first. The 3120 crank is shared with the 3120K cut-off saw, where that hole feeds lubricant to the sprocket bearing. Leave it open and you will chase a leak that is there by design. Electrical tape plus a hose clamp is the field method.
3. ACTION Pressure to 7–8 psi (~0.5 bar). Watch the needle.
4. ACTION Vacuum to 5–7 inHg. It must hold too. A case can pass pressure and fail vacuum — seal lips seat one way and lift the other. Skipping the vacuum half is the classic way to miss a bad seal.
5. INSPECT Leaking? Soapy water around the crank seals, the case joint, and the cylinder base. Bubbles locate it.

UNVERIFIED The pressure figures above are cross-manufacturer norms, not Husqvarna's own. Take the authoritative numbers from workshop manual 101 88 55-26, p.114 and treat the table only as a sanity band.

1. ACTION With the carburettor assembled, pressurise the fuel inlet to about 7 psi.
2. INSPECT Falling pressure points at the inlet needle, the metering diaphragm, or the pump diaphragm.
3. INSPECT With the carburettor wet, spray around both shafts. Bubbles there mean worn bushings.

TOOL Carburettor pressure test procedure: workshop manual p.100. Rebuild kit is 503 59 74-02 (Walbro K12-WG).

This is the real test for the scratched butterflies. Air leaks around the shaft, not past the plate face. A visibly scratched plate on a tight shaft is fine. A pristine plate on a sloppy shaft will run the engine lean and cook it. Plate scoring is largely cosmetic; shaft play is not.

1. ACTION Grip the carburettor body. Push the throttle shaft perpendicular to its axis, both directions.
2. INSPECT Any perceptible rock or click means worn bushings (502 10 31-01, two off). You are feeling for radial play — rotational free-play is normal.
3. ACTION Repeat on the choke shaft.
4. INSPECT Cross-check running: at idle, spray a little carburettor cleaner at each shaft end. An RPM change confirms the leak.

The muffler on this saw was fitted with a shop-made bare-steel gasket — no fibre or composite layer. Something drove somebody to cut one. The mating face is the suspect: it carries a white deposit ring and visible erosion around the oval port.

1. ACTION Clean the mating face back to bare metal.
2. INSPECT Lay it on a surface plate or float glass with engineer's blue.
3. INSPECT Full contact — a new gasket cures it. Dished or eroded — the leak returns whatever gasket you fit.

TOOL The IPL lists no separate muffler-to-cylinder gasket in the MUFFLER group. The sealing gaskets live in CYLINDER PISTON (503 13 13-01, 503 13 46-01) and in the master set 503 14 25-01. Discard the shop-made plate.

WARNING FLYWHEEL NUT IS RIGHT-HAND THREAD — COUNTERCLOCKWISE TO LOOSEN.
The clutch is the left-hand one. Do not confuse them.

If the nut will not move under firm, steady pressure, stop and re-check rather than escalating force. Getting this backwards destroys the crankshaft threads.

1. TOOL Remove the spark plug and fit piston stop 502 50 33-01 in the plug hole.
2. ACTION Remove the two starter pawls.
3. INSPECT The ignition module can stay in place if you are only removing the flywheel. Do not disturb the air gap unnecessarily.
4. CRITICAL Loosen the nut only until it is flush with the end of the shaft — do not remove it. It catches the flywheel when the taper lets go, protecting the threads and your hands.
5. TOOL Fit the puller: 502 50 26-01 with holding tool 502 51 49-01 and bolt set 502 51 54-01. Thread the two M5×25 bolts in evenly, then tighten the centre bolt while holding against rotation until the taper releases.
   UNVERIFIED The "approx. 10 turns" figure often quoted for this step comes from the 362XP/365/372XP manual, not the 3120's own Electrical System section. Thread depth is not transferable between puller sets — run the bolts only as deep as they need to seat squarely, and check they cannot bottom out.
6. ACTION Remove the puller, nut, washer and flywheel.
7. INSPECT Check the flywheel for cracks and check the Woodruff key 735 88 06-00. A sheared key shifts the timing and is a known failure mode on this saw.

Refitting

1. ACTION Clean the tapers on both the flywheel and the shaft. It is a taper fit; contamination there causes slip.
2. ACTION Locate the flywheel, turning it gently until the key mates with the shaft recess. Fit washer and nut.
3. TOOL Set the ignition air gap to 0.30 mm with feeler gauge 502 51 34-02, measured only at the two lower pegs of the module.

UNVERIFIED Flywheel nut torque: the 3120 has its own torque diagram in the workshop manual, with values keyed positionally to an illustration. That figure is deliberately not reproduced here because it cannot be tied to a specific fastener from the manual's text alone. Read it off the Saw 3120 torque page. For scale only: the 372XP calls for 35 Nm and the 346XP 25–30 Nm — the 3120 is a larger engine, so do not assume either. Absent beats estimated.

SCOPE This step is deliberately a stub. It will be written out properly once your Step 1 result exists and says it is needed — writing a full case-splitting procedure for a case that may never be opened would be guesswork dressed as guidance.

What is established so far:

• Clutch is left-hand thread — clockwise to loosen. Full procedure is on Phase 3.
• Crankcase dismantling tool: 502 51 61-01.
• Crank seals / bearings: 503 25 00-02.
• Master gasket set for reassembly: 503 14 25-01.
• SAFETY The workshop manual warns that the crankcase halves are hot when replacing crankshaft bearings — use protective gloves.

A flywheel-side or clutch-side seal can usually be replaced without splitting the case. Only a leaking case joint or a rough main bearing forces a split.