STIHL TS 420 Parts: Spark Plug, Air Filter Kit & Wear Parts

Cut-off saws live in the dirtiest air on any jobsite, so the STIHL TS 420 goes through air filters and spark plugs faster than anything else. Across STIHL cut-off saw repairs in our records, the NGK BPMR7A $8.61 plug is the most-installed part by a wide margin (around 90 repair orders), followed by the 4238 140 4404 air filter kit $26.08.

Quick reference

TS 420 air filter: 4238 140 4404

STIHL's improved-performance air filter kit 4238 140 4404 fits the TS 410 and TS 420 (and the TS 440, 480i and 500i). Concrete dust is abrasive enough to wear a cylinder quickly if it gets past the filter, so:

• Check the main filter daily on the job; tap it out, don't blow compressed air through it from the clean side.
• Replace it when it won't clean up or shows any tear or soft spot.
• Never run the saw with the filter cover cracked or missing.

TS 420 spark plug

We install the NGK BPMR7A on STIHL TS-series cut-off saws more than any other part. A plug that's been run with a dirty filter is often glazed or fouled; replace it whenever you replace the filter and you'll avoid most hard-starting complaints.

Hard to start? Fuel pump, primer and starter

• Fuel pump: the TS 410/420 fuel pump $11.95 is a common replacement on saws that pull fuel poorly or won't prime.
• Primer bulb: cracked primers are common on saws that live in trucks; the Rotary 9948 primer bulb $4.83 replaces Zama 00570.
• Starter: worn ElastoStart rope assemblies make starting hard — 1122 190 3400 $31.52 (4.5 mm).
• Gaskets: if you're opening the engine or carb, the TS 410/420 gasket set $26.00 covers the seals you'll disturb.

Fuel

The TS 420 is a 2-stroke. Mix fresh fuel with STIHL HP $18.46 or HP Ultra $22.76 2-cycle oil at the ratio on the bottle and in your manual. Stale fuel is the second most common reason saws come in hard-starting.

Daily TS 420 checklist

1. Clean or swap the air filter.
2. Inspect the wheel for cracks or missing segments; check the guard.
3. Check belt tension and the water kit (wet cutting).
4. Fresh, correctly mixed fuel.
5. Replace the plug at every filter change.

Common TS 420 symptoms

• Hard starting after a day of dry cutting: dirty filter and fouled plug — replace both.
• Bogs when the wheel loads up: air filter, then fuel pump and carburetor.
• Pull rope hard to pull or won't return: starter assembly or rope.
• Fuel smell, won't prime: cracked primer bulb or fuel line.

Parts in this guide

NGK BPMR7A Spark Plug 5850$8.61AIR Filter Kit (usa) TS 410/420/440/480i/500i$26.08Fuel Pump TS 410/420$11.959948 - Primer Bulb For Zama$4.83Elastostart Assembly 4.5mm Dia$31.52SET OF Gaskets TS 410/420$26.00Stihl HP 5.2 OZ 2 Cycle Engine Oil 6 Pack$18.465.2 Oz. Ultra Fully Synthetic 6 Pack 2 Cycle Engine Oil$22.76

FAQ

What spark plug does a STIHL TS 420 use?

NGK BPMR7A is the plug we install most on STIHL TS-series cut-off saws; confirm against your manual.

What air filter fits the STIHL TS 420?

STIHL air filter kit 4238 140 4404 fits the TS 410 and TS 420 (also TS 440, 480i and 500i).

Why is my STIHL TS 420 hard to start?

Most often a dirty air filter and fouled plug, then stale fuel, a weak fuel pump or a cracked primer bulb.

Related guides

• STIHL Chainsaw Chain Cross Reference (Pitch, Gauge & Links)
• STIHL Chainsaw Won't Start? The Parts We Replace Most (After Sitting or When Hot)

Repair statistics in this guide are aggregated from our service department's repair records. Part numbers come from manufacturer parts lists; always confirm fitment with your model and serial number.