Piston kit, 38mm, for the Stihl 4117-series FS90 and FS96. Replaces Stihl part number 4117-034-0500. Contents: piston, two piston rings, wrist pin and circlips. The engine is 28.4cc. Before you order, check which FS90 you have. Stihl built two completely different machines under that name and both use a 38mm piston. The 4117 series, which this kit is for, is a conventional two-stroke. Nothing on top of the cylinder but the plug. The 4180 series is a 4-MIX engine with a valve train, and its 38mm piston is 4180-030-2007. If your machine has a rocker box or a valve cover on the cylinder, it is a 4-MIX and this kit will not suit it. Same bore, different engine, and they do not interchange. The catalogues carry separate parts diagrams for FS90-4117 and FS90-4180 for exactly this reason. Also worth knowing what is not a 38mm machine. FS100, FS110, HT100, HT101, HT102, KM100 and KM110 are 40mm. FS130, KM130, HT130 and HT131 are 43mm. Numbers for the parts sheet: piston ring 4117-034-3000, two per piston; wrist pin 4117-034-1500 at 10 by 7 by 30; circlip 9463-650-1000; needle cage 9512-003-2340; cylinder only 4117-022-0501; cylinder with piston 4117-020-1200. Piston kit only. Cylinder, needle bearing, gaskets and crankshaft seals are not included. Pressure and vacuum test the crankcase before you run the new piston. An air leak at the crank seals or the intake boot is what scores these, and it will score a new piston as readily as it scored the old one.
- 38MM PISTON KIT - Piston, two piston rings, wrist pin and circlips. Replaces Stihl part number 4117-034-0500. For the 28.4cc two-stroke engine used in the 4117-series FS90 and FS96.
- THERE ARE TWO DIFFERENT 38MM FS90 PISTONS - READ THIS FIRST - The 4117 series is a plain two-stroke. The 4180 series is a 4-MIX engine with a valve train, and its 38mm piston is 4180-030-2007. Same bore, different engine, no interchange. If your machine has a rocker box or valve cover on top of the cylinder it is a 4-MIX and this is the wrong part.
- WRONG BORE MODELS TO AVOID - FS100, FS110, HT100, HT101, HT102, KM100 and KM110 are 40mm. FS130, KM130, HT130 and HT131 are 43mm. None of those take a 38mm piston, however similar the machine looks.
- NUMBERS FOR YOUR PARTS SHEET - Piston ring 4117-034-3000, two per piston. Wrist pin 4117-034-1500, 10 by 7 by 30. Circlip 9463-650-1000. Needle cage 9512-003-2340. Cylinder only is 4117-022-0501, and cylinder with piston is 4117-020-1200.
- PRESSURE TEST BEFORE YOU RUN IT - An air leak at the crank seals or the intake boot leans the mixture and scores a new piston the same way it scored the old one. Pressure and vacuum test the crankcase first, and check the impulse line for hardening while you have access to it. Cylinder, needle bearing, gaskets and crankshaft seals are not included.