Pop-up piston kit, 49mm, for the Stihl 039 and MS390. Replaces Stihl part number 1127-030-2005. Contents: piston with a raised crown, ring, wrist pin and circlips. A pop-up piston has a domed crown that sits closer to the cylinder head at top dead centre, tightening the squish band and raising compression. It is an aftermarket performance part - Stihl does not list a pop-up piston, and 1127-030-2005 is the standard factory number this one replaces. Fits the 039, the 039 Farm Boss and the MS390. All are 64.1cc on a 49mm bore. The ring is 1127-034-3007, 49mm by 1.5mm. Worth noting because most of the Stihl range in this size uses a 1.2mm ring and this one does not. The 1127 series contains three different bores and the saws are hard to tell apart: 46mm - 029, 029 Super, MS290 - piston 1127-030-2003 47mm - MS310 - piston 1127-030-2007 49mm - 039, MS390 - this one And the MS311, despite the name, is not in the 1127 family at all. It is the 1140 series at 47mm. Because a pop-up changes squish clearance, measure it on assembly with a solder test through the spark plug hole and choose base gasket thickness from what you read. Too tight and the dome will contact the head. Raising compression raises combustion temperature too, so set the high-speed mixture slightly rich and run the first tank gently. Piston kit only. Cylinder, needle bearing, gaskets and crankshaft seals are not included.
- POP UP PISTON KIT, 49MM - Piston with a raised crown, ring, wrist pin and circlips. The dome tightens squish clearance and lifts compression. Aftermarket performance part - Stihl does not catalogue a pop-up, and 1127-030-2005 is the standard factory piston this one replaces.
- FITS THE 039, 039 FARM BOSS AND MS390 - 64.1cc on a 49mm bore. The ring for this piston is 1127-034-3007, 49mm by 1.5mm - note that is a thicker ring than the 1.2mm used on most of the range.
- THE 029 AND MS290 ARE NOT THIS PART - They are 56.5cc on a 46mm bore and take 1127-030-2003. The MS310 is 59.0cc on 47mm and takes 1127-030-2007. Three bores in the same 1127 series, and the saws look alike. Measure before you order.
- THE MS311 IS NOT IN THIS FAMILY AT ALL - It is the 1140 series, 47mm, and takes 1140-030-2002. Despite the similar name it shares nothing with the MS310 or the 039.
- MEASURE YOUR SQUISH ON ASSEMBLY - A pop-up changes squish clearance, so check it with a solder test through the plug hole and set base gasket thickness from what you measure. Too tight and the dome contacts the head. Richen the high-speed setting a little and take the first tank gently. Cylinder, needle bearing, gaskets and crank seals are not included.