Pop Up Piston Kit Stihl MS391 49mm 1140-030-2003 64.1cc

Pop-up piston kit for the Stihl MS391. Bore 49mm. Replaces Stihl 1140-030-2003, superseded to 1140-030-2008. Supplied with rings, wrist pin and circlips. SPECIFICATION Bore - 49mm Stroke - 34mm Displacement - 64.1cc WHAT A POP-UP PISTON IS. The crown is raised, so it projects further into the combustion chamber at top dead centre. That reduces chamber volume and raises compression. Stihl does not catalogue a pop-up piston for this saw - the factory part is a flat top. This is a performance variant and it is sold as one. MEASURE YOUR SQUISH CLEARANCE. A raised dome consumes squish clearance. If the base gasket has also been deleted, the two changes stack and the crown can contact the head. Check the clearance with a length of solder through the plug hole before final assembly. Deleting the base gasket and fitting a pop-up piston are alternatives, not a combination. THE NEAR MISS THAT CATCHES PEOPLE. The Stihl 039 and MS390 are also 49mm, also 34mm stroke and also 64.1cc - identical on paper to the MS391. They take piston 1127-030-2005, a different part family entirely. Measuring your bore will not tell the two apart. Go by the model on the saw. THE 1140 FAMILY BY BORE MS311 and MS362 - 47mm - 1140-030-2002 MS391 - 49mm - 1140-030-2003, now 1140-030-2008 - this kit MS400 - 50mm - 1140-030-2014 Check the cylinder through the exhaust port before fitting a piston alone. A raised compression piston in a tired cylinder is a waste of the part.

• FITS THE STIHL MS391 - 49mm bore, 34mm stroke, 64.1cc. Replaces Stihl 1140-030-2003, which Stihl has since superseded to 1140-030-2008. Either number on your parts diagram means this piston. Supplied with rings, wrist pin and circlips.
• THIS IS A POP-UP PISTON, NOT THE STOCK FLAT TOP - The crown is domed so it projects further into the combustion chamber at top dead centre, which reduces chamber volume and raises compression. Stihl does not make a pop-up piston; the factory part is flat topped. If you want the engine exactly as Stihl built it, this is not that piston.
• MEASURE YOUR SQUISH BEFORE YOU RUN IT - This is the warning nobody selling pop-up pistons gives, and it matters. A raised dome eats squish clearance directly. If you also delete the base gasket you are stacking two compression increases and the crown can touch the head. Do one or the other, measure the clearance with solder before final assembly, and do not assume.
• NOT THE MS390 OR THE 039 - and this is the trap. The 039 and MS390 are ALSO 49mm, ALSO 34mm stroke and ALSO 64.1cc, and they take a completely different piston, 1127-030-2005. Identical dimensions, different part family. Bore measurement alone will not save you here - go by the model on the saw and the number on your diagram.
• THE REST OF THE 1140 FAMILY - MS311 and MS362 are 47mm and take 1140-030-2002. MS400 is 50mm and takes 1140-030-2014. MS391 is the 49mm one. All four share the 34mm stroke, so they look alike in the box and only the bore separates them.