Piston Kit Fits Jonsered 820 and 830 Chainsaws, 52mm Bore

Piston kit for the Jonsered 820 and 830 chainsaws, 52mm bore. The kit contains the piston, rings, wrist pin, circlips and the small end bearing, which is a complete top end for one cylinder. The cylinder, base gasket and crankshaft seals are not included. The 820 is the 80.7cc saw built from 1984 and the 830 that followed shares the same 52mm bore. Measure across the crown of the old piston before ordering - the Jonsered 920 and 930 are 54mm and the 630 is 48mm, and a saw of this age may have been rebuilt before. A scored piston on these saws is usually the result of an air leak at a hardened crankshaft seal, so pressure and vacuum test the crankcase before reassembly or the new piston will go the same way.

• FITS THE JONSERED 820 AND 830 - Both saws run the same 52mm top end. The 820 is the 80.7cc machine built from 1984, and the 830 that followed it shares the bore. If the saw has lost compression, starts hot and dies, or you have found scoring down the exhaust side of the cylinder, this is the kit that puts it back together.
• WHAT IS IN THE BOX - Piston, piston rings, wrist pin, circlips and the small end bearing. That is a complete top end rebuild for one cylinder. It does not include the cylinder itself, the base gasket or the crankshaft seals, and a saw that scored a piston usually wants the seals as well - see the last bullet for why.
• 52MM IS THE NUMBER THAT MATTERS - Put a caliper across the crown of the old piston before you order. 52mm is the 820 and 830. The Jonsered 920 and 930 look like the same family and are 54mm, and the 630 is 48mm. Ordering by model name alone is how the wrong piston ends up on the bench, because a saw of this age may already have been rebuilt once.
• WORK OUT WHY THE OLD ONE FAILED FIRST - A scored bore always had a cause. On a saw this old it is almost always a hardened crankshaft seal or a cracked impulse line leaning the mixture out, and it will take the new piston exactly the way it took the old one. Pressure and vacuum test the crankcase before the saw goes back together.
• WHILE IT IS APART - Square each ring in the bore with the piston crown and measure the end gap with a feeler, and check side clearance in the groove while you are there. A ring that is tight at operating temperature breaks a land. Clean the exhaust port of carbon, fit a new base gasket, and torque the cylinder down evenly in a cross pattern.