The only piston style available from Onan for the B & P series engines is shown in the photo on the upper left side. Part# 112-0264, casting # 112-0277 and use's Ring Set Part # 113-0310. Available in STD, .010, .020 oversize.
If you only have one bad piston and it looks exactly like the one pictured on the upper left with casting # 112-0277 no problem. If your pistons look like the three other styles you will need to replace both to avoid an out of balance engine.
Onan manufactured several styles of pistons for the B & P series engines over the years. The most common are shown here. The two pictured on the bottom look identical but have different casting numbers and use different ring sets. Always check the thickness of the compression rings before ordering!
Part # 113-0310 Ring Sets have .057" thick compression rings.
Part # 113-0314 Ring Sets have .077" thick compression rings
