It's not always the o-ring. Don't forget to replace the tiny oil seal, down inside the cast housing, that the pinion shaft slides into. Take out the speedo drive unit, punch out the tiny split pin, slide out the gear and shaft, use a drill bit that fits loosely into the housing. Insert the drill bit while twisting it hard by hand. The sharp tip will bite into the oil seal. Then yank it out. Hopefully the old seal comes up with the bit. Assembly is reverse of removal. I like to use a tiny bit of RTV on the outside of the new seal as a lube and sealant. Use the butt end of a snug fitting drill bit to press the new seal down. It's a bit confusing at MSA: Speedo Pinion O-Ring, Large, 70-83 Z/ZX All, 84-89 ZX Manual Code:20-1552 Price: $1.27 Speedometer Pinion O-Ring, Small, 70-86 Z/ZX Code:20-1550 Price: $3.38 One is an O-Ring and one is an oil seal. You need both when you try to fix the leak.