the 281 never used thin rings. the 181 thin rings are hard to find and usually rather expensive. i do have a source of them. send me a PM to remind me to get a price monday if you want.
the 281 is the same diameter, but diffrent piston. went to a modern thick ring non full skirt piston. if you have a piston like whats pictured below you have a 181 piston. 281 and 181 pistons interchange with no problems, same pin, height and all that.
thanks for the information....it is interesting...do you have any idea why they went to a non-full shirt?...Have you had time to check on the thin rings?..thanks