Yeah both ATAC'ers Twisty but are differnet types of ATAC systems (85 is like the 84 125 with the extra valve body in line with the exhaust port, different back mounts 84 has the mounting bracket bolts to motor and the 85 is just straight through swing arm.
Looking in the Clymer book and the pickup off the crank looks the same but the 85 has the same part number as the 86 which is different (go figure). Going to pull the engines down tonight so will let you know the outcome for future reference.
Shane