Use a cloth seamstress tape to measure the O.D. of your dampener and make marks at 90 degrees from TDC, if it is 7.25 diameter then 3.14 x 7.25 = 22.765 inches in circumference then divided by 4 = 5.69 inches from TDC to 90 degrees and then twice that amount to 180 and so on That will give you 90 degrees marks so you can turn the crankshaft one revolution and stop every 90 degrees to get eight pistons at TDC, don't forget that one cylinder will have the valves open on the overlap stroke when the other cylinder will have them close for the compression and firing stroke
Good luck, have fun