Attention:Avoid buyer to place order with wrong model, please Check the product model by photo with your old razor photo. Our 51S 51B razor are compatible with old Version 5 Series 530/540/560/570/590,5643,5644,5646,5647,5751,ContourPro,360 Complete,Activator Series WaterFlex,WF1sWF2s, and Series 8000,such as 8674,8377,8385,8585,8590,8595,8795,8970,8975,8985,8986,8987,8990,8991,8995 Attention: It is not compatible new Generation Series 5 such as 5030s 5040s 5090 5415,5070cc 5020s 5147s 5140s 5190cc ,New Series 5 need replaced by 52B/52s Small investment, peak performance Within 18 months, your shaver will cut an average of 6,000,000 hairs. In accomplishing this substantial task, the cutting parts will gradually wear out and your shave may become less close and comfortable. That's why recommends changing your shaver’s foil and cutter block every 18 months to maintain your shaver’s maximum performance. For an efficient shave every day. How to replace your shaver head? For older models which have a foil and a cutter block as two independent parts you will need to change the parts in two steps: 1.First remove the foil, either by pressing the release button if your shaver has one, or by pulling the foil. Next rotate the cutter block by 90 degrees and pull it off, holding it at the ends to avoid touching the blades. To fit new parts, simply carry out the same procedure in reverse.