It are different sizes from the Hubertus knives avaiable. Important to choose the correct spring is the closed length in cms of the Hubertus knife. A Length Equivalents (length converter) is on our web page in the service section. It are from Hubertus the following closed lengths avaiable: 8 cm, 9 cm, 10 cm, 11 cm, 12 cm, 14 cm, pen blade and the Guardian knife with a extra spring model. If you are not sure which knife you have, please take a look to the Hubertus knives on our webpage. At the automatic knives are the last two digits from the product number the closed length. You can send us also a picture from your knife and the closed length in inches or cms and we will tell you which knife you have. So that you order the correct spring because we want that you get the correct spring. For any questions, please write us an e-Mail! A description how to get the old spring out of the knife and the new spring in the knife is at the attached link.