Weihrauch piston seal fitments vary considerably. Some seals spin and are loose on the piston others are very tight and have to be forced on. Scratches created when removing the old seal should be filed off, the seal should be spun on the piston a few times. A tight fitting seal will bow out the rear lube band causing it to be bigger than required. This will in turn pull the sealing edge in making it smaller than required, not a desirable situation. In an effort to combat this the internal dimensions of the seal have some clearance. Measure your seal across the rear band and the sealing edge record the measurements. Now fit the seal on the piston remeasure and record. Did the measurements stay the same? Or did they increase or decrease? If the measurements stayed the same and the seal fit is not excessively loose no further action is requited. An excessively tight seal that distorts the seal is a problem. The solution is to either remove some material from the inside of the seal or some material from the piston. A loose seal is generally preferred as it is easily rectified. Teflon tape can be wrapped around the seal fitment or a circle of brown paper can be placed on the piston and then the seal fitted. The paper should be big enough so it wraps around the whole fitment. By adding tape or paper ( I prefer brown paper) the seal can be made to fit nicely. Keep measuring the rear band and the sealing edge and note the difference the paper shims make. In this way the rear band can be increased until it is close to the cylinder diameter. Once the paper is the correct thickness spray it with hair spray and fit the seal. The sealing edge will sometimes decrease as the rear band increases. The piston should be a able to be easily pushed down the cylinder without excessive force. The seal will wear in over the next 250 shots chronograph testing should not be done until after 250 shots have been fired and the seal has worn in and any excessive lube burned up. The seal should be lubricated with a small amount of molybdenum disulfide grease on the rear band. Excessive lubrication or lube on the seal face will cause a dieseling condition. Moly Grease can also be applied to the piston, this will work its way on to the cylinder walls and provide on going lubrication.