Thought that I would share my experiences. Piston retention crescents need to be fitted to your receiver as there are too many types encountered for any receiver manufacture to compensate for. ORF has taken a smart position and milled the slot somewhere in the smaller end of the spectrum. You can approach this problem in two ways. One Recomended by ORF. Clamp the small disc with pliers and grind the outside of the oval down till it just sits flush with the barrel hole. 2) what I did before I spoke with ORF. Use a Dremel tool grinding disc and deepened the slot till the crescent disc sits flush. The other note I have to add is the upper receiver rails that the bolt slides in. On one receiver it closed into battery. The other one required the right rail to be relieved slightly. ORF shared with me that this is intentional as I found out different bolt sizes.Another place that you might need to custom fit. It seems that there are a lot more variations out there between kits than you might emagine. I took my barrels to have them measured and got two different hole sizes on the receivers. Having built quite a few guns in my day I would like to say that ORF really did their research on this VZ-58 receiver. This is however not a drop in and shoot project and requires skill and a little experience in gunsmithing. Having said that you might get lucky and not find these problems, but if you do hopefully these notes help. I am delighted with my purchase of the ORF receivers and recommend this product to all that are up for a really great firearm. Or you can wait a few weeks and Todd and staff will complete one for you! :mrgreen: