I just used a grinder, it probably isn't perfectly straight but it doesn't need to be. I use the 'cone' half with the cone removed on the barrel side as you want the union half with the most burst disk contact area on the barrel side because that's the side the disks are pushed against. If you use gaskets or o-rings to seal the disks, you only need them on the 'chamber side' of the union and not the barrel side.true, haven't just been able to fins proper sized union that doesn't have the "cone". I sanded down but it isn't perfectly straight so getting it seal is really a pain
My latest method has been to stack o-rings inside the union half attached to the chamber (the union half with the external male threads) so that the last o-ring protrudes slightly out of the inside of the union half; providing a seal for the disks. The disks and then stacked on top of that and then the barrel union half is placed over the top. It doesn't matter if the barrel union half is perfectly flat after modification in this case.










