In this step, you'll need to create pilot points to drill the center holes on the end caps. It's important that this step be done, otherwise the piston may not fit correctly and the injection nozzle will look screwy. To get the most accurate center, we will rely upon the accuracy of the threads, so you'll want to use a nipple as the support.
If you have a lathe, all you'll want to do is to insert a 3/4" non-galvanized nipple into the lathe, put on a cap, and drill the hole. I don't have a lathe since they cost more than I'm willing to pay, so I create an adapter for use with my drill press. Here are the steps:
- Find the biggest dowel that your drill press will accept.
- Cut a short piece of the dowel off.
- Wrap the dowel in masking tape so that it'll fit tightly into a 3/4" nipple.
- Insert the dowel into the drill press / hand drill.
- Insert the smallest 3/4" nipple you can, on top of the dowel adapter.
- Screw a cap onto the nipple-adapter.
- Place knife / blade onto cap while spinning.
With respect to step #3, it may require a long piece of tape. Once you finish, you may want to wind it back onto the tape-roll.
With respect to step #7, the rotation tends to push the cutting blade towards the center of rotation. If this doesn't happen, you may need to (roughly) level the cap-tops and try again. A fresh blade from a utility knife works great, though I used some sewing scissors.