I have a gas dryer that was converted to propane and I now need to convert back to Natural Gas. The problem is that I can't find a source for the NG orifice.
What I want to do is to modify the orifice I have for the NG. My calculations which are based on the energy content and supply pressure of NG vs propane say that I need to increase the cross-section area of the orifice by a factor of 3.7 which translates to an increase in diameter of about 2.9. I think this is about right but would like to verify it somehow. If anybody know what the propane to NG ration is I would greatly appreciate it.
I also understand that the propane conversion included a pin that forces the on-board pressure regulator full open so I need to remove it and replace with a vented plug.

Please don't bother answering this question if you are just going to rant about how I will blow up my house and burn down the neighbourhood. I know what I am doing and have a plan to check and double check the flame size and quality plus I will observe many ignition cycles to verify reliability.
I will be oversizing the hole using a reaming tool in a machine shop so the bore will be round, smooth and parallel to the desired gas flow.

Most orifices are universal so try picking up a conversion kits at an appliance store or hardware store. The unit usually comes set up for NG and then Propane orifice is in small card on the back of the unit near the burner, check to see if the last guy who converted did not save the old orifice in the card. That would be SOP for me.

Yes, I looked all over for the NG orifice and it's not there. That's good advice for anyone doing a conversion. Put the old orifice in a small envelope or zip lock and duct tape it inside the unit next to the burner.
I have a lead at a used appliance place nearby. The guy thinks he may have something in his junk pile but he's not in a hurry to dig it out for me.

Ask him if you can check his outgoing junk pile. Most shops pitch their trade ins after awhile.