Yes, Grant, the orfices are much smaller for CNG. I've been looking for
a long time to find out those sizes as I want to convert a nice little BBQ
to natural gas and nobody can give me a size for the new hole. I'd
think there's a formula or chart to use to go from a given size propane
orfice to go to natural gas....



Bill
There appears to be too many variables to give you an answer.
From:
http://www.iccsafe.org/cs/codes/2004...es/FuelGas.pdf

"To select correct orifice size for rated burner
input. The selection of a fixed orifice size for any rated
burner input is affected by many variables, including orifice
coefficient, and it is recommended that the appliance
manufacturer be consulted for that purpose. Where the
correct orifice size cannot be readily determined, the orifice
flow rates, as stated in the tables in this appendix, can be
used to select a fixed orifice size with a flow rate to
approximately equal the required rated burner input."

But I think that there is enough information in that PDF file that you could
get pretty close by comparing your existing orifice size. There are 32 pages
of information and table for your perusal.

Hope this helps.

Lane