12mm flat seat Spark Plug indexing washers are available from Jacobs Eletronics, part# 380815.
But could not find them on Jacob's USA website, seems like the company has been swallowed up by the MR GASKET Corporation, but found them on the Australian website;
http://www.jacobselectronics.com.au/PricesGrp2Cat6.htm
http://www.jacobselectronics.com.au/accessories.htm


Cheers..........Agro