The gasket set for an OS FP .40 will suit.
Larrybtoys on *Bay sells replacement gaskets for many OS engines. Send him a message to see if he has any to suit yours.
http://www.ebay.com/sch/m.html?_ssn=...&_nkw=OS&rt=nc
The back plate gasket for the Magnum GP .40 (FP .40 clone), which also suits the Tower .40 and TT .42, will also suit:
http://www.ebay.com/sch/m.html?_odkw...0&_sacat=0late
Otherwise you will need to make your own. It is easy and an old playing card is perfect gasket material. A sharp exacto knife, a 2.5 mm hollow point punch and an ink stamp pad and you are good to go.
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