Just top up the dashpot to the top of the piston that the plunger goes into. It doesn't matter too much if you put a bit too much in because the screw cap has a smale hole so any excess will be forced out when the piston goes up. My recommendation is to use ATF but any oil that is thinner than engine oil will do. One of my Club Triumph friends used a mix of 20W50 engine oil and 3-in-1 type oil in his racing Spitfire in his dashpots (he was running the 1500 lump with the SU HS4s at the time). The reason for using a thinner oil is to improve response when the piston moves up when the throttle is opened.
West Yorkshire Area Organiser & forum moderator
Meetings take place on the 1st Wednesday of every month at The Hollies Sports & Social Club, 12 Hough Lane, Bramley, Leeds, LS13 3NE
Dolomite 1850 auto (NYE 751L - The rolling restoration)
Citroën C4 Grand Picasso 2.0 HDi Exclusive (MA08 WCL - the modern)
Former stable of SAY 414M (1974 Toledo), GRH 244D (1966 1300fwd), CDB 324L (1973 1500fwd), GGN 573J (1971 1500fwd), DCP 625S (1977 Dolomite 1300) & LCG 367N (1975 Dolomite Sprint) plus 5 Acclaims and that's just the Triumphs!
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