Magnum XL 52 RFS Blue -R/C 4 Stroke Engine (RC) by Magnum. $183.99. Weight: 15.7 oz Exhaust thread size: M10 x 0.75. Bore: 23mm. Displacement: 52ci (8.53cc). Practical RPM: 2,000 - 10,500. Stroke: 21mm. This is the perfect engine for ""40"" size two-stroke aircraft which can handle the slightly bigger size and weight of a four-stroke engine. This is probably the best sub-60 size sport four-stroke ever designed!.