OMEGA DIECAST PISTONS - PLUS 020

Yes, we suggest plateau honing with a cross hatch angle of 35-40 degrees. After honing with either conventional (#220 - #280 grit) or diamond stone (#325 - #550 grit), finish by smoothing the surface with a fine-grit conventional abrasive (#400 - #600 grit) or sweep the bores with a flexible brush or a nylon bristle plateau honing tool. This is necessary to get rid of jagged peaks and folded and torn material. Your machine shop may have different preferences.

These are designed to be press-fitted onto standard production conrods for the 1275 A-Series engine. That includes Cooper S, Midget, 1275 GT and all A+ engines, plus SPi and MPi. If you have aftermarket steel conrods with the little ends bushed to accept a 13/16" (20.6mm) pin, select the 'Floating pins and clips.' These pistons will not fit conrods with 18mm little ends; for these conrods you will need a set of forged Omega pistons.

This gives the option to run a fully-floating gudgeon pin in the conrod little end, if your conrods are fitted with a suitable bush. So rather than being fixed in place, the pins can move and rotate, with the piston held to the pin by a pair of wire clips. The fully-floating setup is preferred by race engine builders for ease of assembly/disassembly, while there is also less chance of damaging the pistons on assembly. The downside is that we do not keep little end bushes for these standard 20.6mm (13/16") pins, so these will need to be custom made to suit.

These are made to the standard 1275 A-Series compression height of 1.498". At 0.020" oversize, the 1275 engine bore size is 2.800" (71.12mm), but these pistons are sized with skirt clearance in mind, so they will be nominally smaller than the bore.

The pistons are only one part of the equation, so it would be inaccurate to supply a static compression ratio figure without knowing more information on the engine build. If you can email us with the cylinder head chamber volume, head gasket type and the deck clearance figures, we will be able to calculate this for you, and make further recommendations.

Not all diecast pistons are created equal, and the Omega diecast are perfectly well suited to mild competition use, as well as near-standard road engine builds. An Omega forged piston will be stronger and lighter, so we would always upgrade to the more race-orientated pistons when using sustained high RPM levels and compression ratios exceeding 11:1.

The +20 Omega diecast A-Series piston with a flat top weighs in the region of 364g, including the piston ring pack and standard length gudgeon pin. The 6.5cc dish option weighs closer to 346g and the 10cc option 336g. Each boxed set is weight matched within 0.5g. The piston weight may vary from one production batch to another, so please contact us at the time of order for the most accurate measurement.