PRODUCT FITTING INSTRUCTIONS
CST 3030 72.5mm Pistons
CST 3030 72.5mm Pistons
The growing dilemma of sourcing usable 1275 engines has meant a re-think on making those we have/are available go further/last longer. To that end CST is European distributor for the Graham Russell, Russell Engineering, Australia designed range of pistons to extend block life. The 72.5mm range is a 'save' on a +0.060” block that is not too badly damaged, being just +0.014” over +60 and gives an engine capacity of 1342cc. The 73mm option on a standard stroke gives 1360cc capacity, and testing to date has shown no measurable loss of performance over the popular 73.5mm bore 1380cc size. The use of the 73mm piston obviously then allows a further 2 over-bore of that block to 73.5mm.
Manufactured by the same company that has produced the excellent 998 flat top pistons and now also the 1098 flat top pistons using the same, super-grade, extra-high silicone content material specifications, these pistons have an optimised crown and dish (6.5cc) shape for improved squish/burn. The A-series generally gives better performance with a larger piston dish and smaller chamber volume. It also means that an otherwise standard 1275 cylinder head can be used initially where required as there is no problem with compression ratio. The piston is lighter than most cast pistons of this size and a lighter wrist pin is used to enhance this. The wrist pin is also supplied oil from the oil control ring land rather than relying on 'splash' from whirling components.
The piston ring groove design has stepped away from convention, utilising specs distilled out of intensive testing and are as used on the very latest high-performance pistons for mega-powered Japanese engines. One feature of this design is that the rings do not retract fully into the groove – this is NOT a design fault!
Ideally, the top and second compression piston rings should be gapped to give 0.004” per 1.0” of bore diameter. Some ring sets come supplied with larger ring gaps, up to 0.020”. This is not a problem. The manufacturers tired of complaints for customers who damaged their engines because they didn't bother setting the ring gaps correctly. To avoid this completely they decided to increase the rings gaps. Industry leaders tests have shown that gaps in the order of up to 0.025” make no difference to performance. On engines I have dynoed with such ring gaps, post dyno compression tests and leak down tests (checked on all dynoed engines as a matter of course) showed excellent readings in both cases. The oil control rings should just be assembled to the piston as they are because the assembly is designed to work at it's compressed in-situ form. This is not measurable with the piston out of the bore.
The higher silicone content and advance heat treatments used also creates a more stable piston which dramatically reduces expansion under combustion/running conditions. Consequently lower piston to bore clearances are required. These pistons are manufactured for race use so will give correct piston to bore clearance on the relevant bore size. Suggested finished honed bore to piston clearances are as follows -
Road/fast road - 0.0015” to 0.0017” (0.0381mm to 0.0432mm)
Sports/super sports - 0.0018” to 0 0.002” (0.0457mm to 0.0508mm)
Race - 0.0022” to 0.0025” (0.0559mm to 0.0635mm)
Turbo road - 0.002” to 0.0025” (0.0508mm to 0.0635mm)
NOTE - The piston rings are position/orientation specific:
The top ring is a moly-faced/shiny silver type that is non-directional.
The second ring fits with the markings uppermost