PRODUCT FITTING INSTRUCTIONS
CST 3030 73 & 73.5 Pistons
CST 3030 73 & 73.5 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 (10cc) 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)
It is highly recommended that the bore are offset maximise the remaining central wall between bore 2 & 3. Offset these two cylinders by 0.015” away from each other.
NOTE - The piston rings are position/orientation specific:
The top ring has a chamfered/bevelled inside edge and fits chamfer/markings uppermost
The second ring fits with the markings uppermost
NOTE – THESE PISTONS HAVE THE SAME COMPRESSION HEIGHT AS THAT OF THE HIGH COMPRESSION, A+ PISTON AS USED IN THE MG METRO. ON OLD-STYLE BLOCKS, MG MIDGET BLOCKS, AND BLOCKS THAT HAVE BEEN RE-MACHINED PREVIOUSLY – A TRIAL BUILD IS ESSENTIAL TO SORT PISTON TO DECK HEIGHT OUT.
I frequently hear the phrase ' big-bore engines tend to be unreliable long-term because of head gasket problems'. Which is complete rubbish, even when non-Payen head gaskets are used. These big-bore engines can be supremely reliable - providing the are machined and built correctly in the first place. Something that continually astonishes me doesn't happen even when engines are built by some of the self-proclaimed A-series 'experts'.
The main problem stems from the fact that when the head is torqued down and compresses the fire ring around the top of the bore, it reduces the internal diameter of the gasket to around 72.5mm across the centreline of the bore - i.e. between the cylinders. This is not much of an issue where 73mm pistons are used since all pistons are a smaller diameter at the crown, and even smaller across the wrist pin axis as they are also slightly ovalised. However, where the larger 73.5mm and 74mm diameter pistons are used, unless steps are taken, the piston crown will clip/hit the gasket where it over-hangs the bore. The obvious and unavoidable conclusion is the head gasket will fail - either quickly or after some time. But it will fail.
There are a couple of ways to prevent this from happening, rather than hoping to get lucky. First is to make sure the block is bored using the head gasket to centre the re-bore on rather than the original bore centres. These original bore centres can be anywhere between spot-on central to the gasket to absolutely miles out. Not much of an issue on the usual over-bore sizes of +0.020", 0.040" or 0.060" - hence the factory's lack of attention to detail at getting the bores set better. So when getting your block bored to take the much larger piston diameters, make sure you supply the company with a head gasket with instructions to centre the re-boring to the gasket, across the centreline/wrist pin axis plane. If they are familiar with or are generally seen as A-series specialists, just ask the question and make sure they are doing this.
Where blocks have already been straight bored out using the original bore centres, position a gasket on the block using several head studs, but especially picking up on the gasket locating stud holes - these are sited at the short stud between cylinders 3 and 4 and long stud behind cylinder 1. These head stud holes in the gasket are 3/8" diameter, all the others are 7/16". With the gasket located in this way you can now asses how much and where the gasket fire ring over-hangs the bore, bearing in mind it will over-hang a little more once clamped down by the head. If you are lucky and there is barely any over-hang and evenly all round the bore, you will get away with it dependant on piston position at top dead centre. If it over-hangs more one side than another, front, rear or side to side, then steps need to be taken to ensure no gasket failures occur because of the piston hitting the fire ring.
One option here is to make sure the pistons are sited a short way down the bore from the block face when they are at top dead centre - 0.010" to 0.015" would be plenty. To a trial build to establish piston to block face distances. If the pistons are less than 0.010" down the bore, then the piston crowns will need machining down to make sure they clear the piston. The main problem here is that you have to have the pistons fitted to the rods to be able to make the measurement in the dry build. Not everybody has build pins - wrist pins with a section ground down to be a slide fit in the con rod - that can be used as 'floating' pins in the rods/pistons during the trial build. The vast majority of folk will be using the original type interference-fit wrist pin to rod set-up. Also - once the pistons are properly fitted to the rods, not all machine shops are capable of machining the piston crowns with the rods attached to the other end. The answer here is to get the piston crowns machined before they are fitted to the rods. The best way of doing this is to machine either a reasonable chamfer or a small 'step' in to the piston crown edge to allow for the gasket over-hang.