Pistons - Whys and wherefores
For many, trying to make an informed decision about which type of pistons they should use for their application is a very difficult thing to do. Mostly you have to rely on guidance from a very few sources such as engine builders and Mini spares specialists, who are most likely to recommend whatever it is they use or sell. And that is not necessarily a bad thing where those companies have strong reputations for good quality products. But even those specialists may not know all that much about the pistons they use or sell. Particularly since trying to get any really useful information out of the manufacturers concerned is practically impossible. Try contacting AE Hepolite technical. Getting hold of someone takes real perseverance. Having achieved that meagre goal, enquiring about piston specification gets those ‘lemon sucking’ sorts of noises – anybody would think you are asking for the Holy Grail! So I’m going to try and cut through the techno-babble to help you identify and know what is what.
‘Piston, n. sliding cylinder fitting closely in tube and moving up and down in it, used in steam and internal combustion engine to impart motion’. The pocket Oxford dictionary’s description of the subject in hand. A bit minimalist, and not awe-inspiring. What a piston does, and the savage environment it does it in should be. Revered even.
For a start - if it wasn’t there, there would be no drive whatsoever. It is responsible for drawing in and helping to contain the all important fuel/air mixture - the ‘charge’ - for combustion (the controlled burn all too many refer to as 'explosion') that gives motion. The more combustion contained, the more power there will be to provide motion.
The temperatures and loads it sees are extreme and practically instantaneous – going from fairly cool (incoming fuel/air mixture), to ‘nuclear fusion’ (combustion, measuring close to two thousand degrees C) within fractions of a second. That is 16.66 times a second at an average engine speed of 4,000rpm. Respect.
To add flavour, the piston top (crown) is running at massively different temperatures to the bottom (skirt), and the cylinder wall (bore) temperatures also vary. These differing temperatures cause varying amounts of expansion/distortion of both piston and bore. So now the piston has to vary its own size to be able to cope with a ‘tube’ that is continually varying in dimensions to maintain a good ‘fit’. Excessive clearance will allow the piston to wobble about causing premature wear of both piston and bore and reduce charge entrapment (i.e. blow-by will exist through inconsistent and poor ring seal at the bore wall). Insufficient clearance will massively increase friction, leading to seizure of the piston in the cylinder. Disastrous.
The crown has to be immensely strong. The pressures it has to withstand compressing the charge, then enduring the rapid burn are enormous – we are talking well over ten thousand pounds per square inch. The clearance between piston and bore is sealed off using a set of metal rings (piston rings), carried by the piston. These rings are seated in grooves (ring lands) machined in to the piston. To be effective, these are placed near the piston crown – right in the ‘extreme environment’ area – and fairly close together. The piston has to be strong enough to withstand the enormous pressures of combustion, and continual fretting of the rings as the piston hurtles up and down the bore. Then there’s the extra shock loading imparted by any detonation (pinking) caused by incorrect ignition timing or poor fuel quality (octane rating) or simply weak/incorrect fuel/air mixture.
Generally, various types of aluminium alloys are used to make pistons. The types used, and how, are what makes certain pistons superior to others. I said at the start that we are not going to get ‘heavy’ with the tech side - avoiding the likes of beryllium/aluminium and MMC ( Metal Matrix Composite) because you are unlikely to come across this stuff used for the venerable A-series engine, but cover the basic whys and wherefores that are the most relevant to making an educated decision about which is best for your application.
Probably the most ‘coveted’ by the ill informed. Like ‘billet’ when referring to crankshafts. Forging pistons, apart from the following information, became the preferred option for race cars in the same way dog-box gearboxes did - it was a far cheaper and quicker process where limited production numbers were concerned and where technical changes were made frequently to make use of new knowledge/performance increases. One forging can be used to make several piston types for various different engines; each dimensional change can usually be catered for simply by machining the piston to a different specification. With a cast piston - each new style generally means a new die - very expensive and time consuming.
Forging imparts a more consistent molecular structure within the alloy used, thus allowing thinner sections (less ‘meat’) to be employed to achieve its strength for minimal weight, although the finished article is generally softer than a cast equivalent. Strength for strength, a forged piston is generally lighter through extensive finish machining. Part of what makes them more expensive than cast items. Sounds the one to use, but…
Pro's – withstands higher compression ratios for less weight than many cast variants. More tolerant of shock loading from incorrect ignition timing (detonation). Will absorb ‘debris’ (broken valves, spark plugs, etc.) without instantly disintegrating - basically more ‘flexible’ - so feasibly reducing total engine destruction. In which case they could be construed as being cheaper long term than cast pistons where racing is concerned. At least you have a chance of making the chequered flag!
Cons – alloy type used (astonishingly still most commonly RR58 developed by Rolls Royce in the early 1930s for the renowned Spitfire engine) expands more so needs greater running clearances making them unstable and therefore noisy (‘piston slap’) until up to correct running temperature. This causes premature wear when used in 'street' engines, especially because in street use the correct and required running temperature needed to attain proper working tolerances is rarely reached. They are difficult to use well, needing experienced machining knowledge to obtain maximum performance. Over-rich mixtures creating bore washing accelerates wear. Ring lands wear quickly. Very expensive to manufacture. Basically they just will not do the mileage. May be just what is needed in an out and out race engine where budget allows, but effectively useless in a road orientated or high mileage competition car.
The most prolific method of manufacture for a vast majority of pistons. Consequently far more development has gone into this method than any other for the masses – if for no other reason than its relative cheapness to manufacture. As the name suggests, they are made by pouring or injecting molten alloy (usually aluminium based) into a mould. A major benefit being you can put the metal where you want it, reducing finish-machining costs. The biggest problem is maintaining correct and exacting temperatures when pouring/injecting the molten metal – this takes seriously expensive high-tech equipment and huge experience. Getting it slightly wrong causes an inconsistent molecular structure that weakens the piston. Definitely not good! Just because they are cheaper does not mean to say they are not capable in a performance or race engine - modern products are a far cry from those made in the early 1960s. The specification, quality and consistency of the alloy used usually depends on piston application and size of manufacturer. So, where does that leave us with this?
Pros – alloys and heat treatments used produces a much harder, stiffer piston. Very low expansion rates mean tighter tolerances, so vastly reduced noise and wear for street and high-mileage competition cars. Easier to use to good effect, maintains shape better and the ring lands are far more durable. Much cheaper to manufacture, therefore generally cheaper to buy.
Cons – generally does not tolerate incorrect ignition/fuelling too well leading to failure in abnormal circumstances. Does not absorb debris, causing fracturing and failure, and further possibly greater damage to other components. Higher strength types tend to be slightly heavier than forged equivalents, although latest technology has seen a dramatic improvement in material further improved with specific heat treatment processes to help reduce weight/strength differences.
Developed by AE Hepolite, this combines all the plus points of both forging and casting pistons, whilst minimising the effect of the bad points. Just before the alloy poured into the mould solidifies totally, the casting is impacted. Simple to say, but complex to do accurately and consistently - the temperature of the alloy from liquid form to impact point has to be very carefully monitored. Again we are talking serious technology. Unfortunately AE have judged the process to be too costly to continue with. Their continuous development program enabling them to produce cast pistons of a very high quality, making squeeze casting redundant as far as they are concerned.
Piston weight is another obsession for some. Obviously the lighter the component the better – providing it will stay in one piece. To try and put this into some sort of perspective, the spectrum of different weights across the range of big bore A-series pistons from standard to 74mm bore sizes is around 20 grams. That is about three and a half 20-pence pieces. Not much, eh? I would say that pistons weighing up to about 395 grams are fine for continual engine speeds of up to 8,500rpm. Running higher rpm really needs a lighter piston. More to reduce wear than anything else. But there again, a carefully designed, stable piston with the right ring pack, in the right place, will give more power than an inferior light one! And there are not many A-series engine specs that will be making performance worth running to 8,500rpm for. Not for very long anyway unless seriously well built with lots of exotic and expensive parts. Skirt finish should not be very smooth. A ridged-machined finish is far better as it helps retain oil to protect the skirts against scuffing/galling at start up.
As you can imagine, the whole ring thing is a complete and separate science all on its own, so we will just look at what is relevant to us.
Getting the best ring types in the optimal positions is of massive importance. Not only are they responsible for preventing the combustion pressures from zipping down between piston and bore, and keeping the oil being sprayed about ‘downstairs’ from getting into the combustion area, they are also creator of the biggest percentage of friction in the engine. Friction consumes power – the best rings will maximise power out-put whilst keeping friction down to tolerable levels. Poor quality and design rings will reduce power and increase friction.
Fortunately, the main piston producers/ring manufacturers have this pretty well sorted out these days. A majority of the rings fitted to the relevant pistons are more than up to the job. The only problem seems to be just where they should be. Many years of experience, particularly in racing and high performance road engines, have distilled this down to a certain pattern, starting with the top ring being around 7mm down from the crown for many, though 5 or even 4mm is considered optimal. OK for a race engine where mileage is not an issue. Any further down than this and detonation can result due to excessive amounts of un-burnt fuel collecting in gap down the side of the piston crown and above the ring where mixtures and ignition timing have not been properly sorted . Any higher up and they are in danger of damage from combustion and also making the piston too thin between ring land and crown will cause piston destruction from detonation and the sheer brute force the burning charge exerts on the crown/crown edges.
When in motion, the piston turns from a relatively fairy weight of around 390 grams to a behemoth weighing in at several tons. The higher the engine speed (rpm) the more it weighs. Consequently the pin needs to be strong and stiff to stop the pistons hurtling out through the cylinder head! Conversely it also needs to be as light as it can be and still perform the aforementioned duties – its weight contributing to the pistons all-up weight. The strongest lightweight pins tend to have tapered bores at each end or parallel bored with thin wall sections. Mass-produced pins are usually parallel bored with quite thick walls (over engineered). Material is generally a type of ‘tool steel’ - extremely robust is a mild description – carefully ground to give a super-smooth and ultra-straight finish.
A majority are now interference fit in the small end of the rod. Years ago this was referred to as ‘press fit’, meaning they are pressed in to and through the small end of the connecting rod. A practice doomed to all kinds of errors – some very costly - unless done with specifically designed tools and experienced hands Providing the fit is good, this method is perfectly capable of performing without hassle even in race engines. Some say they ‘fall out’, causing massive damage to bores, pistons and rods alike. Simply not so – the fit can not have been correct in the first place (i.e. badly built), they are regularly used in race engines turning in excess of 9,000rpm. In fact, until the late 1980s/early 1990s far fewer problems were experienced in A-series engines using the press fit method than either circlips or buttons – be they PTFE or aluminium. This was largely due to incorrect material type used, badly manufactured parts, or ham-fisted assembly. In short, the interference fit method is far less ‘fussy’. PTFE is fickle, and must be the right grade. Aluminium must be accurately manufactured. Both difficult to make well for taper-end pins. Fortunately several of the specialist piston manufacturers have finally got the circlip deal sorted out. I personally prefer fully floating pins, and have been using these for many, many years. Making the buttons to do so is a hassle and costly. I am now generally going the circlip route.
Don’t get mugged. Forged pistons are only necessary in serious, high rpm, race engines. Current hi-tech, hi-spec cast pistons are more than adequate for everything else, and are proving their worth in a vast majority of motor sport where mental compression ratios can not be used because of fuel limitations. Not to mention being loads cheaper. I have been using cast pistons in my own as well as customer race engines for decades with no issues caused by piston quality shortcomings.
For further information see 'Pistons - Favourable features for maximum performance'