1 reply to this topic

[087dave]

Hello all, i am rebuliting a 998 and have been offered slipper pistons so,
what are these slipper pistons and how better are they from flat top ones

Thanks in advance

[Wright&Wright]

Slipper piston
A Piston with the lower edge cut away so that the Piston skirt is short on the two sides not used as thrust surfaces. Such a design lightens the piston making it easier to accelerate and decelerate, wastes less power, and is easier on the bearings. Cutting away the skirt also allows the Connecting rod to be made shorter but leaves enough room between the Counterweights and the pistons so the overall height of the engine can be reduced.

That's the exact definition from google.
By the sounds of it, it's a lightened version of a standard piston, which makes sense I suppose. They'll probably be omega if they're for a Mini.

Keith Calver (Full accreditation goes to him) :

And I'd actually read this fully and properly by the way, it'll be extremely important information from a reliable source.

Perhaps the 'peak' of piston design is the current race-type items as used by the likes of F1, etc. - what amounts to little more than a large disc joined to the con rod by a short 'tail', having only two piston rings (one compression one oil ring) and weighing only slightly more than yer average feather. You have to see/feel one to appreciate the design - based on what has become commonly called a 'slipper' piston. Not because it looks like your fireside 'comfies', but because of it's low-drag design having no sides to speak of making if very 'slippery' in use. A feature carried over into the road-car market and after-market performance pistons. The Mini/A-series being no exception.

The first pistons available in this 'T' shape were manufactured for Abingdon ST for Mini Miglia/998cc engines. A pukka forged slipper piston made by Cosworth. You had to use S rods with these though as no sides precluded circlip or button retention for the wrist pin. The main hassle here was having to assemble the piston to the rod, remove the rings, feed the piston in from the bottom of the bore, then poke the piston out the top of the bore, and re-assemble the rings before making fast with the ring clamp to actually finally fit the assembly complete back into the bore. Strip down was the exact opposite. A fair bit of hassle.

Latter days saw the introduction of the slipper shape to the A+ engines, first fitted to the Metro, in an attempt to reduce piston drag/weight to maximise performance and economy for the road engines. It wasn't long before folk started using them in performance street and competition engines - believing what's good for the likes of F1 must be good for the A-series. Much in the way many other performance-related items have been applied from one automotive application to another entirely different one without any real understanding of the subject, consideration or testing/development work.

To cut this long story short - remember what I said right at the start about bore and crank flexure? F1 engines have supremely stiff block assemblies with very short-stroke, supremely stiff crankshafts. They simply do not suffer from flexure to a degree that affects ring seal at high rpm (and we-re talking 18,000rpm here). Even when considering the fact the engine and gearbox assembly is what they call a 'stressed member' - it carries the whole of the rear suspension as well. The A-series doesn't enjoy anything like this stiffness.

I am not saying the current crop of slipper pistons are a waste of time in an A-series. They are perfectly capable when used in a situation they were designed for. And I have used them (the MG Metro versions - the best of the bunch construction-wise) in race engines with success. It's just not the best option for our venerable A-series. This was recognised by Rover when they went into production of the Metro Turbo engines, and latterly the injection Mini engines where full-skirt pistons were used.

So - where an option exists a full-skirt piston is likely to produce more effective and more consistent ring seal, especially at higher rpm than a slipper type because of the crank/bore flexure problem. And this is anything over 6,500rpm on a regular and protracted basis, i.e. not just a quick 'visit' to that rpm peak on an odd occasion.

Edited by Wright&Wright, 09 October 2010 - 07:57 PM.