Well done Erik! You are indeed if nothing else a persistent blighter!
This job may not quite prove to be a case of “Here be Dragons” but it is a minefield of unknowns – the more you think about it the more there are! In the end, I suspect, it is going to be a case of “suck it and see”.
Weight reduction is going to be the really important job to tackle. For the metrically challenged the difference is just a tad over 4oz. That is a lot of extra inertia. At least we have the expectation, (it is to be hoped!), that the quality of the material in the new piston will be superior to that of the original. This is the thing that may save the day as far as weight is concerned for improved material quality will allow for some skimping on wall thickness.
In many ways it is a pity about the existence of the lower ring grooves as they are going to complicate matters. I’d be inclined to dispense with the lower pair of rings; I will forgo my reasoning for the sake of brevity. I do not think that the empty grooves will present a problem.
I would not be inclined to shorten the piston, at least not yet. Time then to sit down, measure and draw up both pistons to see where the excess metal is. Given some extra mental gymnastics it would be possible to calculate exactly how much thinning would be needed. With that information to hand some careful boring following the outside contour up to the gudgeon pin bosses would seem to be in order. The gudgeon pin bosses, at least visually, seem to compare favourably with the original and in any event any reworking in that area, I can say from experience, is more than a trifle difficult. There may be some scope for metal removal in the accessible areas under the piston crown if that proves to be thicker than the original.
I am not so sure about an oil recess in the piston, it would certainly help reduce a little bit if weight but the side thrust on the piston is what it is and a reduction in area will only serve to increase the unit loading on the remaining part of the full diameter, not really a problem with modern lubricants. If any relieving on the outside is to be done it is my feeling that it should be restricted to an area around and below the gudgeon pin holes.
The ideal shape for a piston is of course a true cylinder at working temperature. To allow for this the shape at ambient temperature is not, the difference is dependent upon the quantity and manner of heat flow through the piston until rejected via the cylinder walls and the lubricating oil, (this a 4stroke remember). That will vary with engine design; each is different and is the subject of much research particularly on high volume production units.
The most important working area of the skirt is 90º to the crank axis. Look at a piston for a modern exotic engine, the skirt in the area of the gudgeon pin bosses has all but ceased to exist and even at right angles is only large enough to stop the darned thing looping the loop! The piston skirt itself has little to do with the actual sealing of the combustion chamber, the job it does do is hold the piston vertical in the bore and stop it flapping about, this is essential if the rings are to seal properly, any rocking of the piston in the bore is very detrimental.
There is much, much more that can be said and speculated about but this post is quite long enough already. Sufficient to say that once the weight has been bought down I would be inclined to try the piston as it is. After a careful period of running inspect the piston and if necessary relieve any tightness. To this end it should be possible, once thoroughly degreased, to deposit a thin layer of copper on the iron with a copper sulphate solution. Due to the contrasting colours of copper and iron any high spots will show up more clearly.
Thanks as always for the elaborate reply David!
Concerning the weight I think cutting of the lower oil ring only and skimming the wall behind the lowest compression ring and the skirt below the pin will give me a considerable weight loss already.
The new piston also has a slightly larger compression height. About 1,6 mm. And the crown is also a bit thicker. So if neccesary I could also skim the top of the piston a bit. But I will leave that as a large resort because if the piston fits with the higher compression height I would not mind as this would give me a slight gain in performance. And with modern day petrol raising the compression a bit would not be a problem I think.
I think your suggestion of just trying the piston after weightreducton seems like a sensible idea. Lifting the cylinder every now and then to keep an eye on high spots is quite easy with it being an easy accesible side valve.
What do you, and others, think is a good clearance to begin with? The 0,06 mm I posted in the image (being about 0,0025 inch)? Or more/less?
I can see how removing the bottom groove is attractive from a weight saving point of view but my objection would be ‘cos of that old guy Pythagoras or one of them there Greeks blokes what had something to say about levers. The point being that the shorter the skirt the less rocking control it can exert. You may have noticed that most piston skirts just clear the crank webs, there is a good reason for that.
I doubt raising the compression ratio will have any really noticeable effect but it will increase the loading on the bearings and octane ratings are one the slide anyway. On the other hand taking 1.6mm off the crown will account for 40 grammes of the weight you are looking for; the question is which is worth more?
The piston will achieve a higher temperature than the barrel but as both are cast iron the differential expansion will be low, I’d be inclined to try it as it is, it’s easier to take something off than put it on!
Do try the copper sulphate trick, apply it by brush or a bit of rag, if it doesn’t colour the iron its probably because it needs more degreasing. It’ll do no harm and comes off easily.