I have ordered a minimum set of pistons today, which is 12 number or 6 sets in total. The price for this batch has been fixed at a special developmental price, the next batch will cost £350 for a complete set of two pistons, rings, pin and clips.



They are expected to arrive around the 18th June 2012, and I will add more details as I get them, but the picture above shows the basic design. The actual piston is based on a 1950’s car forging, uses readily available motorcycle rings, but the cylinder bore will be 85.3-85.4 mm, depending on the ring set I can get for this batch.

They will be higher compression capable, lightweight 3 ringed pistons, with a 3/4” or 19.05 mm pin diameter. This will require a special interference fit conversion bush to be made for the connecting rod little end, which is an additional cost item for the early adopters.

I managed a little more of the engine, after getting sidetracked with the Whatton Boring Bar. Here’s the older design M6 on the left, and the newer M8S rear bearing support casting on the right. There is a difference in hole diameter at the inlet, and more of a restriction at the other end too.





This is the left hand M6 casting after being modded at the inlet side, and also at the outlet

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The hole has been smoothed and with a dummy bearing fitted you can see the difference in surface area, versus the older design underneath. What is not apparent is the hole is only 3mm in diameter just under the bearing on this M6 casting.





With the improved design of the oil circuit, especially the mod to the camshaft gallery, more oil will flow into the bearings and also be at a higher pressure. Next up the oil light piston and the rear seal mods.

Panhard pistons are quite heavy and bulky items compared to todays’ equivalents, examples shown below, but that’s because the piston has to be domed to make the compression high enough due to their hemispherical combustion chamber layout, whereas modern day pistons have minimal surface areas applied to the piston and combustion volumes, thinner rings and latterly thermal coatings, which allow for even thinner sections.



The biggest limiting factor is the close proximity of the top ring to the end of the steel liner, and as shown below the top ring is quite far down the piston, around 10mm from the top deck.



These engines are from the junk pile, but they still have their uses, as this piston was cut up to show the typical cross-sectional areas that exist within the original. There are more than one type of Panhard piston used in the flat twin engine, but this is the later 4 ringed type, that was cut up today.







Why is this done? I need to understand why the naked Panhard piston weighs 505 grams, when my target weight for a heavily domed piston is sub 400grams, which is double the weight of a single cylinder Supermotard piston from 2002, and probably three times heavier than a complete MotoGP piston!

PANHARD PISTON REDESIGN

I need to replace the pistons and rebore Brian’s cylinder, as this engine will represent a rejuvenated and refreshed Panhard engine using mostly OEM parts. The original pistons are only available in certain sizes, and the design of them isn’t really good practice nowadays.



There is little that can be done about the domed area or the piston pin position, because these are factors of the combustion chamber, and also the crankcase limits using longer rods. Obviously you can shorten the cylinders, but it is a lot of work, that few people will want to pay for. Under examination, it is fair to see that the tall & shallow OEM ring construction is at odds with the more modern thin & deep thinking. The oil scraper functions have been analysed and there are several upgrade options that can be used too. Piston skirt design, and gudgeon or piston pin location strategies have all changed, as well as different materials and coating technologies, which all enhance performance or give longer life when used in marginal conditions. So ideally finding a modern day piston that would fit would seem to be the answer, but although I have known about several pistons that I can use, they all have potential issues around the top ring placement and the fixed liner.

On reflection it would be more logical to design a replacement then, but what benefits would this give?

Obviously, this will give a slight capacity increase, as you are essentially reconditioning the standard Panhard liner, which has a finite thickness, but what about the tapered bores. Hmmm, we’ll pass on this for a minute while considering other points.

Fundamentally the piston should be a lot lighter, which will reduce crankshaft loads and prolong the life of the rotating elements at higher rpms. The ring technology has changed, so these get lighter and generally use combustion pressure to effect a seal. The thinner rings also have less friction and can reduce crankcase compression blowby, but the cylinders have to be finished to a higher standard for the rings to seal correctly.

There is a possibility that an offset piston pin can be used, which reduces piston noise when the engine is warm, and more importantly reduces the shock loading on the connecting rod, by spreading the reciprocating load changes over a fractionally wider area of rotation. The slight offset or bias to the exhaust side of the piston also reduces cylinder bore wear, but this is not used in racing, because the engines are maintained more frequently, so most forged pistons don’t use this.

Lastly, there is a wealth of coatings technology that can be applied, although this does add expense to the final product, it does offer protection from nasty combustion events, and does improve reliability.

These doodles are some of my thoughts from today, and obviously I can reduce the sections some more, but the target weight of sub 400grams looks like it will be achievable now.










I need to do some finite element stress analysis to take it further, but hopefully those following this will understand the direction the engine is taking.

PS The tapered bores will be for another blog post.

I was going to remove the rear NU209 main bearing on the engine, but needed to make a small adaptor to measure the end float on the crankshaft before I do so. This particular engine has a little bit of movement in the rear crank pin, and I need to find out why it has moved.

The early M6 back plate is on the left and the later M8 is on the right.



The holes are bigger on the later one, which means more oil can be supplied to the rear bearing. After measuring these it equates to an improvement of 15% over the earlier design.



Brian’s engine will be increased further still, to match the front crankcase drilling. In the meantime I am waiting for another plate as Brians is damaged around the rear seal area, and it looks like something, a fastener possibly, has got between the flywheel and the rear plate!

Next up to bring Brian’s engine up to date with the oil supply modifications is restoring the earlier oil supply method, but this time bypass all the negatives.

The later engines didn’t have this option, as can be seen here.



The original engines had this oil feed visible above the camshaft bore at the very front of the crankcase, which is shown below. As the camshaft rotates this drilling is filled two times per camshaft revolution, and after this it passes underneath the bearing and jumps into the slingers. Some oil splashes back and passes through the main bearing, but the rest is accelerated by the slingers on the crank web, and passes through the big end bearing before exiting and splash lubricating the little end.



Unfortunately this hole is too small, and it led to a 50% reduction in flow rate over the rear, which is why I mod the front like so.



It might not be obviously apparent from the pictures, but the hole size is now 6mm diameter, whereas the one above is just 4mm. This new oil feed now matches the camshaft aperture, so that all openings, front and rear are timed equally at the rotating surfaces. The 4mm diameter hole of the earlier design will have to be enlarged to 5mm, as the rear oil feed is actually 5mm in diameter internally, but at first it is more important to get the hole in the right position, and work from this.



The internal oil feed pictured above now opened up to 5mm diameter, which is 50% bigger than the OEM drilling on the earlier engines. This new size matches that of the original rear bearing feed, which should equalise the flow rates. Next up, modifying the aluminium rear main bearing support casting to take a double lipped oil seal.