Yamaha OEM pistons

[earywig]

The JE pistons look "meatier" than stock, or the stock pistons look more "high performance". Maybe that's why the cylinders wear out?

Actually, you're closer than you think, the following may be of interest.

The stock piston is a diecast slipper type, specific to the TDM/TRX. As Yamaha would've needed to purchase thousands of these, it makes sense to diecast them as the unit cost of production would be cheaper, even when you factor in the cost of a diecasting tool.

The JE piston has been manufactured from a standard 'jampot' style forging hence the meatier look; the forging is not specific to the TRX, but a large number of other applications as well. The cost for a fully formed forge tool is around £4000,(at least it was when I last requested a quotation), not very economic for the likely aftermarket quantity involved.

Note how the sidepanels on the JE piston, (the cutouts at each end of the gudgeon pin axis) have been machined. This is purely done to remove excess material and reduce the weight of the forging to enable the use of a short, lightweight pin. You'll also notice that the inside of the skirt has been machined as well, down to the level of the ring belt. The function of this is to further reduce the mass of the piston.

The width of the skirt has no bearing (no pun intended) on the rocking of the piston in the bore, that's why the stock piston has a narrow skirt - designed to reduce friction losses.

It's a combination of the skirt length/bore diameter/top land diameters which control the amount of piston rock, which happens radially about the gudgeon pin axis, not axially along it, (unless the gudgeon pin/conrod small-end fit is like a pr**k in a bucket). Note also that the diameter of the piston along the gudgeon pin axis is smaller than the diameter perpendicular to it, in other words it's oval at room temperature. When the piston is at running temperature, (about 250°C) it changes shape to be fully circular. I programme a CNC piston turning machine at work which can apply a total of 255 different ovals along the profile of the skirt. This is because the temperature gradient from the crown of the piston changes over the piston length.

Finally, the tapered bore gudgeon pin is used as an additional attempt at reducing reciprocating mass.

Hope the info helps.

[steve speed]

that was a very good bit of info earywig, thanks

[HansJ]

Someone who knows something, instead of guessing like me!

Would You mind elaborating a bit? Why is it that the lesser piston area, if it reduces friction, doesn't mean that the wear will be greater? Smaller swept area sounds like higher pressure per area? And this leads to excessive oil consumption? And when it starts to wear down the cylinder bores, the piston will be more prone to rock, speeding up the wear process?

And re the pins, that surprised me, if I recall it correctly, that the JE pins were not tapered in order to save weight. But there may be other reasons for not tapering them I guess (here I go again...)

Edit: OK, I compared the pics and now I get it, You mean the pistons for sale have been modified! But I still don't get it re the cylinder wear/rocking?

[earywig]

Ok, here goes, but please bear in mind that the opinions raised stem from my experience with racing engines.

Why is it that the lesser piston area, if it reduces friction, doesn't mean that the wear will be greater?

I didn't say that the wear wouldn't be greater - but vehicle engine dynamics is outside of my field of expertise. I should explain that most of what I have learned about pistons is in part self taught, but the finer points of piston design were imparted by a very talented design engineer colleague I worked with before he left to go to Mercedes Performance Racing Engines.

Smaller swept area sounds like higher pressure per area and this leads to excessive oil consumption?

Actually, the link to excessive oil consumption has more to do with blow-by past the rings. But this is a double edged sword. We found when developing the V10 Cosworth TJ engine that ring groove topology had a big influence with regard to consumption, coupled with ring radial compression, (i.e. the amount of ring pressure on the cylinder wall). It was a fine balancing act to achieve. Good oil consumption = high radial ring pressure = good blow-by, but at the expense of power, due to aforementioned friction losses.

Excessive oil consumption = low radial ring pressure = excessive blow-by, but more power due to lesser friction losses.

This probably has nothing to do with the TRX, but as I say this is my experience with high revving engines. We had the same problem with the BSB Virgin Yamaha pistons.

And when it starts to wear down the cylinder bores, the piston will be more prone to rock, speeding up the wear process?

Wear is a function over time and duty cycle. It's also tthe case that piston rocking also has an effect on the ring orientation in the bore; but it's the piston that wears faster than the bore. The top lands have a big influence on keeping the piston straight. If you're able to look at a used piston, look on the top land for a polished area. That's because the land is in quite hard contact with the bore, i.e. doing it's job - if there's little or no wear then the clearance is too great, exacerbating the rocking problem. It also increases what's known as crevice volume, which reduces the compression ratio and creates the possibility of detonation.

For your info, which you probably knew already, the skirt profile is barrel/straight/barrel shaped along its length, which helps to constrain the piston orientation in the bore.

And re the pins, that surprised me, if I recall it correctly, that the JE pins were not tapered in order to save weight.

That sounds correct. The shape of the bore in the gudgeon pin, as well as the length, OD and ID diameter have a large influence on the stress in the piston gudgeon-pin bore. You should've seen the FEA results we got on some designs

Phew, I'm putting the kettle on.

Edit: In my humble opinion, I think Yamaha were aware of the piston rocking scenario - which is probably why they used a barrel shaped top compression ring.

[Quan-Time]

WOW ! cheers for the info. that was fantastic !

[wicky]

brilliant stuff Earywig, now i knew ya were more than a F1 fan judging by yer ickle ditty at the borrom of each post.

Cosworth - Last of the independents - Killed by the FIA.