Modern Vespa

Hello again all,
Here I come again with some more novice questions.

So..... pistons today and in the image you will see two versions.

One on the left for px and one on the right for p125x. Both 3 port. the p125x model has a bigger gap between the crown and first ring. while the px does not.

Now. I keep getting told that p125x and px are essentially the same. So I am a bit confused which to take. Can anyone shed any light on the differences and what they would mean?

Also, would anyone know if the distance from the con rod pin to the crown differ between the two?

Thanks so much in advance
chief

PX125 piston seems to have L-profile ring and that is the reason for it being closer to the crown. No difference in function though.

I don't know about the other dimensions being different.

Possibly over the years we ground more off the barrels while porting, so needed to move rings up further

Get the right one, newer design and with better rings. The dykes ring (L type) is an old and outdated design…

Chief, the Dykes equipped PX125 has a compression height of 36.6mm and crown height 5.3mm. The rectangular ring PX150 has a compression height of 34.8mm and crown height of 7mm. Of course they are also different diameters for their respective bores.

I'm not sure why you are comparing the two pistons as they are not interchangeable unless you are thinking of oversizing your cylinder to 150cc?

While the Dykes ring shows superior sealing ability (gas pressure acts on two large surfaces horizontally and vertically), and because it is closer to the crown it offers better control over port opening they don't fare well at high rpm due to high rigidity, which is why they feel out of favour.

Aren't both the pistons in the photo 125 pistons?

Both Pistons are for 125cc

The GTR is 125 cc and the 3 port version is also known as "old px", which should be the same as for the p125x.

The px is the later version. I'm particularly interested in this question as both p125x and px125 are supposed to be pretty much the same. however, if that's the case what would be the impact of using one or the other piston?

I thought one may be taller, as I thought I may have had a lower compression with the px piston in my p125x.

Chief

The Dykes ring pistons were fitted in all PX125E models, until the last made. The ring is supposed to seal better and run cooler.
I have one in my 125

OMG, they are, I have no idea where I saw PX150 in this post, but I thought I did. Never mind.

Thanks Mr pull my finger anyway. Good you took time to respond.

So, then can we assume that I might have better compression with the dyke ring version?

Which leads me onto another question. I have one of these dyke numbers in the garage. The thing is, it's a bit scored.

Is this usable? or should I dump it? I can always put another ring on it.

Totally useable. Run some sanding paper over the scatches, re-ring and call it a day.

"A bit scored".... you haven't lived if you think that's knackered!
I don't think I've ever pulled out a piston that good!

Hole in piston?


Just weld it up

Or put a bolt in it!

Morning Chaps,
thanks for all the pics and responses. I didn't notice they came in until today.

very reassuring! I'm gonna re-ring the bugger and let her rip