[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4)

Ollies930 a écrit:

Jan and Frits,
have you ever tried a piston that has a radiused edge rather than the reasonably sharp edge usually used? There might be some benefit in breathing of partially closed ports. Additionally it might help create more cooling of the piston crown above the ring lands. Of course the rings might have to be lowered somewhat depending on the radius.

We tried some different radiuses, on the flowbench.
The bigger the radius the better the flow!
But there was no HP gain, only loss.
Of course the cilinder head was modified to maintain the same compression ratio!

hi frits i have new questions. if you remember many pages back i talked of that ktm where the piston side skirting was cut very high and as a consequence uncovered the aux exh ports at tdc. and not only that but it prevented the aux window from being made larger in the lower portion because that would just increase the size of the shortcircuit hole.

why it was intentionaly designed this way is a mystery to me but i think i have a cure for the problem. i found a piston from a different engine that should work like a charm. the side skirt is alittle longer so it covers the full window at tdc and allows the window to be made bigger in the lower portion. but theres one problem. its 1.25mm smaller in diameter than the original piston. this is your typical mx cylinder with nikisil plated and no sleeve. i called a plating company today and they said .38mm thick is the very most they could apply the nikisil. so im trying to think of a different way. do you think having a alluminum sleeve made and plated would be a good idea ? im sure i could have a iron sleeve made just as easy. whats the best way do you think ? thnx again

For what it's worth, I enquired about removing iron sleeves from a 4-stroke and replacing them with plated alu. It is a known and successful technique. There might be issues on a 2-stroke because of the ports reducing stiffness when pressing in the sleeve...

GrahamB a écrit:

For what it's worth, I enquired about removing iron sleeves from a 4-stroke and replacing them with plated alu. It is a known and successful technique. There might be issues on a 2-stroke because of the ports reducing stiffness when pressing in the sleeve...

The main problem will be to ensure, that no nicasil can get between cylinder and sleeve.
Why don´t you use a custommade piston?Last time I asked [Vous devez être inscrit et connecté pour voir ce lien] the price was about 220€ for a custommade one.
And I am very sure that wont be paying more than for resleeving and recoating your cylinder.

Tim Ey a écrit:

Why don´t you use a custommade piston?... that wont be paying more than for resleeving and recoating your cylinder.

Even it a custommade piston was more expensive, I would advise against fitting a sleeve.

For what its worth, a friend of mine has a stock ktm 380 engine, with a new con-rod, all correctly installed, cylinder replated because it was getting oval and the two Wossner pistons that he bought(for all most 250€ each) never did more than 5L, they have a skinny skirt and no ribbing inside, the result was the same with both, oval pistons and a seize, has he was breaking in the engine it caused no damage to the new plating, but none the less, it was a pretty expensive experience, Wossner always called user error.

Senne s a écrit:

Where the aprilia exhaustport bridges relieved?
Is it a necessity with auxiliary exhaust port bridges?
This is what I mean:
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Hi Senne S

In a centre bridged exhaust port the problem is that the bridge (unlike the surrounding cylinder) is poorly cooled and will expand more than the surrounding material. As a result if the bridge is finished without any relief the bridge will begin to protrude enough into the bore to heavily mark the piston and cause the top ring to become trapped, with consequent damage.

two things are done to alleviate this problem:

1. The centre bridge is relieved with what approximates to a 3 mtr. radius.
2. The piston ring groove is relieved with a small 45 degree chamfer on the front part only.

Once the bridge is relieved as the piston approaches the top of the port on the upstroke it relies on the rings being "fed into position" by the top edges of the port. For this reason the top of the port edges should have a gentle slope downwards from the centre outwards.

As Jan says, this is not necessary on the bridges between the main and sub exhaust ports in an "eyebrow" type port arrangement.

Senso, we had the same problem you describe with Wossner pistons in one of our CR250 kart motors. First and last time that I used them!!

Best Regards

Ian 

Frits Overmars a écrit:

Even it a custommade piston was more expensive, I would advise against fitting a sleeve.

I deliberately avoided mentioning a certain piston manufacturer's name....

I to had problems with woessner's.

fitted new ones in my KR1, and first real ride out one cylinder stopped working : part of the ring was gone, and a bridge under the C-port was gone (strange bridge only found in KR1 cylinders, KR1S didn't have it anymore).
Only after ordering a new one, I found the problem : on the 3 Woessner's, the pin holding the rings is "about where it should be", but ot exactly, it was different on all 3 of them.
This meant one end of the ring could just entere the C-port and that did the damage.

The belgian importer send them all 3 back, together with an original piston and cylinder of mine with an explanation of the problem, and they just dismissed any responibility , allthough the evidince was handed them on a plate. Never again   
Luckily the importer is also the Wiseco importer, and they stepped up and said I could chose any piston I'd like to replace them.

Ian Harrison a écrit:

2. The piston ring groove is relieved with a small 45 degree chamfer on the front part only.

you meen like this angle, the width of the exh window roof ?  and if so then how will it affect the window opening as the piston uncovers it ?


[Vous devez être inscrit et connecté pour voir cette image]

happy birthday Mister   

Hi Brokedown

No, the top and bottom edges of the ring groove are chamfered, not the top of the piston as you have drawn.

Just a short distance across the exhaust port centre bridge

Happy Birthday to Jan Thiel.

Best Regards

Ian  

Happy birthday to you great legende, and long life at you.Zeze

happy birthday jan. i dont know you personally but thnx for all the info you shared around here. its helped me tremendously   

Happy Birthday Jan :)

Happy Birthday Jan, and many thanks for passing on your knowledge and experience.

happy birhday mr Thiel, i hope your dessert was good!!! 

gelukkige verjaardag !

Mr Thiel, Overmars
we can see (part 1), on the drawing for the RSA, the output off the transfert A, is a divergent,while the output off the B is slightly convergent........   WHY THIS DIFFERENCE ?????

Thanks a lot Jan and Ian for the answers, I really appreciate your help and your knowledge!

Happy Birthday Jan and thank you for sharing your knowledge over these years. 
I know that all the readers of this forum value this very much - good health    

Can you tell us when Frits' birthday is as he has hidden this well from the forums well over the years?


best regards
Allan.

Yes, happy birthday Mr. Thiel and much more years in good health.


Do someone know, how much temperature the pistonring (125cc) can reach in full load, exhaust temp. is 630 °C ?

Thank you

Jan would you clear up the google translation please. Mainly the part about the 250

Quote from: gash on June 6, 2014, 21:06:25
Indeed, a good plan .

here is a picture of the carburetor of Aprilia RSA125 (Dell'orto VHTC42) nicely made, but in the beginning I had a problem with fuel level understood.

Yes Jaap. "What problems formations" that you can say!
Each Dell'Orto is problematic formations regarding the fuel supply!
There simply is not enough fuel in the float chamber, the bore is too small!
In RSW float needle Keihin was used.
That was a 'legal' standing carburetor just to good, but only in a 125cc.
At 250 a gas was used, otherwise it went wrong.
I think there are a lot of riders 250 cases in fast corners by 'enigmatic' lockups.
So too poor to low float level.
The RSA carburetor was slanted, and that exacerbated the problem really.
After 2-3 years of problems here also showed the gas station, the only good solution.
I thought it was an incredibly stupid carburetor.
The throttle was really the only nice!
He also took over 1000 Euro .....

The translation is as bit wrong.
'gas station' should be fuel pump!
With a fuel pump the problem of too little fuel flow was solved.
First on the 250, and later on the RSA.
Nearly all Dell'Orto carburetors have this problem!

Indeed the throttle slide cost more than 1000 Euro.
With such a narrow slide they could have made a far better carburator.
With interchangeable front and rear parts.
So that a lot of experiments could have been done.
I think this was a very stupid thing to do!