[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 3) (Locked)

Jan and Frits,seeing these larger cylinders I am curious what you feel to be the practical limit on bore size given a proper ignition,stroke,cooling system,pipe and other factors.There are few limits on singles in many classes of club racing here in the US and it be nice to able to use your wisdom and experience to take advantage of this.

Frits Overmars a écrit:

If you start changing con rods anyway, put in a proper length, like 2.2 * stroke, or at least 2* stroke.

yes i agree 100%. i do have alittle experience changing to longer rods because i tried it one time before. both rods used the same diameter wristpin (20mm) so there was no issues there. but the longer rod had 2mm bigger lower pin. so i decided to have the crank wheel hole enlarged rather than use a stepped pin. so far the crankshaft has worked well with no problem even though people told me enlarging the crankwheel hole is very difficult   

original

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comparison

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longer rod installed

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but regarding this new engine im talking about. it uses 125mm rod (24mm lower pin, 18mm wristpin). i think i seen a 135mm available that will fit right in because i believe it has the same wristpin and lower pin. but obviously that will only put the rod ratio at 1.875.  

so if i go with the longer rod of 140mm or 144mm, the 140 also has a 24mm lower pin but 20mm wristpin. the 144 has 26mm lower pin and 20mm wristpin. i dont see enlarging the crank wheel hole as a big issue because i have experienced that before. but what about the problem of changing 18mm wristpin to 20mm ? i guess the easiest solution would be find a larger bearing or have a special bearing made if thats possible. other wise the only solution i see is enlarging the hole in the piston, if it wont weaken too much the structural integrity of the piston. frits do you have some advice with this type of situation, either finding or making a larger bearing, or enlarging the piston hole for a bigger wristpin ?

brokedown a écrit:

this says 6-12% MORE power !  but i think just more snake oil   

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From your other posts, am I right in thinking you are perhaps looking at a 250 single?
If so I wouldn't put that thing anywhere near it!!   
When that groundstrap "cage" falls off due to vibration and detonation, you will be looking at a cylinder repair if your lucky. Same applies to a plug with a heavy single groundstrap like the NGK Iridium BR10EIX
You want a proper race plug with a fine wire side-electrode.
As for 6-12 Hp increase . . . . . not even in your widest dreams.

When using a long rod try and find a piston with a low timing edge height. This also helps with stability, especially in a bridge ported engine (which I think yours is not??), but anyway . . . .

Best I can find for you is a Maico 490 rod at stroke*1.902 It's 137mmL/22mm little end eye/25mm x 64mm big-end pin (so you'd have to do the crank bore) and either 17 or 20mm wide depending on year (the later Katooms, like the CR250 are 17mm. A 1996-99 KTM 250 piston will slot straight on and has a 17.7mm timing edge height as against the 29mm that I'm guessing you got now. So your running at 125+29 = 154 and the suggested combo would be 137+17.7 = 154.7, so you'd actually only need an extra 0.7mm of base gasket/spacer plate. You'd just have to check it would all work with regard ring peg position, piston cut-outs, etc. They are components that I never used.

Apologies . . . . not 125 and not Aprilia GP, but I just got on-a-roll!

best Regards

Ian  

I guess I could ask this here... what has proved to be faster on the track? the 125cc 2 strokes... or the 250cc 4 strokes in the moto3 class?

Ian Harrison a écrit:

From your other posts, am I right in thinking you are perhaps looking at a 250 single?
If so I wouldn't put that thing anywhere near it!!   
When that groundstrap "cage" falls off due to vibration and detonation, you will be looking at a cylinder repair if your lucky. Same applies to a plug with a heavy single groundstrap like the NGK Iridium BR10EIX
You want a proper race plug with a fine wire side-electrode.
As for 6-12 Hp increase . . . . . not even in your widest dreams.

When using a long rod try and find a piston with a low timing edge height. This also helps with stability, especially in a bridge ported engine (which I think yours is not??), but anyway . . . .

Best I can find for you is a Maico 490 rod at stroke*1.902 It's 137mmL/22mm little end eye/25mm x 64mm big-end pin (so you'd have to do the crank bore) and either 17 or 20mm wide depending on year (the later Katooms, like the CR250 are 17mm. A 1996-99 KTM 250 piston will slot straight on and has a 17.7mm timing edge height as against the 29mm that I'm guessing you got now. So your running at 125+29 = 154 and the suggested combo would be 137+17.7 = 154.7, so you'd actually only need an extra 0.7mm of base gasket/spacer plate. You'd just have to check it would all work with regard ring peg position, piston cut-outs, etc. They are components that I never used.

Apologies . . . . not 125 and not Aprilia GP, but I just got on-a-roll!

best Regards

Ian  

hi ian. its 293cc. 72x72. and no i wouldnt use one of them snake oil spark plugs. it was posted only for a laugh   

like i said earlier im pretty sure theres a 135mm rod (original is 125mm) from another ktm that will fit directly in my engine with no modification, so using the maico rod of 137mm and boring the crank holes makes little sense for only +2mm extra length. but like frits said, if im going to bother changing rods then why not try and get the ratio to atleast 2*, which is a great idea.

but the only obstacle is the upper wristpin size. original is 18mm. 140mm rod is 20mm (old ktm rod), 144mm rod is 20mm also (honda rod).  so i need to research if theres a larger bearing that would allow the smaller wristpin (18mm) to be used in them longer rods. i dont think a bearing manufacturer would be keen to make one special bearing for me but perhaps im wrong ? only other option is boring the piston hole bigger but i never heard of anyone doing that and really im not so sure its a good idea but im not a engineer so i dont know.   actually ill do some more research, maybe theres a 140mm rod (or close to it) that uses 18mm upper pin.

ya its not aprilia related ( i wish there was a forum we could ask frits general 2t questions so not to slow this forum down with stuff not related to aprilia) but these questions are 2stroke related and i hope no one minds that we discuss 2strokes. i would like to have a rsa125 engine but i dont think any will show up in my local newspaper for sale any time soon   .  frits's help has been priceless and if there was a way to repay him i certainly would   . and also the help jan and other members has provided is worth 1 millions dollars   

williamsmotowerx a écrit:

I guess I could ask this here... what has proved to be faster on the track? the 125cc 2 strokes... or the 250cc 4 strokes in the moto3 class?

This year the 250 4strokes are slightly faster, and have about the same HP as a 125 2stroke...
And now they also have better tyres.

Jan Thiel a écrit:

This year the 250 4strokes are slightly faster, and have about the same HP as a 125 2stroke...

And much better tires than the two-strokes had back then....

Hello

I know it was already said but I can not find it.

which sheet thickness has the titanium exhaust from the RSA?

thanks Manuel

Manuel Rainer a écrit:

which sheet thickness has the titanium exhaust from the RSA?

0,6 mm. You can find it on the picture below: ".024" is the thickness in inches.
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brokedown a écrit:

Ian Harrison a écrit:

From your other posts, am I right in thinking you are perhaps looking at a 250 single?
If so I wouldn't put that thing anywhere near it!!   
When that groundstrap "cage" falls off due to vibration and detonation, you will be looking at a cylinder repair if your lucky. Same applies to a plug with a heavy single groundstrap like the NGK Iridium BR10EIX
You want a proper race plug with a fine wire side-electrode.
As for 6-12 Hp increase . . . . . not even in your widest dreams.

When using a long rod try and find a piston with a low timing edge height. This also helps with stability, especially in a bridge ported engine (which I think yours is not??), but anyway . . . .

Best I can find for you is a Maico 490 rod at stroke*1.902 It's 137mmL/22mm little end eye/25mm x 64mm big-end pin (so you'd have to do the crank bore) and either 17 or 20mm wide depending on year (the later Katooms, like the CR250 are 17mm. A 1996-99 KTM 250 piston will slot straight on and has a 17.7mm timing edge height as against the 29mm that I'm guessing you got now. So your running at 125+29 = 154 and the suggested combo would be 137+17.7 = 154.7, so you'd actually only need an extra 0.7mm of base gasket/spacer plate. You'd just have to check it would all work with regard ring peg position, piston cut-outs, etc. They are components that I never used.

Apologies . . . . not 125 and not Aprilia GP, but I just got on-a-roll!

best Regards

Ian  

hi ian. its 293cc. 72x72. and no i wouldnt use one of them snake oil spark plugs. it was posted only for a laugh   

like i said earlier im pretty sure theres a 135mm rod (original is 125mm) from another ktm that will fit directly in my engine with no modification, so using the maico rod of 137mm and boring the crank holes makes little sense for only +2mm extra length. but like frits said, if im going to bother changing rods then why not try and get the ratio to atleast 2*, which is a great idea.

but the only obstacle is the upper wristpin size. original is 18mm. 140mm rod is 20mm (old ktm rod), 144mm rod is 20mm also (honda rod).  so i need to research if theres a larger bearing that would allow the smaller wristpin (18mm) to be used in them longer rods. i dont think a bearing manufacturer would be keen to make one special bearing for me but perhaps im wrong ? only other option is boring the piston hole bigger but i never heard of anyone doing that and really im not so sure its a good idea but im not a engineer so i dont know.   actually ill do some more research, maybe theres a 140mm rod (or close to it) that uses 18mm upper pin.

ya its not aprilia related ( i wish there was a forum we could ask frits general 2t questions so not to slow this forum down with stuff not related to aprilia) but these questions are 2stroke related and i hope no one minds that we discuss 2strokes. i would like to have a rsa125 engine but i dont think any will show up in my local newspaper for sale any time soon   .  frits's help has been priceless and if there was a way to repay him i certainly would   . and also the help jan and other members has provided is worth 1 millions dollars   

Brokedown. The rsa shows up on fatbaq from time to time

brokedown a écrit:

Ian Harrison a écrit:

From your other posts, am I right in thinking you are perhaps looking at a 250 single?
If so I wouldn't put that thing anywhere near it!!   
When that groundstrap "cage" falls off due to vibration and detonation, you will be looking at a cylinder repair if your lucky. Same applies to a plug with a heavy single groundstrap like the NGK Iridium BR10EIX
You want a proper race plug with a fine wire side-electrode.
As for 6-12 Hp increase . . . . . not even in your widest dreams.

When using a long rod try and find a piston with a low timing edge height. This also helps with stability, especially in a bridge ported engine (which I think yours is not??), but anyway . . . .

Best I can find for you is a Maico 490 rod at stroke*1.902 It's 137mmL/22mm little end eye/25mm x 64mm big-end pin (so you'd have to do the crank bore) and either 17 or 20mm wide depending on year (the later Katooms, like the CR250 are 17mm. A 1996-99 KTM 250 piston will slot straight on and has a 17.7mm timing edge height as against the 29mm that I'm guessing you got now. So your running at 125+29 = 154 and the suggested combo would be 137+17.7 = 154.7, so you'd actually only need an extra 0.7mm of base gasket/spacer plate. You'd just have to check it would all work with regard ring peg position, piston cut-outs, etc. They are components that I never used.

Apologies . . . . not 125 and not Aprilia GP, but I just got on-a-roll!

best Regards

Ian  

hi ian. its 293cc. 72x72. and no i wouldnt use one of them snake oil spark plugs. it was posted only for a laugh   

like i said earlier im pretty sure theres a 135mm rod (original is 125mm) from another ktm that will fit directly in my engine with no modification, so using the maico rod of 137mm and boring the crank holes makes little sense for only +2mm extra length. but like frits said, if im going to bother changing rods then why not try and get the ratio to atleast 2*, which is a great idea.

but the only obstacle is the upper wristpin size. original is 18mm. 140mm rod is 20mm (old ktm rod), 144mm rod is 20mm also (honda rod).  so i need to research if theres a larger bearing that would allow the smaller wristpin (18mm) to be used in them longer rods. i dont think a bearing manufacturer would be keen to make one special bearing for me but perhaps im wrong ? only other option is boring the piston hole bigger but i never heard of anyone doing that and really im not so sure its a good idea but im not a engineer so i dont know.   actually ill do some more research, maybe theres a 140mm rod (or close to it) that uses 18mm upper pin.

ya its not aprilia related ( i wish there was a forum we could ask frits general 2t questions so not to slow this forum down with stuff not related to aprilia) but these questions are 2stroke related and i hope no one minds that we discuss 2strokes. i would like to have a rsa125 engine but i dont think any will show up in my local newspaper for sale any time soon   .  frits's help has been priceless and if there was a way to repay him i certainly would   . and also the help jan and other members has provided is worth 1 millions dollars   

Skidoo MXZ600 is 72x72 and has a 20mm wristpin

CRECY a écrit:

Skidoo MXZ600 is 72x72 and has a 20mm wristpin

excellent work crecy. '03-'10 appears to be 72mm with 20mm pin.  ill look into it.    

hey frits let me ask something. regarding the wristpin location in the piston, whether its high/middle/low,  does the height location have much importance and if so where is the prefered hieght ?

and i remember jan mentioning the better crown cooling with a domed piston.  aside from the cooling affect, will a flat top work as good as the domed or is there even more drawbacks to a flat top ?  just wondering,  in the event flattop is my only option.  thnx

brokedown a écrit:

...regarding the wristpin location in the piston, whether its high/middle/low,  does the height location have much importance and if so where is the prefered hieght ?

The bulk of the piston mass is above the pin. Therefore I prefer a high pin because it will make for a lighter piston. It will also allow for a longer con rod.

Citation :

i remember jan mentioning the better crown cooling with a domed piston.  aside from the cooling affect, will a flat top work as good as the domed or is there even more drawbacks to a flat top?

There is no "aside from the cooling effect". A hot piston crown will cause charge dilution and detonation, so piston cooling is paramount.

Frits Overmars a écrit:

brokedown a écrit:

...regarding the wristpin location in the piston, whether its high/middle/low,  does the height location have much importance and if so where is the prefered hieght ?

The bulk of the piston mass is above the pin. Therefore I prefer a high pin because it will make for a lighter piston. It will also allow for a longer con rod.

Citation :

i remember jan mentioning the better crown cooling with a domed piston.  aside from the cooling affect, will a flat top work as good as the domed or is there even more drawbacks to a flat top?

There is no "aside from the cooling effect". A hot piston crown will cause charge dilution and detonation, so piston cooling is paramount.

Frits perhaps if you have another spare moment you would educate us on how & why the mass Vs pin position can be beneficial and or detrimental to the end game

hi

which height has the piston of the RSA from mid of piston pin to the upper piston edge?

in my TM KV 95 (125 kart engine 54x54, 4) I have a piston with 29mm height and 110mm long rod. the precompression is 1-1.3. I would like to use a 120mm rod and a 10mm spacer. i think the precompression would bring me a bit.

or is it better in this case when I'm looking for a piston, where the distance between pin and crow is 10mm lower. because of less weight, but then i have the same pre-compression.

Frits is there a obtimale proportion between rod length and distance between piston pin and crow?


Thanks Manuel

RAW a écrit:

Frits perhaps if you have another spare moment.....

Not in 2013; I'm involved in a project that is taking up all my time.

Manuel Rainer a écrit:

which height has the piston of the RSA from mid of piston pin to the upper piston edge?

I'm not at home so I can't get to my archive and I can't anwer you question, Manuel.

Citation :

in my TM KV 95 (125 kart engine 54x54, 4) I have a piston with 29mm height and 110mm long rod. the precompression is 1-1.3. I would like to use a 120mm rod and a 10mm spacer. i think the precompression would bring me a bit. or is it better in this case when I'm looking for a piston, where the distance between pin and crow is 10mm lower. because of less weight, but then i have the same pre-compression.

I would certainly choose the long rod. And then you can try the spacer and the new piston, or the old'piston without the spacer.

Citation :

Frits is there a obtimale proportion between rod length and distance between piston pin and crow?

I dont think so. Just get a long rod and a piston with the pin as high up as you can find.

Frits Overmars a écrit:

RAW a écrit:

Frits perhaps if you have another spare moment.....  

Not in 2013; I'm involved in a project that is taking up all my time.

I hope it is aalt or fos.
Its been a long time sinds

hi frits i havent seen much about your 24/7 idea in a while. but theres something i thought of the other day and maybe somebody else has already thought of it. but regarding the method that opens the flap, what about a small electric magnet ? perhaps even electricly charged at a certain rpm from the ignition ? but this is really a topic out of my league but figured i would mention it anyways. seems like you said the flap wouldnt need to open suddenly so maybe the magnet isnt a good idea because maybe it would open more abruptly. or maybe theres already a far better solution somebody found   

brokedown a écrit:

hi frits i havent seen much about your 24/7 idea in a while. but theres something i thought of the other day and maybe somebody else has already thought of it. but regarding the method that opens the flap, what about a small electric magnet ?...  seems like you said the flap wouldnt need to open suddenly so maybe the magnet isnt a good idea...

I said the flap wouldn't need to open gradually; suddenly is fine. Magnets are no good when they need to cover a certain distance and need to be small. But a common servo motor, like the kind that moves an exhaust power valve, will do the job. A single power valve motor can even perform both jobs simultaneously, as the inlet flap needs to move at about the same rpm as an exhaust power valve.

ok sorry i didnt recall exact what you said about the flap. nearly 150pages got my brain steaming   . i figured the magnet might be a crazy idea

hey frits im sure alot of us would like to make a 24/7 inlet valve for our own engines but we might not have the time or tools at the moment so we are forced to use just the normal reed valve. if you was in our shoes would you use just the standard reed cage design or a design like what the v-force uses ? and if the engine operates at 10k rpm do you have a suggestion what a good reed thickness would be ? thnx

brokedown a écrit:

hey frits im sure alot of us would like to make a 24/7 inlet valve for our own engines but we might not have the time or tools at the moment so we are forced to use just the normal reed valve. if you was in our shoes would you use just the standard reed cage design or a design like what the v-force uses ? and if the engine operates at 10k rpm do you have a suggestion what a good reed thickness would be ? thnx

Broker, what motor are you running, and in what class are you riding ( driving ) ?

Have you done any dyno testing ?

If yes, what were the results ?

brokedown a écrit:

...would you use just the standard reed cage design or a design like what the v-force uses ?

I have insufficient experience to answer that.

Citation :

if the engine operates at 10k rpm do you have a suggestion what a good reed thickness would be ?

That depends on a lot more than just the rpm. Fibre material (steel, glass, carbon), fibre weaving pattern and orientation, reed lenght; unsupported reed width, cage angle, type of limiter, crankcase volume, etc. You'll just have to test it.
And remember: the softer the reeds, the leaner the air-fuel mixture will become, so with every reed change you will have to reset the jetting.

[/quote]
And remember: the softer the reeds, the leaner the air-fuel mixture will become, so with every reed change you will have to reset the jetting.[/quote]

Frits , could you explain why softer reeds are leaning the air-fuel mixture ? Thanks.