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

I also found that rising the auxiliary exhaust ports to the same level (196°) as the main port(s) gave the best power, with gains in the whole usable rpm range, especially at peak. It works well on our engines (about 30 rwhp @ 100cc), but I guess on true high-performance, 125cc engines the story is different.

Then I went further, and have risen the auxiliaries by another 3° (I have exceptionally long exhaust aux ports). I gained exactly nothing, but what was interesting I lost also nothing. Perhaps the positive and negative effects of such arrangement cancelled out each other?

Suggestion. The flow in the exhaust is not restricted. However you may find that there is a restriction elsewhere. Find it and you will have some gains

hey guys i have conducted a test that somebody might be interested in. after hearing good things about the green putty i tried it in a gas engine and it did well. so i had to test it against methanol   

heres what i did. cured it 24hrs then put in methanol for 3 days. the result was it turned into a softer compound like rubber. i was going to go one step further and attach some to a small piece of aluminum and see if it dislodges but after seeing the results of the first test i decided there was no point in further testing.

the putty wouldnt exactly be submerged in methanol in the engine but i think the exposure would still cause similar affects to the results i had from my test. i wanted to have the worst case scenario results and thats why i submerged it for a long period.

i have some devcon F that i may do a methanol test with soon. ill report back when i have any results

RAW: Whenever I tried the main exhaust port higher than the 195-196° mark, I always had noticeable loss of power, peak power remained about the same, but in the lower rpm the losses were quite substantial. When I have rised the auxiliary exhausts to about 199°, I expected to see a loss of power, especially at lower rpm, but that did not happen.
Due to cylinder studs, my auxiliary ducts are about 8mm longer than they would be if they were "straight". So in my theory this produced a longer but of lesser-amplitude exhaust pulse.
So I roughly calculated that I need to open the aux about 3° before the main port in order so the pulses would converge where the aux duct meets th main exhaust duct.
I shall try next time with the auxiliaries about 196° and the main port lower and see what happens.

hi frits can you offer some advice ?  i have this cylinder (ktm) of 89mm diameter and with the original piston at tdc i made a red line marking the side skirt. you can see it sits very high and offers no chance to make the aux windows larger in the lower section above the A and B ports and makes a escape hole from crankcase to pipe also. i found a 89mm honda piston (blue line) that will fit in good and has lower skirt on the side which covers the aux window and provides more covered surface area to enlarge the aux windows above A and B. my question is the honda piston is a wiseco and was intended for a sleeved iron bore with clearance range of about .066-.076mm. do you see any reason why this honda piston wouldnt work in this new cylinder with the same clearance even though the cylinder is nikisil with no sleeving ?   thnx   


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brokedown a écrit:

...the honda piston is a wiseco and was intended for a sleeved iron bore with clearance range of about .066-.076mm. do you see any reason why this honda piston wouldnt work in this new cylinder with the same clearance even though the cylinder is nikisil with no sleeving ?

No I don't. But then I have next to no information; there's a lot more to be said about pistons apart from their clearance.

Looking forward to hear from you what could be said about pistons    

Thank you for sharing your knowledge with us   

The pistons will have different shapes, due to the different thermal expansions of the aluminium or iron liner, the pistons are not round, they are oval, or even a stranger shape, so they might no play well together.

On a side note, does anyone here knows where to get BC1-1442 SKF bearings(in Europe), or if some of you guys have a personal stash of them, or are they "common" items that can be ordered from any SKF dealer?

In the world of viel-takt, a piston for an iron bore has about twice the clearance of one for a nikasil bore of the same diameter to allow for the differential expansion. Look up specs for the Hayabusa or Gladius (interchangeable 81mm pistons for Nikasil, one forged, one cast) and for the SV650 (steel liner, cast, smaller diameter).

Hi Senso

They are a fairly common used as 125cc main bearings. Should be available from an SKF bearing distributor or via specialist kart/motorcycle suppliers.


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Best Regards

Ian 

Thanks for the fast response, my friend appreciates your help.

Now, if I'm not abusing, I have an odd engine at home, a rareish yamaha lcde 80 originated from a Spanish TZR 80 bike, it as a cast iron liner, that was bored to 49mm, and due to rush and not paying attention the piston and the 49mm bore are toast, is there any piston on earth with 50mm nominal diameter, with one(or two segments) with 0.8mm thickness(i can get 1mm rubbish), made to run on a 12mm small end pin?
I have been searching but there seems to be a gap between 48mm from the old yz and cr's and the now super used 54mm on the 125 engines.

There is the Kawasaki KM100 witch have a 49,5mm bore, but I don't remember the pin size. It should exist in reparation size in 50 and 50,5mm.

This little bike is very well-known in USA, I even received a used engine in one month (surface transport by US Postal) 50$ for the engine, 50$ for the shipment...

To my mind, the tzr80 engine is identical to dtlc2 80 engine.
Pistons are still available from Yamaha France , nominal size and two overbore sizes ( 49, 49,25 and 49,50)
Wiseco also sells pistons ( look pistons for yz80 1976-1980 - aircooled engine but same pistons)
and offers many oversizes, from 49mm to 51mm.
Dont try low cost pistons for sale on .bay and similar, there are made of bare aluminium and wear
Immediatly out of the box, plus rings gum

Thanks for the suggestions.
But isn't the yz80 a nicasil plated liner?
The tzr80 is a cast iron cylinder and liner, and the rings might not like to work in the steel liner.

There are barikit replacement pistons up to 50mm, but they look so crappy   

Barikit may look crappy but emot is verry happy with them.

It must be obviously a cast in iron liner, dumb me, if it was a nicasil plating they would not have sizes in increments of 0.5mm.

I know that barikit can make good looking pieces, and as a good service of welding and nicasil plating, but their standard pistons look like meteors that failed quality control, I know that barikit makes custom order pistons and they look much better.

Look what happened to the rings after 2-3l of gas:
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For now I dont have a photo of the piston, but its a thick lump of metal, very soft aluminium, and all the edges MUST be de-bured before install, not everyone has that kind of care and the result is a dead bore.

senso a écrit:

Thanks for the fast response, my friend appreciates your help.

Now, if I'm not abusing, I have an odd engine at home, a rareish yamaha lcde 80 originated from a Spanish TZR 80 bike, it as a cast iron liner, that was bored to 49mm, and due to rush and not paying attention the piston and the 49mm bore are toast, is there any piston on earth with 50mm nominal diameter, with one(or two segments) with 0.8mm thickness(i can get 1mm rubbish), made to run on a 12mm small end pin?
I have been searching but there seems to be a gap between 48mm from the old yz and cr's and the now super used 54mm on the 125 engines.

hi senso

in the 100 ccm kart engines there are 50 pistons in a cast iron liner.

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there are vertex racing pistons around 50.

KE/KH100 has a 14mm pin.

search these sites, you'll find lot's of possibilities (I did and was posting them, but made an error and sadly deleted everything, but from memory best possibilities are : Soes LT80 quad, RM80, CR80).

Mitaka, has lot's of details of piston's on this site :

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also dimensions of con-rod's etc (http://www.mitaka.co.uk/catalogue_index.htm).

also Wiseco has lot's : [Vous devez être inscrit et connecté pour voir ce lien] just go through all the catalog's

GOL doesn't have so much piston's , but they do have a nice search function : [Vous devez être inscrit et connecté pour voir ce lien]

Namura : [Vous devez être inscrit et connecté pour voir ce lien]

The namuras around here are know to break the rear skirt in half, the mitaka if we go by the internet are made of cheese, but I know at least one case of success with them, so I take all the internet opinions with a grain of salt.

The yz80 from 1976 to 1979 fits the bill and wiseco has them, for a sweet price of 100€, which is not so good, but at least they have them in 0.50mm increments from 48m to 51mm.

All in all, I'm amazed to all the responses you guys have, I went from no option to a lot of good candidates   

as KR1 piston's are no longer available, most use the Mitaka's and on the KR-forum in the UK no one has complained about them.
still : there site is the best with the most dimensions given, so can look there and then order somewhere else   

senso a écrit:

Thanks for the suggestions.
But isn't the yz80 a nicasil plated liner?
 

the yz80 i seen was iron liner and im sure it was original. kx80 has 50mm piston. but the problem i think is all the cr80,rm80,yz80,kx80 all have 14mm pin

i have cylinders with iron liner and nikisil. the cold piston clearance is very similar of both but i dont know the true clearance when everything is hot. still i think my idea to use a piston intended for iron liner will work fine in a nikisil bore. i talked to wiseco and they said both pistons have the same ingredients so i dont think there will be a problem

Senso
I don't doubt what you say. But barikit seems to have a decent reputation and i wouldn't now why they sell something that ruins the reputation.
Must be marketing of some sort.

S

Hi Everyone,

Fascinating reading as always   

I am still working on my yamaha tzr 250 3xv SP. I am going to use TZ250 4DP pistons as they are single ring and the ring peg design is better than the TZR pistons (I had a ring peg failure a few months ago but that's another story). The pistons are flat top with a shallow ~12 degree chamfer around the edge of the piston around 3-4mm wide. This piston crown profile is the same as the TZR pistons hence another reason for using the TZ pistons as they are an easy fit. I'm guessing the 12 degree chamfer is to help the incoming charge adhere to the piston crown, sort of a halfway house between a dome piston and a fully flat top?

I am busy setting up the squish and I have seen that the 12 degree chamfer in the cylinder head is wider than on the piston. This means that currently the squish band is tighter at the edge of the piston (on the chamfered part) than the flat part. The bore is 56mm and the stroke is 50.7mm. Fitted combustion chamber volume is ~8.7cc and I am running the bike on 98 octane unleaded with no detonation issues whatsoever. The diagram below depicts the situation:

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In my head I am thinking that I should either machine the piston or the head so that the two profiles match. In order to get a constant parallel squish from cylinder wall to the end of the squish band. At the moment I am able to get 0.6mm squish clearance on the chamfered part, but this leaves 0.8mm squish clearance on the flat part. Did yamaha do this for a reason? I believe the 4dp TZ head has the same situation but this is only by reading on the internet, I haven't measured a head myself.

Would I be best off matching the piston and head profiles in order to get a uniform squish clearance?

Also, the edge of the piston crown has a small (~0.5mm), steep chamfer running round the edge. It makes it tricky to define port timings as I can't decide whether to measure at the extremities of this small chamfer or to ignore it and use 12 degree chamfered part that is closest to the cylinder wall? Any help/guidance would be appreciated.

Regards,

Dan McBryde

maccas a écrit:

...The pistons are flat top with a shallow ~12 degree chamfer around the edge of the piston around 3-4mm wide.... I'm guessing the 12 degree chamfer is to help the incoming charge adhere to the piston crown, sort of a halfway house between a dome piston and a fully flat top?

The Aprilia RSA piston has an edge angle of 8,17° (due to a 190 mm dome radius). So I think your 12° edge angle is better than halfway.

Citation :

the 12 degree chamfer in the cylinder head is wider than on the piston. This means that currently the squish band is tighter at the edge of the piston (on the chamfered part) than the flat part.

A tighter squish gap at the edge is not much of a problem (it would be if it were the other way round).
But I'm afraid your continuing head squish area will, through the Coanda effect, tend to pull the flow away from the piston crown, which is bad for piston cooling.

Citation :

Did yamaha do this for a reason?

Martin Ziegler was the first to modify the completely flat Yamaha racing pistons of that era on behalf of the Robers Grand Prix team, with great results. Yamaha just followed suit.

Citation :

Would I be best off matching the piston and head profiles in order to get a uniform squish clearance?

I think so, provided you do not need to reduce the 12° angle too much.

Citation :

the edge of the piston crown has a small (~0.5mm), steep chamfer running round the edge. It makes it tricky to define port timings as I can't decide whether to measure at the extremities of this small chamfer or to ignore it and use 12 degree chamfered part that is closest to the cylinder wall?

I know the problem, Dan. I use a 1 mm gauge, bent at 45°, that I pinch between the piston edge and the port edge in such a way that the bent end is vertical (parallel to the cylinder bore). Then I measure from cylinder top deck to piston edge and subtract 1,41 mm from the measured value. It works for every port as long as its axial angle is not steeper than 45°, even when both the piston edge and the port edge are chamfered or radiused.
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Hi guys,
Very good idea Frits,the 45° bended jauge.

If i want reducing the volume of the lateral transferts Frits, any good advise or solution?
Thank you