Aprilia ETX 125 (Don't expect to go motocrossing...)
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@SpookDog Ey?
Oh yeah I guess I haven't really shared this. HUNDREDS of pounds. I'll get some photos up in a second.
@markus-w No this is a stroked rod, 5mm longer than the stock rod.
@mhbikesnbits I actually have several RS 125 rear master cylinders, but I just got back in from a ride out now and the rear master cylinder doesn't feel like it's working so...might just buy a new genuine one.
On my DTR I run the stock Nissin master cylinder.
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@markus-w No, it's stroking the engine by a whopping 5mm. (that's sarcasm).
So the problem with big bore kits is that two strokes prefer "Square Engines". What this means is that, in an ideal world, the bore matches the stroke.
The Polini 160 kit has a notorious drop in over rev, stroking the engine back out so that it is now the same size as the bore, will enable the engine to make more torque and rev harder.
The price of the parts are pretty astronomical, especially considering I barely ride the thing. It's more of a thought experiment.
The Italkit 140cc I'm running at the moment is just incredible! The power delivery on that thing is such an adrenaline rush.
This is a long term project, and I'll probably never drop it in, but be nice to build this stroked polini, the eventual displacement will be 166cc, it's a real minimal stroke. It should hopefully enable the engine to retain some of its revvy characteristics.
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@Calum yes I know about bores and strokes but obviously installing a longer con-rod will not change the stroke, the only way to alter the stroke of any engine is by altering the crankshaft, even then you'd need to fit a longer stroke crank to give you more cc's, and for it to work you'd need a shorter con-rod to prevent the piston smashing into the head. Or am I missing something as it looks like there's some sort of spacer under the head? The only reason I know of for installing a longer con-rod is to allow for a shallower hence lighter piston.
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@markus-w Dude
Uhhmmm yeah you're absolutely right. I feel like such a dinglebat.Well, this is a long term project so more thinking is required! I'm not achieving very much here am I???
Thanks for correcting me, I don't know what I will do with these parts, more thought is required.
So it's still 160cc's, which is fine I am not interested in my displacement, but there are gains to having a longer rod.
Note here my failings were thinking longer rod meant long stroke which is NOT correct.
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@Calum no worries bud. As I said though I have heard about long rods being used to allow for a more shallow therefore lighter piston but thinking about it that must be on 4 strokes as the piston height is needed on a 2 stroke to close the ports. You had me thinking for a while there though. Anyway, sounds like you're going all out on this one. Quite a rare bike I believe too. Look forward to seeing the progress.
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@markus-w Yeah, well the intent on this bike is some green lanes. My RS is the all out one which is where the Polini 160 kit and Italkit rod is coming to play. But I was out on the RS last night and I am well happy with thr Italkit, it's superb. This setup is just something to build and put on the shelf.
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@Calum said in Aprilia ETX 125 (Don't expect to go motocrossing...):
@markus-w Dude
Uhhmmm yeah you're absolutely right. I feel like such a dinglebat.Well, this is a long term project so more thinking is required! I'm not achieving very much here am I???
Thanks for correcting me, I don't know what I will do with these parts, more thought is required.
So it's still 160cc's, which is fine I am not interested in my displacement, but there are gains to having a longer rod.
Note here my failings were thinking longer rod meant long stroke which is NOT correct.
I think everyone is achieving a lot here.
Reading this thread made me remember that conrod length is a major factor in most two-stroke tuning equations as it affects peak piston speed and whereabouts in the engine's stroke it occurs. It can also alter the position of the piston in the bore at a particular point in the the stroke.
On a two-stroke this is obviously critical due to port timing. Does this big bore kit come with the 5mm longer rod? Because if that's the case, it may be that the kit manufacturer has decided the power characteristics of the original 125cc engine are best preserved by maintaining the relationship between bore and stroke when increasing cubic capacity. So after you've put it together and set up jetting etc., it should still rev like the original 125cc engine but with more power.
Conversely, the DTR 170 Athena kit has a good reputation but is not particularly "scientific" in that it just increases the bore size which is probably why users report their top-end power gains aren't spectacular. "Piston speed is higher across TDC and lower across BDC with a short rod as compared to a long rod used on the same stroke." And just bolting on an Athena 170 effectively shortens the stroke from what Yamaha intended. Considering almost all cylinder filling/evacuation on our engines occurs between mid-stroke and BDC, this can seriously disrupt this process meaning the engine is unable to rev as well as the original 125cc engine (probably why a properly derestricted KMX125 had a reputation for being faster than a KMX200 back in the day).
This article is about big V8s at the dragstrip and so of limited relevance to us in some ways, but it explains well the concepts of bore/stroke ratio, peak piston speed which is what we're interested in. Not sure I've understood it fully so bear with me; I broke both my legs in December and have had 3 hip replacements since then. Hopefully the final one last week and the anaesthetic is still working its way out of my system!
https://www.chevyhardcore.com/tech-stories/stroker-engines-long-short-connecting-rod-length/
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@HOTSHOT-III I am going to consult my Graham Bell Two Stroke Performance Tuning. This was always my intent with this Polini kit build. To fabricate and do something unique and learn. But I'm glad I have been informed now so I didn't go away buying more bits before reading.
So I need to do a blog on the Athena kit. I've spent the last few weeks dialing mine in, after owning it for like 2+ years...
It's running better now than it ever has, including TZR RR Belgarda engine days, and my Mick Abbey 3MB Ported days.
This Athena kit would seriously benefit from some porting work IMO. My main issues with the Athena kit is by 10k it is totally out of puff, but I may need to richen that top end back up. Once I've dialed it in, I'm taking it to a rolling road and I'll report back with my verdict. But with the modifications I've got on my DTR it's now a little wheelie machine and I absolutely love it. The main thing is, it and I are a lot heavier than we were 12 years ago when I bought the bike, the extra torques has been really useful to me. But now I've dialed in the powervalve/ignition/jetting settings a bit better, the bike is running lovely. I'd expect if BHP isn't mid 20s then the jump in torque will justify the expense paid.
I also got my Athena kit from Germany for around £400. When you consider what's in the box, cylinder, studs, powervalve, head, skull cap, gaskets, piston, bloody good value for money. Consider the Italkit 140cc bore is £350 for a bore... But the Italkit makes $$$ power
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@Calum I have the Athena 170 kit as well, just haven't fitted it yet, and got mine from a similar source. Definitely good value for money as you say for a brand new top end, in fact I was so impressed when it arrived I decided to try an Athena 125 kit as well which will be going on the bottom end I rebuilt recently.
Plan is to build that bike for the road/greenlaning to avoid insurance/DVLA hassles, and use the 170 kit's extra cubes/torque to make another bike into a proper little Enduro bike and enter it in an event locally. I've already got some XT600 fork springs/15W oil and a YSS shock which improved the suspension off-road no end so the only thing I'm worried about is the combination of lower gearing and increased capacity leading to overheating hence my "Bigger Radiator?" thread a few weeks ago. Another idea is to get a couple of small computer fans, make up a bracket and wire in a manual switch.
I think the basic DTR engine platform is well up to this as GPX use it in the TSE250R and everyone's raving about that ATM. Half the price of a brand new KTM but still expensive. Would be very satisfying to rock up at an Enduro with a 30-year-old bike with more setup options than one of those!!
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@Calum said in Aprilia ETX 125 (Don't expect to go motocrossing...):
@HOTSHOT-III I am going to consult my Graham Bell Two Stroke Performance Tuning. This was always my intent with this Polini kit build. To fabricate and do something unique and learn. But I'm glad I have been informed now so I didn't go away buying more bits before reading.
So I need to do a blog on the Athena kit. I've spent the last few weeks dialing mine in, after owning it for like 2+ years...
It's running better now than it ever has, including TZR RR Belgarda engine days, and my Mick Abbey 3MB Ported days.
This Athena kit would seriously benefit from some porting work IMO. My main issues with the Athena kit is by 10k it is totally out of puff, but I may need to richen that top end back up. Once I've dialed it in, I'm taking it to a rolling road and I'll report back with my verdict. But with the modifications I've got on my DTR it's now a little wheelie machine and I absolutely love it. The main thing is, it and I are a lot heavier than we were 12 years ago when I bought the bike, the extra torques has been really useful to me. But now I've dialed in the powervalve/ignition/jetting settings a bit better, the bike is running lovely. I'd expect if BHP isn't mid 20s then the jump in torque will justify the expense paid.
I also got my Athena kit from Germany for around £400. When you consider what's in the box, cylinder, studs, powervalve, head, skull cap, gaskets, piston, bloody good value for money. Consider the Italkit 140cc bore is £350 for a bore... But the Italkit makes $$$ power
I'll happy to help you with info on that blog.Full of experience from the kit and the tuning I did.
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@Calum hiya pal just yesterday I bought myself a 1999 aprillia etx I’m used to my 4 strokes just wondering if u can give me any info on how reliable they are and what piston size they take just Incase I blow her up
any help would be appreciated all runs and rides mint atm running it’s at 30/1 is that the best ratio for them a aswell ? -
@18tylacollins Hi bud,
As described in the blog, this is a heavily restricted version of the RS 125 and even the RX/SX/MX series of motorbikes.
As such they suffer from the same problems as those bikes but on a much smaller scale. Such problems are well documented on the net.
The main thing to bear in mind is that these bikes are old and will need maintenance. Since you're coming from a four stroke to this bike, the thing you need to keep an eye on is the balancer shaft cogs which drive the oil and water pump. These are nylon and will eventually snap, starving the engine of oil and causing a seizure.
It is because these engines are so restricted that actually the maintenance schedule for this is incredibly drawn out and therefore need seeing to less frequently.
For all your engine needs I'd recommend PJME.
My ETX is currently stripped down to the frame for a full nut and bolt restoration so I can't tell you how reliable they are to live with, it was never my intent to run this as a daily whip.
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@Calum the balancer shaft we’re is that located buddy and how would I know it’s gone back or going bad befor the worst
