Rotax 129 256 questions

John Tirado · Joined: 17 Nov 2010 Posts: 3

Hi All.

I am a Rotax 129 fan and have done a lot of independent research regarding this power plant and maintenance/upkeep. (Not much out there nowadays). I know that there is a real deep knowledge base here and I'd like to tap into thatif I could.

I have gotten help from Riley in the past and thank him immensely for his help but, I really would rather not pester with relentless, sometimes stupid, questions. I know he frequents here and my questions here will allow him to answer at his discretion as well as tap into the forums knowledge base

I apologize if these questions sound general or novice but my application is in a bike (Aprilia RSR125). There is absolutely nowhere to go for these kinds of questions in the bike world and I thank over and over the Kart guys for keeping me going.

Some q?

In reference to cranks
Counterweighting

I have two cranks here both 113mm con rods that are tungsten counterweighted. I have another 110mm that has the counter weighting as an expanded lobe variety/ Milled as one piece. (not sure the terminology)

Is there an advantage of one over another? Does the integrated style tend to" wobble/run out" at high RPM?

The "aprilia spec/GP?" (113mm) cranks also incorporate a splined shaft to hold the balance gear whereas the 110(kart) crank incorporates a pin. I would think that the pin version is inherently weaker but have noticed the 256 cranks and variant use a similar pin method.

The 110 crank incorporates a 20mm big end pin. Aprilia/spec 115mm connecting rods use 20mm big end pin. I'm assuming that a 115mm con rod can be pressed into these counterweights.

I have thought that I might be able to use a 256 crank (front) in the 129 motor but do not have enough knowledge base to know if this is possible. I really don't want to drop a $$ for a new 256 front to gather the specs... especially if I can't use it.

I am aware of width differences piston differences and some other peculiarities between the cranks/ motors casings etc and am prepared to work around these issues.

Thanks in advance for any help

John

Riley Will · Posted: Wed Aug 08, 2012 11:20 pm Post subject:

Hello John,

Once upon a time, maximum primary compression was deemed a requirement. This help produce bottom end power but sacrificed top end and over rev. When they made the cranks thinner they decreased primary compression but lost the required balance factor. This is when we started to see the use of Tungstem alloys that brought the desired balance factor back to a useable level (~58%) for a single cylinder.

The tandem twins run a much higher balance factor due to their ability to counter balance each other. A Rotax 256 crank will not work in your 129.

Reciprocating weight changes the balance factor. Crank pin diameter and piston assemblies have a massive effect. Rod length has a less of an effect but still changes it. The 20mm crank pin allows for a smaller OD connecting rod big end which then requires less Tungsten to achieve the desired balance factor of the designer.

I caution anyone thinking about over boring because of the substantial change this makes to balance factor due to increase piston assembly mass.

The Aprilia RSA 125cc engines used both 20mm and 22mm crank pins in its development. They also use 120mm connecting rods on a 115mm piston design which allowed the designer to easily increase crankcase volume by 5mm making a massive change to primary compression. They run ~675cc's of crankcase volume as measure by having the piston at TDC, cylinder on, and pouring thru a small hole in the top of the piston. The RSA routinely revved to 15k if desired.

If required, I have a brand new Rotax 129 crankshaft in my possession;)
_________________
Riley Will
BRC Engineering
rwill@brceng.com
(403) 216-0630 wk.
www.brceng.com

John Tirado · Joined: 17 Nov 2010 Posts: 3

Wow! that's a tremendous amount of info in a short paragraph.

Thanks tons Riley. You always come through.

This opens a host of new questions that I really should talk direct to you about. I have a 22mm pin crank that I would like to put a 115 or 120mm rod in but now I see that it's not necessarily a press and go as the current balancing may not be applicable ... If I remeber correctly the 120mm utilizes the 22mm big end pin.

Thinking the best thing to do is contact and send off to you to work your magic.

I have yet to get the new motor up and going utilizing the new BRC cylinder Wink

Recently acquired a 129 Kart/roadracer motor complete and have been using the RSR spec motor to reverse engineer this power plant to a more updated version with stand alone ignition and new pipe.Also machining a few odds and ends to bring it up to the Aprilia spec (albeit older). Intention is the new cylinder, pipe, valve, Crank ( I have 10.. 115mm pistons on the shelf). Thinking Keeltronic stand alone.

I have a rolling road dyno on site now and hope to compare both engines to see what the difference between the two will be.

Ive been working with Lozza Stolk over at 2 stroke institute who has been a tremendous as well.

Going has been slow as of late....too many pokers in the fire.

I love this stuff

PM me the details on that crank ... Wink

Barrie Raterink · Joined: 12 Aug 2012 Posts: 2

Hey

I am Barrie from the Netherlands, and also rebuilding a wonderfull 129 for bike use.

When necessary i can help you with proper exhaust designs and head calculations. Please PM and I can help you.

But are there people that also have proper usable cilinders? the chrome liner i dont care if it's worn or so. But the exhaust port walls unbroken, and untuned? Powervalve or not doesn't bother me at all. Just an genuine 129 cilinder... or a blank. That needs to be worked.

My engine looks perfectly fine but the cilinder doesn't match with my intentions it has to be perfect.

thanks!

Barrie

Riley Will · Posted: Tue Aug 14, 2012 3:20 pm Post subject:

I have 2 perfect 129 cylinders.
_________________
Riley Will
BRC Engineering
rwill@brceng.com
(403) 216-0630 wk.
www.brceng.com

Barrie Raterink · Joined: 12 Aug 2012 Posts: 2

Thankyou! *already mailed*

Well are there people in here that retrofitted an 129 cilinder with powervalve and how did they do it?

I have an own milling machine etc but there are several ways, first milling and then filing?

Barrie

Riley Will · Posted: Wed Aug 15, 2012 9:17 am Post subject:

129 cylinders have the casting material removed internally so it is not possible to make them into a power valve cylinder. The 130 cylinder has the power valve.
_________________
Riley Will
BRC Engineering
rwill@brceng.com
(403) 216-0630 wk.
www.brceng.com

John Tirado · Joined: 17 Nov 2010 Posts: 3

Hey Barrie
Sorry for not seeing this. I expected all responses to be sent directly to my phone and got so busy that I never sat down to take a look. A friend just sent me the link. I was starting a blog in reference to our bikes but then thought........ How many guys would actually post there????

Thinking I should get back to work on it!

Anyway, thanks for the offer of help! I pm'd you and look forward to working/chatting with you

John