Long connecting rod kit for an RM 125?

HI, im looking for some tips for a future project. Has anyone heard of a long rod kit for anything other than a banshee. Im considering getting a 4-5mm longer connecting rod for my 98 RM but i can't find anything on the internet thats related. Does wiseco or hot rods make custom connecting rods? I am not trying to increase the stroke , just the rod length. I understand the head will need to be milled for clearance.

Thanks for the input

Tony.

Best bet will be to find a rod that fits the diameters and length you need, and have the cank built up with that rod kit.

Wiseco specs your 98 RM125 rod as

100 mm eye-to-eye

28 mm big end

19 mm wristpin

You'll need to match the pin diameters unless you want to get into custom stuff. From Wiseco's con-rod sheet it looks like only other RM125's match those diameters (the other manufacturers use larger diameters on one or both eyes).

88-96 RM125

105 mm eye-to-eye

99-03 RM125

102 mm

04-06 RM125

110 mm

Now, you'll need to do more than mill the head to make this work. You'll need a cylinder spacer to keep the ports in the proper location since the piston will be higher in the cylinder at any given crank angle. Putting in a 2, 5, or 10 mm longer rod without changing port locations will MASSIVELY change the port timing, and most likely be a huge step backwards in performance.

What are your goals for this engine that make you want to persue a long-rod?

FYI

My 05 YZ250 uses a 125 mm long rod. Before I had it the bike was built up with a rod from a 2000 KTM 250sx, which is 129 mm long. There is an aluminum spacer under the cylinder, along with an additional base gasket to make up the longer rod. On top of that my bike runs a domed piston from a 03+ KTM 250SX, and a heavily modified head.

Maintance is about the same as a stocker, cept I can't buy an aftermarket crank when it's time for a lower end rebuild. Last time I had Crankworks in Tempe, AZ redo it with a connecting rod kit for the KTM. Came out perfect, couldn't ask for better service. Gaskets are all stock, piston and rings have to be for the KTM as well.

Was this spacer plate custom made? I was planning on having the head milled along with having the ports raised because i wouldn't have to worry about extending the rod that connects to powervalve mechanism along repositioning the bolts that secure the cylinder head and the frame. If i had a spacer plate that was the same thickness of incressed length of the connecting rod would i still need to have head work? if not, a spacer plate seems like the way go

Also, which length do you recommend I should with from the longer stock connecting rods?

The plate is custom made. I've read that Cometic (the gasket company) will make plates to order.

Raising the ports will be MUCH more work than extending the powervalve linkage and ovaling a few mounting holes. Just grinding the ports out to be taller will again, throw port timing out the window, as the overall height and duration will be changed drastically. To do it properly you would need to raise the ports and weld in the bottoms, then hope that you can get decent angles on everything. Will require a lot of work.

Again, what's your goal for this setup? Trying to build more low end? Peak hp? What's your exprience with wrenching / tuning? I ask the last one, because going for a major engine design change doesn't make sense if the bike hasn't even tested with correct jetting, or if the characteristics wanted can be found with some porting and a modified head.

As for rod length to go with, depends on a lot of what you are intending to do. Again referencing the Wiseco connecting rod guide ( http://www.wiseco.com/PDFs/RodSpecs.pdf ), it looks like Honda stuck with 104 mm for more than 2 decades. Kawasaki flipped around 104, 106, and 109 mm, and Yamaha has stuck to 105 mm.

Here's some explination / theory on how rod length effects a 2 stroke.

http://www.stahlheaders.com/Lit_Rod%20Length.htm

2strokezuke,

What specificaly are you trying to accomplish?

Remember a longer rod does not increase displacement, it does however increase the duration (time) that the piston stays closer to TDC and BDC. This can have its advantedges . Also when a longer rod is used -- you lower the primary compression ratio (compression ratio in the cases) This can be corrected with the proper use of epoxy.

I have plenty of expereince on wrenching on these bikes, but not so much when it comes to porting. If i was going to raise the ports i would send it to a professional rather screw in up myself... but the spacer plate seems to be the way to go. I thought a long rod kit would be a good first engine mod because it creates a more ideal crank angle which produces more torque throughout the entire powerband( correct me if im wrong) along with reducing stress on engine components.

I have all the basic tune ups and the jet and needle specs dialed in but still want more power. I don't want to have it ported for only top or bottom end power because im planning on racing but haven't decided let between MX or harescrambles.

Since a longer rod dwells longer at TDC, would i need to advance the timing

Pete payne

When i split the case i noticed the previous owner had filled the space directly under the crank with epoxy. There was no evidence that he did this to fix a crack. The reason for the epoxy always perplexed me, is this what you were talking about?

Edited by 2strokezuke

Tony,

the previous owner had filled the space directly under the crank with epoxy

Does the epoxy extend across the case junction, leaving the cases epoxied together? If so, I won't comment on that decision :ride: but I found a solvent for cured epoxy that might help you get the cases apart for the crank work:

http://shop.rings-things.com/cart/pc/viewPrd.asp?idproduct=19997&idcategory=1988

On a side note: I wonder if the new Yamaha 'backward' engines are trying for the best of both worlds, using piston offset for increased leverage duration instead of a longer rod?

I think if I had a modern 125 and wanted more power, I would find as much info as possible on the KTM 125 FRR road-racer motor. 55hp isn't bad for a 125. :lol: I'm not suggesting you try and copy that motor, but there's gotta be something going on in there you can use.

http://www.ktmpress.com/Bike-Spec.5359.0.html

Ray

Ray

The epoxy was placed on the sides of case separately so i am able to split the case. It completely fills in the empty space that is located directly under the crank. Is this used to increase the primary compression? What kind of effect does primary compression have on the power curve?

Thanks

Tony.

Tony,

Is this used to increase the primary compression? What kind of effect does primary compression have on the power curve?

Yes, I imagine that's why it was done. From what I understand about primary compression, in a well-designed engine it has the greatest effect when the engine is running "off the pipe", i.e., when pipe action is not pulling mixture from the crankcase through the transfers. If a motor is already getting good scavenging action across the rpm range using the existing P.C.R./porting/pipe, increasing the P.C.R. should mainly result in additional charge loss out the pipe. On the other hand, if there are instances where scavenging is not so good, additional P.C.R. might improve cylinder filling and power output, provided the porting arrangement can make good use of the additional charge pressure in the deficient rpm ranges.

According to the Bell book, motors have been designed around high P.C.R.'s which had very high power outputs, and extremely narrow powerbands.

In a modern motor, I think I'd want to do some pretty extensive computer simulation before filling in the crankcase, but I'm just cautious that way.

Ray

rayivers,

You got it right ! The better the pipe design the better it will scavenge so a larger volume in the cases can be beneficial to some point . And for some one who keeps the revs up in the upper zone. but there is a lot of area in a 5mm spacer plate that is under the cylinder.

I normaly measure the volume before and after the spacer plate is added and adjust the volume accordingly. It can get tricky as to where to add that much epoxy to get the compression ratio back up in the cases .

Pipe's that work good with big case volumes tend to be big and fat in the mid section by virtue of the size of the divergent cone. On a bike packaging it can be a problem.

ooboy, looks like im going to have to get rid of that epoxy next time i have the case split.

Ray

What was that solvant you were speaking of.

Pete Payne

So the idea is to matched the volume of the spacer plate with the volume of epoxy you add under the crank?

Also, how does a longer rod change the power curve? I just assumed it slightly increase the torque throughout the revs.

Thanks alot for the input guys!

Tony.

Thanks, Pete!

Tony,

The solvent is called Attack, and apparently it's designed for removing cured epoxy (there are other solvents for uncured). I haven't tried it myself. Here's the link again:

http://shop.rings-things.com/cart/pc/viewPrd.asp?idproduct=19997&idcategory=1988

Also, how does a longer rod change the power curve?

Without performing the mod on a bike I'm already familiar with - which I haven't done - I really can't tell you.

If this helps at all... a theoretical motor with an infinitely-long rod would vary continuously from minimum to maximum acceleration. Leverage would be more constant than a short rod. A piston acceleration graph would look like a sine wave.

A motor with an infinitely-short rod will have long periods of low piston acceleration followed by shorter periods of high piston acceleration and maximum leverage. An acceleration graph would resemble a 'clipped' or half-sine waveform.

If I were planning to try this, I'd want to simulate the effective piston leverage during the combustion/gas expansion phase, when added long-rod leverage would actually yield benefits.

I realize you're looking for "a long rod will give you more ______ at ______ rpm" kind of answer, but it really depends on the motor. If the original rod was quite short relative to stroke, you might get a noticeable increase in torque across the band along with possibly a changed motor 'feel'.

Ray

Edited by rayivers
...If this helps at all... a theoretical motor with an infinitely-long rod will have long periods of low piston acceleration followed by shorter periods of high piston acceleration and maximum leverage. A piston acceleration graph would resemble a rectangular waveform with rounded edges.

Ray

The piston motion for an infinitely long rod would be a perfect sine wave.

Edited by adynes

not that im some engineering guy but i did ride an 03 rm and an 04. in 04 they switched to the longer rod motor. it seemed to rev up a little quicker and pull more through the mid to top. the few years before they were trying to get a motor to have strong midrange but without success with the shorter rod.

.....

Edited by adynes
The piston motion for an infinitely long rod would be a perfect sine wave.

I believe you're right for acceleration, too - my bad. :ride:

The following chart shows piston acceleration for 100mm to 10,000mm rod lengths using an 87mm stroke, and definitely shows a progression towards a sine shape:

piston_acc.GIF

It's weird how a 144mm (stroke x 1.66) rod keeps the piston nearly motionless for about 90 crank degrees.

Ray

Haha, that's funny. I just googled that same graph! I go back to my original statement. That's what I get for second guessing myself. :ride:

It's weird how a 144mm (stroke x 1.66) rod keeps the piston nearly motionless for about 90 crank degrees.

Ray

It wouldn't be motionless, just constant acceleration. What's really wierd is the third derivative of position, Jerk: the rate of change of acceleration. I can't quite wrap my head around that one! I think it's important in camshaft lobe design though.

positon/velocity/acceleration/jerk

At one time I had spreadsheets made up for all this stuff...

Jerk: the rate of change of acceleration.

Hey, I think that's the characteristic of a 2-stroke motor I like the best. :ride:

Ray

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