XS650: Steffis’ Crank Problem   Leave a comment

Steffi is swiss-very, and that’s not a bad thing. He has a passion for XS650s too, and that’s not bad either. But he’s been having a problem.

2008:  at around 275K, harmonic resonance triggered a synaptic alert. The cause, crank pin pitting. The kids’ future incomes were leveraged for an NOS conrod and rollers from Yamaha. No crank pin was available so an aftermarket one was bought from Bela, also  swiss-makes nice original stainless steel exhaust systems for the XS.

Bela says he can harden the pins to any depth you want. He reckons that Yamaha optimised material hardnesses to minimise wear and when you harden material any deeper it loses elasticity possibly leading to pitting if the tolerances aren’t increased too…0.06-0.07mm.

After 10K more, pitting again discovered on the pin.

The pitting was consistant between 0° and 30° after TDC indicating a problem with timing the combustion pressures, at lower engine speeds the maximum combustion pressure is concentrated at the beginning of the cycle. At 60 Bar with 80mm pistons combustion pressure easily tops 3 Tons. Reducing compression ratio wasn’t an acceptable option however.

A rod-kit,  rod, pin, roller-cage, thrust washers, was then ordered from those we don’t talk about.

 

533 Conrod kit

 

After 12K, pitting was found in the big-end.

Parts were replaced under warrantee.

Another 12K, more pitting in the big-end.

No more warrantee cover.

Replaced with an unknown Ebay  €50 crank…so far with 25K problem-free km.

Now Steffi can’t just sit around, and being swiss-well they build watches after all-has something against unreliability. He also clocks-up a high annual mileage. The prospect of regular crank-stripping was not appealing.

Big-end bearing pitting commonly occurs around 200k. And then the recent pin problem. Age and the increased performance stresses from running his home built electronic ignition and the 80mm pistons meant cranks were lasting about 80k, just 3 yrs riding…not enough. He went looking for solutions. Right to the horses’ mouth-the bearing man.

Conclusion: ..The original Yamaha big-end bearing configuration ( 29-39-22 mm inside,outside,width ), appears to be designed for higher engine revolutions with the 13 x 5mm rollers leading to extreme surface loading at low to mid ranges. Add poor lubrication and cooling and there is room for improvement.

Task: ..Increase the number of rollers. Improve lubrication by slotting the big-ends and opening the oil gallery nipple jets that direct oil onto the crank as it rotates.

 

The alternatives

 

Original: …

  • 29-39-22mm (inside,outside,width) and 13 x 5mm Rollers. ( shown is a 533).

Folan:..

  • 30-38-18mm and 17 x 4mm rollers. ( shown is a 447).
  • Conrod and crank pin are machined to fit the bearing.
  • Conrod is slotted, oil gallery nipple bored to 1.5mm.
  • High volume oil pump required.
  • Expensive, seen in 980cc sidecross motors so there’s not much data on high kms.

Steffi:…

  • 31-39-22mm and 19 x 4mm rollers. ( shown is a 533).
  • Selfmade bearing cage @ 1.77mm spacing, hardened and copper coated.
  • Slotted conrod and increased crankcase oil nipple jet.
  • Crank pin machined, hardened and ground.

Bearing specialists claimed 19 rollers give better surface-loading at lower engine speeds but greater thermal loading at higher speeds  whereas 17 rollers are better for extended full-gas riding. Other considerations were caged or uncaged, and which diameter roller.

Diameter end points were 5.0-3.5mm with 4mm deemed optimum for Steffi. Although uncaged rollers are known to have higher carrying capacity the internal frictions are 4 times higher and roller close-packing inhibits oil flow through the bearing-perfect for my swingarm.

Nipple jets-one each side-enlarged

 

On with the job.

Unfortunately production 17 x 4mm roller cages to fit the big-end ID..29mm, and width..22mm, couldn’t be found so were fabricated, hardened and copper coated.

 

Machining the cage

 

Grinding the pin

 

Pins were machined, hardened and ground to fit the bearing cage.

The original 533 conrod was slotted and the oil gallery nipple jet enlarged.

 

Finished product

 

Copper-coating helps prevent carbon diffusion. The effect – while the big-end is ca 62 HRC, the rest is ca 54 HRC, not so brittle.

Well, the target is longevity and reliability in a slightly tuned, high mileage, mountainous environment. The absolute minimum is 3 years or 80K whichever is the longest.

Happy riding………………

Sources:    here and here


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Posted December 30, 2010 by xscafe in Motor - Crank

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