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Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 8378 times)

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #90 on: Feb 12, 2018, 13:59:18 »
At some point between 1980 and 1982 (I think around then) the rockers were upgraded to be the ones used on the 350 quad engines - they are the same part.  The difference is that the original part is a cast item with a nitride hardened surface on the cam contact pad.  Whereas the new rockers/the quad rockers have a (through?) hardened pad on them as a separate part.  Another difference is the tappet screw and nut.  The original has an M6 x 0.75 thread while the replacement part has an M7 x 0.75 thread (first time I have encountered this).  This is why when you buy a new rocker, it comes as a set including the tappet and nut - because obviously these particular pieces are not interchangeable any more. Check out the photos for the differences.

I suspect there were either some wear issues or Yamaha wanted to consolidate parts across similar models.  The wear would have been on the cam and the rocker pads obviously, but I also suspect wear on the tappets and the valve stems was also an issue - as I am sure many of you have seen.  So going up a millimetre in diameter for increased contact surface area on the valve stem (up to M7) might have helped that.

There is also some good information on the Kedo site and other sites about the incompatibility of the different cams and rocker versions - looks like it was the same thing for the rockers in the SR500.  I am pretty sure though that it is no problem to run either rocker with an old cam, but not ok to run old rockers with any new cam?  I deduce that the new cams are harder and therefor the old rockers are not up to the job?

Offline der_nanno

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #91 on: Feb 15, 2018, 05:30:16 »
As an ex-SR500 rider, rockers, rockershafts and cams are a huge problem with these on higher mileage bikes, because the oil-supply to the head is a bit of an issue. That being said, the hardpad-rockers are good to be used with all cams. The older chromed pad rockers do have a central oiling port in the center of the pad, which can (and will) ultimately leave a little ridge on a used cam. If your new cam is nitride-treated, get some 2000-3000 grit sandpaper oil it generously and run the sandpaper over the cam-surface until it brightens up a bit. This will take off the sharp edges of the nitride coating and allow you to run older style softer rockers on SR-cams. (Generally the same rules should apply with the 250, but I only have personal experience on the 500, especially with reground and nitrided cams...)
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Offline datadavid

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #92 on: Feb 15, 2018, 16:55:57 »
Looks like a stellite pad on the late rockers to me, quite like old triumph cam followers.