cams look ok, but guides are pitted - Sportbike Forum: Sportbike Motorcycle Forums
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post #1 of 3 (permalink) Old 04-05-2005, 01:40 PM Thread Starter
Join Date: Apr 2005
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cams look ok, but guides are pitted

I am in Serbia and do not have access to authorized dealers. just pulled the motor out of a gsxf 600 after getting some real grimy oil and am working through it. the camshaft surfaces and hardened surfaces in the head look pretty good, but the removable guides are pretty pitted for some reason. can these be replaced without also replacing the cam with an oversize and grinding down the head surfaces? not sure what machine shop facilities are like here and prefer to avoid if possible. see the attached photo of the guides. Thanks!
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post #2 of 3 (permalink) Old 04-05-2005, 05:20 PM
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I saw this post before and it wasn't clear to me what you meant. I thought maybe someone else might be more familiar with your specific model and could answer.

Your English and spelling was so good it didn't occur to me that maybe you were translating. Is that so or are you a dislocated English speaker?

There is a terminology difference and the part identity was lost in the translation. Those would be the cam bearings, not "guides". The "guides" would be the bronze inserts/sleeves that the valve stem rides in, guiding it and centering the head of the valve in its seat. I wasn't sure what would cause them to pit but now realize you were talking about a totally different part.

Anyway, back to the problem at hand. First, no there are no oversize or undersize cam jounals to repair the head. Typically that damage would be reason to replace the head. It's also not an easy task to machine those bearings with typical machine shop tools. That is usually done with a line boring machine and would not be a common tool for most shops. Even if someone decided to attempt it, it would be a very tedious job, therefore probably very expensive, and the chance for sub-standard results would be great. If it's really bad, a used head that you might find on E-Bay and have shipped to you might be the best solution. BUT..... ALL IS NOT LOST!

From the picture, those look like tiny pits from corrosion caused by dirty oil sitting in one spot over a long period of time. If the rest of the bearing surface looks good, not damaged or flaking from a different cause, like running without oil, then I wouldn't give it another moments thought and just put it back together that way. Cam bearings are very forgiving. A lot of bearing area for the relatively light load. That's why they don't worry about just running them right in the aluminum head. If those were marks in connecting rod bearings, I'd be a little more concerned, but only a little.

If the crank has any pits like that (not the bearing insert but the crankshaft itself), I'd just get some VERY fine sand paper, suitable for wet sanding, no more coarse than 400 grit but as fine as 1000, and polish the journals. Again, I'm assuming occasional pitting, not whole sections that are badly rusted. You don't have to get all of the pits out. Once the general condition of the undamaged surface looks good, stop. The edges of the pits will be sufficiently broken to not gouge into the soft bearing shell surface and the lost bearing area will be insignificant.

I don't know if those grit ratings are international standards but if not, a local machine shop could probably advise you what number grit is appropriate in your standard for polishing bearing journals. Cut a piece of sandpaper from the sheet lengthwise, that's about 3/4 to as much as the full width of the journal, and use it to polish those journals. Apply some kerosene or very fine oil to the paper and journal, and then wrap the paper around, grip each end, pulling it backwards and forwards and moving side to side to polish the full width. Rotate the crankshaft often to assure uniform sanding. While this may sound crude, you'll find that with those fine grits you'll have a hard time measuring ANY dimensional change, even with your micrometers, but will improve the surface finish and remove any oxides that may exist.

I hope that helped and good luck.

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Last edited by Dad; 04-05-2005 at 05:30 PM.
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post #3 of 3 (permalink) Old 04-24-2005, 12:34 PM Thread Starter
Join Date: Apr 2005
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thanks a lot! really helpful and saved me a potentially problematic trip to a local machine shop. sorry about the terminology, the book i have called them a couple different things (or maybe i just got confused). getting into the crankcase now, so if you dont mind may try to pick your brain some more. Thanks again.
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