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Guys,
I think you know me by now, I never try to bad mouth anyone or anything on the Forum. But, to be honest, I'm afraid that someone will use this cam in one of my heads or with the 170cc bore kit.
Last weekend I purchased a Yuminasi Billet CNC SX250 7.1 cam for the Grom off of Ebay. I got it from the US dist. If nothing else, I could see what someone else is grinding for cam timing. I paid $133.00 for the cam, the Honda OEM cam is $28.00. The cam came with no cam data at all, nothing. I'll show the cam timing tomorrow night, the numbers don't make any sense at all. Cam timing can generally be corrected by shifts done with the cam sprocket. but I'm not sure it's possible here. The really bad issue is the camshaft material hardness. The stock Honda Grom cam has a lobe surface hardness of Rockwell "C" scale 50/53. With a roller follower cam, anywhere between 50 & 60 is plenty hard. Chevy racing roller cams are 58 /61 Rockwell "C" scale. With the light spring pressure, you could maybe get away with 40/45 "C" scale. This Yuminashi cam Rockwell'd at 20/23 "C" scale. About as hard as 1010/1018 common steel. I had an idea that something was wrong this morning when I machined a better center in one end, and it machined like butter. I could not remachine the Honda cam, it was too hard. I think all I can do with this cam is to use it when I dyno the modified head this month. It will probably hold up through a few dyno runs, but I would never use it in an engine for any length of time, with higher spring pressures. It will fail, and then you have steel metal shavings all through the engine.
Sorry guys, I hate to be negative.
Brian
 

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We know you're trying to keep it professional around here, so thanks. I'm not surprised at all hearing that. I tend to stick to names I can trust, preferably American made.
 

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i'm thinking chinese fake? doubt any real engine manufacturer would make that mistake?
 

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i'm thinking chinese fake? doubt any real engine manufacturer would make that mistake?
I doubt it, Motorsportsnation eBay account is a US dealer for Yumi parts

I'm just glad I didn't buy this junk, it's becoming clear everything they make is junk, come with zero information and that the company uses the customer to put the parts to a test
 

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thanks for the research and cautionary info.
 

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Brian--

Out of curiosity, have you had your hands on a cam for any of the GY6 engines or really, a cam from any of the chinese clone pitbike engines before? I'm just curious how much harder (or not) this cam is versus those little 50cc engines cams. I've seen some gnarly QC issues on those so hearing this doesn't surprise me given how similar the design is.
 

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is it safe to assume this is the same type of quality to expect in their bore kits?

Too bad, had me kind of excited for those...
 
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