By: Anonymous: オートバイ ()  Wednesday, 09 December 2009 @ 06:55 PM ICT (Read 3540 times)  

My 2008 Yamaha FZ1 just came due for a valve-clearance check at 40,000 km. I personally checked them and found one valve just barely out of specification on the tight side. The gauge would go in but took a nice tug to get it back out. I know that tight valves are a not good condition; but while I'm comfortable checking the clearance, removing the cams to replace them shim is more than I care to get into. I took the Yamaha to a dealer, gave them my measurements and asked for a valve adjustment.

The next day, it came back with the notation. “All valves within specifications.” I spoke with the mechanic, who said that the valve in question was tight at TDC, but, when the cam was rotated off TDC, it was within specifications (he was not the only one who witness this effect), and that installing my suggested shim would throw the clearance to the loose end of the specifications. Is this a valid check? It's a good-running motorcycle, so should I not worry about it? Or do I get a run-around by a Thai mechanic?

By: ThaiDesign (offline)  Wednesday, 09 December 2009 @ 08:13 PM ICT  

I wasn't one of your mechanic's eyewitnesses, so I can't say for sure if either his technique or his prognosis is correct. Actually, neither do I know if you method of checking the valve clearance gave you accurate results.

The Yamaha FZ1 service manual instructs you to check the clearances on cylinder No.1 with the piston for that cylinder at TDC on the combustion stroke. You then turn the crank 180 degrees to check the clearance for cylinder No.2, another 180 degrees for No.4 and finally another 180 for cylinder No.3 (The 4th cylinder before the 3rd is no mistake).

That's an excellent method, obviously, or the factory wouldn't prescribe it. But occasionally, especially with an engine that has quite a few kilometers on it, 40,000 km in a year is a lot, you can get a marginally different reading if the cam lobes are rotated very slightly from those prescribed positions. Ideally, valve clearance should be measured between the tappet bucket and the very heel of the cam lobe, which is 180 degrees opposite the highest point of the lobe.

But because an engine's cam timings is not split symmetrically on both sides of TDC, the heel is not quite directly adjacent to the bucket at TDC. So if you check the clearance at that spot, then turn the cams and check it again between the heel and the tappet, the clearance may be very slightly different. Usually that difference is only a thousandth of an centimeter or less, which is too small to worry about, although it can make a feeler gauge noticeably harder to pull through.

Personally: I believe your mechanic is probably right. But if you still have doubts, you could always check the clearances again using the mechanic's method...Road Cruise



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