Tweet
"Ok, here's the deal."
Ken, I think you may have misread, or misunderstood, either Dell82's or my postings. Let me start from the beginning, so even Hobbyman can understand.
Dell wrote that when the lobe was pointed in the right direction, he got clearance "X". He then rotated the cam a little, and the clearance then was more("X+"). He wanted to know where to take the correct reading.
On our bikes, one takes the reading when the lobe is pointing away from the shim. Period!
But, as we're all aware, I'm not one to pass up an opportunity to run my mouth, so I did, and further confused the issue.
Not all bikes have their clearances checked with the lobe pointing away from the shim. On certain Kawasaki's and Triumphs, for instance, you check the clearances with the intake and exhaust lobes facing away from each other. (You set #1 cyl at TDC, and you check both valves for that cyl. at the same time)
You are measuring at the side of the cam.(the clearance "MAY" be different if you checked with the lobe pointed away from the shim)
To add further confusion... with the #1 cyl at TDC, you also check the intake of #2 and the exhaust of #3.
The engine then gets cranked till #4 cyl. is at TDC. You then check #4's valves, and also the exhaust of #2 and the intake of #3. Then you are done.
This is how the cam is ground... also has something to do with the angle of the valves in the head, etc.
Again, our machines call for the lobes to be pointing directly away from the shim. That should be, based on our cam design, the closest area. If another area of the cam proves to have a bigger gap, that is no matter.
To expound on what you and I both wrote earlier....
Let's say that at the proper point on the cam, the clearance is a perfect .004. If one rotates the cam a little, the gap may widen to say... .010. I believe you are stating that you should shim there.
But, if you shim that area to .004, when the cam rotates, then the original area(the correct area) would then have a gap of -.002 and be holding the valve open.
Thus, one shims at the area of closest tolerance, regardless of what other areas of the cam reads.
Looking at the drawing so lovingly supplied by MRO, one sees that the cam diameter, in theory, should be the same for 75% of it's rotation before the lobe gets involved.
It would be nice if it was... as we wouldn't be having this discussion and I could finish eating my dinner before it gets colder...
But, all cam grinding techniques and machines aren't the same. There are slight differences in the diameter of the cam based on the design. For this reason, our cams, in theory, have their closest point to the shim when the lobe is pointing away from it. (Other cams, when the lobes are pointing to the sides.)
MRO is also correct that valve clearance affects valve timing. Valves set at .004 (closer) will open sooner and stay open longer than valves set at .006(looser)
(technically, the same applies for the air gap on the pickup coils. The closer the gap, the sooner the spark fires.)
Same for the gap on spark plugs. The wider the gap, the later the spark fires, as the coils have to produce a higher voltage to bridge the wider gap and that takes a little more time. This results in a hotter spark, but also will burn out the center electrode of the plug all the more quicker.
Don't try to out-guess the work done by those pointy-headed little Japanese engineers. They knew what they were doing when they designed all the systems on this bike.
Ok.. except for that octopus thing!
Ken, I think you may have misread, or misunderstood, either Dell82's or my postings. Let me start from the beginning, so even Hobbyman can understand.
Dell wrote that when the lobe was pointed in the right direction, he got clearance "X". He then rotated the cam a little, and the clearance then was more("X+"). He wanted to know where to take the correct reading.
On our bikes, one takes the reading when the lobe is pointing away from the shim. Period!
But, as we're all aware, I'm not one to pass up an opportunity to run my mouth, so I did, and further confused the issue.
Not all bikes have their clearances checked with the lobe pointing away from the shim. On certain Kawasaki's and Triumphs, for instance, you check the clearances with the intake and exhaust lobes facing away from each other. (You set #1 cyl at TDC, and you check both valves for that cyl. at the same time)
You are measuring at the side of the cam.(the clearance "MAY" be different if you checked with the lobe pointed away from the shim)
To add further confusion... with the #1 cyl at TDC, you also check the intake of #2 and the exhaust of #3.
The engine then gets cranked till #4 cyl. is at TDC. You then check #4's valves, and also the exhaust of #2 and the intake of #3. Then you are done.
This is how the cam is ground... also has something to do with the angle of the valves in the head, etc.
Again, our machines call for the lobes to be pointing directly away from the shim. That should be, based on our cam design, the closest area. If another area of the cam proves to have a bigger gap, that is no matter.
To expound on what you and I both wrote earlier....
Let's say that at the proper point on the cam, the clearance is a perfect .004. If one rotates the cam a little, the gap may widen to say... .010. I believe you are stating that you should shim there.
But, if you shim that area to .004, when the cam rotates, then the original area(the correct area) would then have a gap of -.002 and be holding the valve open.
Thus, one shims at the area of closest tolerance, regardless of what other areas of the cam reads.
Looking at the drawing so lovingly supplied by MRO, one sees that the cam diameter, in theory, should be the same for 75% of it's rotation before the lobe gets involved.
It would be nice if it was... as we wouldn't be having this discussion and I could finish eating my dinner before it gets colder...
But, all cam grinding techniques and machines aren't the same. There are slight differences in the diameter of the cam based on the design. For this reason, our cams, in theory, have their closest point to the shim when the lobe is pointing away from it. (Other cams, when the lobes are pointing to the sides.)
MRO is also correct that valve clearance affects valve timing. Valves set at .004 (closer) will open sooner and stay open longer than valves set at .006(looser)
(technically, the same applies for the air gap on the pickup coils. The closer the gap, the sooner the spark fires.)
Same for the gap on spark plugs. The wider the gap, the later the spark fires, as the coils have to produce a higher voltage to bridge the wider gap and that takes a little more time. This results in a hotter spark, but also will burn out the center electrode of the plug all the more quicker.
Don't try to out-guess the work done by those pointy-headed little Japanese engineers. They knew what they were doing when they designed all the systems on this bike.
Ok.. except for that octopus thing!
Comment