Valve Clearance Questions.

Hey guys (and gals),

I'm new to the whole 4-stroke scene; just sold my YZ125 and picked up a '02 YZ250F. Ridden it a couple of times and it starts and runs great. However, I wasn't real sure how much maintenance was done on it previously (had multiple previous owners) so I wanted to pull it apart, replace the piston, rings, cam chain, and check the valves. So I pulled it apart last night and I've got some questions for all of you pro's. At TDC (or punch mark on cam's inline with horizontal edge of the head) my valve clearances seem really tight. The smallest feeler gauge I have is a .008", and it is too large to fit in any of the exhaust of intake valves! .008" is approximately .20mm. So questions #1, and #2: Do I need to get some feelers that are smaller than the .008"?? If the .008" doesn't fit, then they are all too tight, correct?

I pulled a valve bucket and looked at the shim. There are no numbers on them (I suspect maybe they have worn off??) Checked it with a micrometer and it is .078", or 1.98 mm. So, does that mean it's approx. a 198 (or 195, or 200) shim? Is that how that works??

So here's what I think I need to do.... get some feeler gauges that go below .008", find the correct valve clearance, measure all the shims, use the chart in my manual, and get new shims. Is this all correct??

Also, I guess I don't understand how the valve clearances actually tighten up. I can understand why they would loosen (with the wearing of the valves), but I don't get why they tighten. Anyone care to explain??

Thanks in advance to any help...

The spec is .10-.15 MM on the intakes and .15-.20 MM on the exhaust. In inches, this is .004-.006" and .006-.008. My exhaust valves are a bit on the tight side as well. I figure that's due to carbon build up between the valve and the seat.

Well I don't know why I was all confused about it earlier, but it seems clear(er) to me now! I bought some thinner feeler gauges, and the exhausts are .006" (.152mm). So I guess that's right on the border line of being too tight, eh? Think I should re-shim?

Riderx, thanks for the reply. I was also thinking about the carbon between the valve and the seat, that was my only logical theory behind valves tightening up.

No need to re-shim. I think the smallest shim would put it out of spec anyways.

your theory doesn't sound very locical to me. If anything carbon build up would make the valve cleaances looser

Your right, I wasn't thinking backwards. It could be from wear on the valve seat.

As the valve and seats wear the valve gets pulled futher and futher into the head making the clearance smaller.

In the first place, although the intake clearance is .10-.15mm, the valve clearance spec for the exhaust is .17 - .22mm (.0067-.0086"). If both your exhausts are at .15mm, they both need to be shimmed. But double check them with a .007" gauge.

When measuring shims, realize that you are attempting to measure less than .001" differences (.01mm=.00039"), so you will need to use a vernier micrometer, and compare its reading of a shim of a known size as a way of checking its (or more likely, your) accuracy.

The measurement of 1.98mm is probably an error in measurement, and the shim is most likely a 200. 250F's are rarely (as in, I've never seen one) shipped with shims larger than 190, so this would indicate previous valve work. Also, you will find shims in sizes other than even 0's or 5's, especially on the first clearance adjustment, or when a professional mechanic has put used shims in during an adjustment. The factory uses shims in .01mm increments, which are not sold at the dealer as service parts, so you might see a 187, or a 178.

"...smallest shim would put it out of spec anyways." It certainly would, the smallest shim is a 120, I believe. Maybe he meant the next smallest? No, it wouldn't put it out of spec, but if it were in spec, it would. But it's possible it could have been at .172mm, and had a 182 shim. You could then have used a 180 to move it to .192. But if it had a common shim like a 185, a 180 would make it too loose. And anyway, if the minimum spec is .17, .172 is in spec, and requires no adjustment.

You will never see clearances loosen unless a valve gets bent, or something gets caught on the seat or stem to keep it from closing. Wear at the face and seat will cause clearances to close up, which is pretty much the only accurate statement in this thread up to this point. In a pushrod engine, the nose of the rocker arms wear at the valve stem because of the wiping action that happens there, and such wear increases clearance. But in the bucket tappet DOHC design, there is no such wear between lifter and valve to speak of, and since the lifter and cam already don't touch each other when the valves are closed, wear at the cam lobe won't do it either. If something really goes wrong, the lifter could be worn enough to increase clearance, but it is very unlikely.

Good thing Gray is here to clear up the bullcrap. :D He should be on the TT payroll. He's one of the only guys I take advice from...including myself. I don't even trust my own advice....maybe I should stop posting advice since it seems most of the advice I thought to be true usually isn't!! :D

Gary--here's a question for someone who is older and YZer!

What is the life expectancy of the middle intake valve on an '02 WR250F versus the the left and right intake valves, and the exhaust valves? My middle intake valve always seems to be super tight. Stock shim was a 179. It took a 155 to get it back to spec whereas the left and right were 178 and 181 stock, and I put 175's in them to spec it. I just reshimmed and went to 170's on the left and right, but here is where I get confused.

Middle intake has a 155 in it. The manual chart says is clearance is 0.00 to 0.04 than I should put in a 145. It won't work--still can't get a .038 gauge in there. I put in a 140 and I am getting close--I can get a .063 in there, but not a .076. The manual says if I have a 140 shim in there, and clearance is between .05-.09, then I should go with a 135 shim? If so, should I then go with a 130 instead so I get closer to .15 at the looser end of the tolerance because it will only get tighter?

Another question--if I go off the chart, then I guess it is too far out of spec and new valves, seating and lapping is in order. What I can't figure out is why are there so many measurments for shims for huge clearances when all our bikes have small tolerances and only get tighter?

I am at a stand still right now--my closest shop is closed tomorrow but I could get a 135 and 130 tomorrow from another shop 25 miles away.

Or is it time to pull the head?

Life expectancy is too dependent on too many variables to give any kind of solid number to it. Mine went about 400 hours, and, as I am seeing here quite often, it was the center intake that as the worst worn. My right intake was pretty bad though.

What happened to you is that you actually went to a below zero, or negative clearance situation. Other than trying a smaller shim and re-gauging, there's no way to know this if the clearance is zero or less. The "less" becomes indefinable. How that happens is that once the valve reaches zero clearance, it begins to vibrate/oscillate against the seat, wearing itself further so that it is actually never in contact with the seat in a static sense.

As I said earlier, there really is no point in shimming a valve that has worn to the point that it needs a shim more than .20mm smaller than the original. They simply wear too fast from that point. It can let you squeeze another ride out of it, but you might also start breaking off chips of the edge of the valve head, and you don't need that.

The range of shims available exists in order to cover a variety of rebuild situations, and allows for more work to be done to the valve seat.

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