Valve adjustment after rebuild

Hi all,

I had my XR250's engine rebuilt by a mechanic over 12 months ago and
haven't had time to put the engine back in or start it. I'm almost
ready to start it, but thought i'd give it the once over before firing
it up. After checking the valve clearances, I found that they weren't
set to spec. The engine is a single cylinder, with 2 inlet and 2
exhaust valves. I found one of the inlet valves to be set at 1thou,
and the other at 4thou. The reason for the rebuild was that one of the
inlet valves burnt out and required replacement. I remember the
mechanic saying he had to grind approx 3thou out of the seat to seal
everything up. Is it possible that he's allowed for this when
adjusting the valves, or are they just incorrectly set?

If the seats were ground, would this ultimately affect the valves lift?

Thanks in advance,

Ben.

 

Maybe the mechanic expected that you would set them correctly. The
valves have adjuster screws in the rocker arms, don't they?

Anytime you grind the valves, you need to compensate for what has been
removed from the valve seats by increasing the valve clearance so it
will be in the middle of the specification range.

Like, if the range was .003 to .005 thousandths of an inch, you would
try to set the valves at .004 and that's very, very close to .10
millimeters.

If the valve clearance is specified only in millimeters, you'll see
numbers like .09 to .10 millimeters. Since a millimeter is .0394
inches, .10 millimeters is .00394 inches and .09 millimeters is .003546
inches. That's less than 4/10,000ths of an inch less, and when you
tighten the locknut on your screw adjuster after carefully setting your
valve to .09 mm, the nut will pull the screw back a little so when you
re-check the clearance, it's .10 mm again.

That's why I just use American feeler gauges, they aren't as close
together in thickness.

Valve lift depends on cam lobe eccentricity and rocker arm geometry.
Grinding valves
sinks the valve head lower in the valve seat and the valve stem sticks
further out the other end of the valve, taking up clearance, so you
have to adjust the valves to compensate.

 

Thanks for those tips guys.

Ben.