afternoon, just before closing time, a customer bustled into the combination
salesroom and office of the Model Garage. "Got any good valve-grinding
paste?" he demanded. "I want the kind that has two compartments - one with
coarse compound and the other with fine."
"I'm sorry, Mr.
Rankin, " replied Joe Clark, "but we don't carry that kind. Why don't you
try the new water-floated paste? Gus Wilson, my partner says it does a dandy
growled. "No substitutes for me! I know what I want." And he left the
Gus Wilson, who
was working on a car near by, had overheard the conversation. "We never seem
to have what that pest thinks he wants," he called out to Joe. "I wouldn't
shed any tears if he never came near this place again. But I have a hunch
he's going to need us to get him out of trouble one of these days."
"He won't get
into trouble this time," laughed Joe. "Valve grinding isn't so hard."
the veteran auto mechanic. "Don't fool yourself. Valve grinding looks easy
and it is easy; but you can make plenty of mistakes if you don't know how to
go about it."
Two weeks later
the phone rang insistently and Joe clamped the receiver to his ear. A moment
later he turned from the phone with a grin.
"Get out the
wagon, Gus," he called. "Your hunch came true. Rankin's stuck at the bottom
of Marley Hill."
Gus drove off
in the service car and in half an hour he returned with the disabled machine
on his towrope.
"Now let's see
what's the matter with those valves, Mr. Rankin," said Gus as he set to work
removing the cylinder head after opening the radiator drain cork.
snapped Rankin, "I'll admit the car is on the blink, but there can't be
anything wrong with the valves. Why, I ground'em myself just a little while
ago. Gus disconnected the hose connections and removed the cylinder head.
Rankin leaned over to look at the valves.
All the exhaust
valves appeared badly burned. Several were so warped that they made contact
with their seats only at one side. The intake valves, on the other hand were
in good shape,
must be defective," Rankin stormed. "I'll make the company replace every one
"Not a chance,"
said Gus calmly. "It's entirely your fault. You put the intake valves in
where the exhaust valves ought to be, and the exhaust valves are doing
"I did it on
purpose," Rankin admitted. "The exhaust valves looked blackened and they
were pitted, so I thought I ought to shift them around - to equalize the
you went wrong," explained Gus. "A lot of cars these days have plain steel
valves for the intake and special exhaust valves that can stand a lot of
heat without warping. The manufacturers have to save every penny in order to
sell cars at today's prices - so there's really nothing to criticize. The
intake valves have an easy time of it compared with the exhaust valves that
are burned with hot gas on every explosion."
Gus removed the
valves and the result was quite evident. Both the surfaces of the valves and
the faces of the seats were blackened and corroded.
"How many times
have you ground these valves?" asked Gus.
"This is the
second time," Rankin answered. "The first time I put the valves back as they
ground'em aplenty," Gus observed , "These valves and seats are worn down as
much as they would be after ten normal grindings. It isn't necessary to
grind so much. You used the coarse grinding paste quite a while before you
started to finish them. The more you grind the more you widen out the seat -
and a wide seat is bad. A narrow seat is better, because there isn't so much
chance for particles of carbon to jam in between the valve and the face of
the seat and keep the valve from closing."
Gus held up a
valve. "See that ridge? You've ground the valve against the seat so much
that the face is worn away, and there's a ridge around the outside edge. If
the valve stem is the least bit loose in the valve guide, that edge is going
to catch on the valve seat and prevent it from closing gas-tight. The same
things would occur after a number of correct grindings unless you had the
shows another trouble. The grinding has made the face curved instead of
flat. Looks as if the valve guide is loose and the stem wobbled in the guide
while you were doing the grinding.
"We've got an
electric valve-facing outfit and I'm going to grind the faces of all the
valves. That will make them practically as good as new. Also I'm going to
ream out the gas passages a trifle, to narrow the seats, so I can true 'em
up without making 'em any wider, and replace one of the valve guides."
gasped Rankin, "I must be some mechanic! If ever I try grinding valves
"I'm not trying
to scare you out of working on your car," smiled Gus. "There's no reason why
you shouldn't do ordinary valve grinding - if you go about it right. It
isn't necessary to grind the valves on a special machine and reface the
seats every time. Such treatment is only needed after a number of regular
should be done every time you scrape out the carbon. Don't let it go until
the valves leak so bad the motor begins to lose compression and runs rotten.
Once the hot gases begin to get past them, the valves burn and pit much more
rapidly. Little and often is the rule for best results. Each time you grind
the valves, take off just enough metal so that they seat all the way round;
and don't forget to scrape the carbon off the face of the valve before you
paste evenly, don't press hard, and never rotate the valve all the way
around. Turn it back and forth through a part of a circle a few times. Then
lift it off the seat, turn it part way round and do a little more grinding."
"How do you
know when the job is finished?" Rankin inquired. "I ground 'em a lot just to
be on the safe side."
mechanic can tell by the feel of the valve and the look of the seat," Gus
replied. "If you're very fussy you can buy a pressure tester like this one."
He fished an instrument out of his tool kit. It was shaped like an inverted
cup with a gage attached to one side and a rubber bulb connected to it with
"You press the
cup over the valve and squeeze the bulb to pump up a bit of pressure," Gus
explained. "If the gage needle stands still, you can be sure that no air is
leaking through the valve and it will be gas-tight.
simple way to test valves. Wipe off all the grinding paste from both the
valve and the seat. Rub enough Prussian blue on the ground part to color it
and press it tightly in its seat without turning it. Take the valve out
again and if you find a complete ring of blue on the valve seat you may be
sure the valve is sufficiently gas-tight."
"Maybe I used
the wrong kind of grinding paste," Rankin suggested. "The coarse stuff cut
fast enough; but it made deep rings in the metal and I had to grind quite a
while with the fine paste to get rid of those rings."
"Joe tried to
sell you the right kind of grinding paste, but you wouldn't listen to him,"
Gus reminded him. "You didn't bother to polish the stems, did you?"
admitted. "I thought the motion of the valve stem up and down in the valve
guide would scrape off the carbon if it got too thick."
that point is what spoils a lot of amateur valve-grinding jobs," Gus
asserted. "The carbon gets burned on the stems of the exhaust valves so hard
that it forms a little ring on the stem just where it interferes with the
valve's closing tightly on the seat. First the motor begins to miss when
it's idling, finally it gets so bad that the motor will only fire on all
cylinders when the throttle is nearly wide open and the compression is high
enough to jam the valves shut in spite of the carbon-caked stems.
thing is that the valves seem to close all right and the compression is
fairly good when you crank the motor over by hand. The valve stems cool off
when the motor is stopped and they contract enough to work right.
valves you've got to adjust them for proper clearance. If you get 'em too
close, the valves won't seat right, the motor may start hard and the valves
will need grinding again too soon.
reducing the clearance between the valve stems and the tappets will make a
motor sound nice and quiet especially if the parts are badly worn.
"On the other
hand if there is too much clearance the motor will be noisy. It's better,
though, to have them a little too loose than a little too tight."
correct clearance?" Rankin asked.
"Depends on the
car," replied Gus. "The service instruction book will tell you what the
clearance should be. If you haven't got a book, set the intake valves with
about three thousandths of an inch clearance and the exhaust valves at five
thousandths - exhaust valves always need more because they get hotter and
the stems expand to a greater length. If it's an overhead valve motor the
clearance should be greater - about five to seven thousandths for the intake
valves and from seven to ten thousandths for the exhaust.
clearance you use, it's mighty important to have all the intake valves
exactly the same; the exhaust valves also should be alike. Then whatever
noise is made by the valve mechanism will be smooth and steady. If one valve
is a trifle looser than the others you are sure to hear the individual
question I'd like to ask," said Rankin. "I've been turning the motor over by
hand to find when the valve is closed so I can set the clearance. As soon as
I see that the valve is closed I get busy with the adjusting nuts. Is that
"Gus answered. "Turn the motor one half a revolution beyond the point where
the valve closes. That is especially important in a motor that has been
driven considerably, because when the valve closes the valve tappet snaps
down on the cam just past the high part that makes the valve open. This
pounding has a tendency to wear the cam at that point and if you do your
adjusting while the tappet is resting on the worn spot, there won't be
enough clearance on the rest of the low part of the cam.
to inspect the valve springs. Weak springs make the motor run rotten at high
speed. The trouble is that a weak spring doesn't close the valve fast
enough, so that it stays open when it should be closed. The faster you go
the worse the valves work. Of course it is easy to test the spring for
weakness, because a weak spring always is shorter than a new one. So just
compare your springs with a new one, and discard any that are noticeably
"Well, if I do
a bum job the next time I grind the valves it won't be your fault," said
Rankin as Gus finished his explanation, "and if you folks told me that
breakfast food made good valve-grinding compound, I'd almost believe you!"