shortcut adjusting valve tappet clearance

1976 CJ360T (two cylinder, single overhead cam)

My owner's manual calls for a 0.05 mm (0.002 in) clearance, to be
checked with a feeler gauge.

The threads on the tappet adjusting screw are 1 mm apart. I am thinking
I can tighten the screw until I feel resistance, then back off 1/20 of a
turn and tighten the lock nut. That ought to be 0.05 mm. Would a good
mechanic do it that way?

 

Nope. First you have to determine 1/20 of a turn which is sort of tough.
Then you need to allow for the nut tightening which increases the
clearance AND also there is some slop in the system. A good wrench sets
the gap so that there is resistance on the gauge when you pull it out.
Some say you should not be able to re-insert the gauge, but I don't do
it that tightly.

 

No. A good mechanic would use a feeler gauge. It's not like they're
super-expensive and/or unobtainable, ffs.

 

I forgot to ask about the tolerance on this adjustment. My manual
doesn't give a range, just a single number. I figure it can't be too
critical, since an extra 1/20 of a turn doubles the value.

 

I guess my question is about how critical the measurement is. Valve
tappet adjustment has always been troublesome for me. It seems that
either I can't get clearance for the smallest gauge (0.015 mm) or I can
get clearance with a much bigger gauge (say 0.076 mm).

 

Oh, for heaven's sake. Let's say you haven't got a 2 thou blade, then.
Right:- to get a clearance of 2 thou without a 2 thou blade.....

1 Get a 4 thou blade.

2 Set the clearance to zero.

3 Set the clearance to 4 thou, counting how many turns of the adjuster
it takes to do that. Let's say, for the sake of example, it takes four
turns.

4 (a) Back the adjuster off halfway (two turns) or:

4 (b) Screw it back in again and open it out halfway (two turns).

If you haven't got a 4 thou blade, but have a six thou blade - follow
the above procedure, but this divide the number of turns the adjuster
needs by three.

If you only have an 8 thou blade.... but I'm sure you have got the
picture by now.

It really, really isn't rocket science.

Correct valve clearances are critical for some bikes and not critical
for others. Can't really be more specific than that, I'm afraid. Just
set them to the recommended gap and you'll be OK.

 

I use an inch-size feeler gauge, instead of a metric gauge.

1.0mm = 0.0394 inch
0.05mm = 0.00197 inch
0.06mm = 0.002364 inch
0.07mm = 0.002758 inch
0.08mm = 0.003152 inch
0.09mm = 0.003546 inch
0.1mm = 0.003940 inch

The difference between, a 0.05 mm feeler and a 0.06 mm feeler is
0.000394 inches. That's less than four 10000ths of an inch (five
10000th's is half a thousandth) and that's a finer adjustment than you
really need to make.

It's really hard to feel that difference in friction as you slide a
feeler gauge between a shim and a tappet, and when you're working with
screw type adjusters, tightening the lock nut is automatically going to
change your
valve clearances, so why fuss with the metric gauges that are ground so
accurately?

No, I've never heard of such a thing. I usually do a Go/No Go
adjustment for clearance checking only, since I want the valves to stay
looser than the minimum measurement, especially with an inverted bucket
and shim valve adjusting set up. I'm happy when a feeler gauge of the
right size or 1/1000th of an inch larger than nominal will go in, and I
start to worry when I can't get the right size or 1/1000th of an inch
smaller between the lobe and the bucket. That means the valve has sunk
into its seat because it's burnt...

You're wrong there. You're flirting with burning a valve with
clearances like 0.05 mm on an air-cooled motor. The valve head gets
hot, the valve stem grows longer, your clearnaces get smaller...

Really tight valves not only burn, the cam lobe gets worn excessively
from the sliding friction of the rocker or inverted bucket tappet...

I'd be using about a 0.003 inch feeler or a 0.004 inch feeler to set
the valves, and see if that sounds too noisy when the engine gets hot.
The exhaust valve typically has to be a bit looser than the intake
valve, as the exhaust valve runs hotter. If the valves are too noisy
hot at 0.003 inch, I'd try 0.07mm to see what that sounds like when the
engine is hot...

I know about 0.07mm being too tight for an exhaust valve, because I
thought I could save myself the hassle of readjusting the valves on my
GS-1100 if I set them at the minimum clearance. I figured the cam lobes
would wear and the clearance would get looser. I was wrong. The valve
seats eroded and the
clearance got smaller as the valves burned. Sixteen new valves at $15
apiece later...

With screw type adjusters, the sharp tip of an adjuster screw with too
much clearance will hammer a crescent-shaped divot in the top of the
valve stem, making it difficult, if not impossible to adjust with a
feeler gauge.

There you have the range of problems you can run into with valve
adjustments. You could still use a dial indicator to adjust divoted
valves, though...

 

Bad day at work, dear? ;-))

 

A 'firm sliding fit' is what I set to.

No jokes, please.

 

1976 CJ360T (two cylinder, single overhead cam)

A good mechanic would gap them the proper way.....period....otherwise he
ain't a good mechanic...

Fwed

 

Some code name for 'dumbass', I suppose.

 

Thanks for your help. I have a feeler gauge.

Not to upset you, but I think you missed the point that one turn is one
millimeter. So two thou is 1/20 of a turn. Now maybe you see why I
feel like I am splitting hairs with the tappet adjustment. By the way,
a half a hair's breadth IS in fact rather close to 0.05mm, the spec for
the intake tappet clearance.

Thanks to all who indicated the danger of burned valves and worn cam
lobes. I will be sure to not get the rockers too tight.

 

I have one, and it set me back $6 the other day.

Closest possible match to FFS at acronymfinder.com was "fumble-finger
syndrome".

 

I normally figure if a guage will fit and the next size up
(.0025" or .003" in your case) won't fit, I've got it right.

Wyncha put a .002" guage under the tappet then tighten it down
until you feel resistance then check it ?

 

The usual advice here seems to be "it should feel about as tight as
inserting the blade between the pages of a phone book".

big

 

No matter the accuracy of the thread pitch, or your accuracy
in rotating the screw, it's all out the window when you tighten the
lock nut.
Like others here have written, I've found it helps to use the
"Go, No-Go" method. When you think you have it set, try a feeler
gauge .001" thicker. (Actually, it probably WILL go through, but
you'll know it's too tight if the adjustment is correct).
You really can "feel" a thousandth of an inch.

 

*Nice*

And true.

 

Hmmm ... the manual says, "NOTE: The checking or adjusting of the tappet
clearance should be performed while the engine is cold. The clearance
may tend to increase as the temperature rises."

 

That part is true.

I think they are trying to say that if you set your valves when the
engine is hot, you may notice that the valves are noisy when the engine
is cold. You don't run your engine cold for very long...

The manual for my Nissan pickup truck specifies adjusting valves when
the engine is *hot*...

You absolutely must have some valve clearance when the engine is hot,
regardless of any temporary clatter when the engine is cold...

Heat makes metal parts expand. The valve head gets hot, heat transfers
to the valve stem, which gets longer, taking up what little clearance
you have between the end of the valve stem and the threaded adjuster's
tip...

When the valve clearance is gone, the valves cannot seat, they cannot
transfer heat to the valve seat, so they get even hotter. The valve
head expands until it does touch the valve seat. But it is so hot from
the lean mixture that results from valves that don't seat properly, it
momentarily *spot welds* itself to the valve seat...

When the valve is pushed open again by the cam and rocker, that tiny
spot weld breaks loose, tiny bits of cherry red hot glowing steel fly
out the exhaust part. When a mechanic inspects the valve, he notices
that the seating margin (which should look smooth and shiny) looks like
mice have been chewing on it...

I have a Clymer manual here for the earlier 250cc to 305cc Honda twins
that were built from about 1959 to 1968, when a new generation of twins
was introduced. My Clymer manual specifies 0.10 mm for both intake and
exhaust valve clearances...

I find it very hard to believe that you could run an air cooled
4-stroke with only 0.05 mm valve clearance without having the valves
expand and take up that tiny amount of clearance and burn...

 

Early Yamaha XS-2 650s specified .002" clearnace, cold, for
both inlet and exhaust. I know of several that ran reliably for quite
a long time.