[DeTomaso] Valve Lash (was "steering rack question")

Daniel C Jones daniel.c.jones2 at gmail.com
Wed Jun 16 12:16:21 EDT 2010


> Dan, so what is your preferred method of setting valve lash on high overlap
> cams?

The valves must be set when the lifters are on the base circle of the
lobe.  At this position the valve is closed and there is no lift taking
place.  The SAE method (a.k.a. the EOIC method for Exhaust Open, Intake
Closed) is to:

 1. Warm the engine.  Once you know the difference between hot and
    cold lash, you can set the lash cold.

 2. Follow the firing order.  You can pick any cylinder but I usually
    start with number one and have a copy of the firing order and
    cylinder numbering in front of me.  I mark the cylinders as I
    complete them in case I get interrupted.

 3. Crank the motor over by hand (socket on the balancer bolt), set
    the intake lash just as the exhaust valve begins to open.

 4. Watching the same cylinder, crank the engine over some more and
    then set the exhaust lash just before the intake valve closes.

The above assumes a solid lifters where you are setting lash but
also applies to hydraulic lifters where you are setting preload.
Although valve lash should be measured with the engine hot, you can
set one cylinder hot then let the engine cool and see what the actual
difference is hot to cold.  Aluminum has a different rate of thermal
expansion so the lash deltas will be different.  A rule of thumb for
When setting cold lash the first time, Crane suggests:

 iron block and heads - 0.002" looser than the hot spec when cold
 iron block and aluminum heads - 0.006" tighter when cold
 aluminum block and heads - 0.012" tighter when cold

These are just starting points and the lash should be checked when hot
and readjusted if necessary.  On a Comp Cams 282S solid lifter cam, Comp
specifies 0.022" lash cold but 0.016" hot.  That assumes iron heads and
block.

Be aware there are "Tight Lash" cams, with lobe designs that have very
short clearance ramps.  "Tight Lash" camshafts cannot deviate from the
recommended hot lash setting by more than +.002" increase, or -.004"
decrease. "Tight Lash" and should be set hot.

In more detail,

1.  Remove the valve covers and pick the cylinder you are going to adjust.
If your cam is already broken in, do this with the engine fully warmed up
(takes thermal expansion into account).  If installing a new cam, use the
cold specs before firing the engine, then re-lash after cam break-in with
the engine hot.

2.  Hand turn the engine in its normal direction of rotation while watching
the exhaust valve on that particular cylinder.  When the exhaust valve begins
to open, stop and adjust that cylinder's intake valve.  When the exhaust is
just beginning to open, the intake lifter will be on the base circle of the
lobe, ready to be adjusted.

3.  With a solid cam, use a feeler gauge to set the correct valve lash.
Place it between the tip of the valve stem and rocker arm.  Turn the
adjust until you arrive at the proper setting then lock the adjuster in
place.  In my TR8, with hydraulic lifters and adjustable pushrods, I
shortened the pushrod until I got zero pre-load (lengthened pushrod
until free play was gone), then went one turn on the adjuster.  I used
a thread pitch gauge to verify my adjuster was 32 turns to the inch.
1/32 = 0.03125" or 31.25 thousands (with the Buick/Rover aluminum V8
30 thousandths cold is a good goal with a high performance hydraulic
flat tappet cam).  If I'm feeling paranoid, I also used a bent wire
gauge under the lifter clip to verify the pre-load.

4.  After the intake valve has been adjusted, continue to rotate the engine,
watching that same intake valve. The intake valve will go to full lift and
then begin to close.  When the intake is almost closed, stop and adjust the
exhaust valve on that particular cylinder.  When the intake valve almost
closed, the exhaust lifter is on the base circle of the cam.  Follow the
procedure in step 3 to set the lash or pre-load.

5.  Both valves on this cylinder are now adjusted, so check the firing
order and move to next cylinder and repeat the procedure.

The SAE method is more accurate than the 90 degree method described in the
Ford manual and the more overlap the cam has or the less piston-to-valve
clearance you have, the more important this method is.

Be aware that some hydraulic lifters like the Crane link bar retrofit
roller lifters bleed down slowly which can make it difficult to adjust.
With those, set them cold after they've bled down for a day.

Dan Jones



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