Engine Rebuild - Prepping the Heads - Step 5
By Ryan Ballou
With the shortblock
completed, we turn our attention towards getting the heads ready for installation.
I'll leave the topic of porting and chamber modifications for the experts,
but there's still plenty to do for the average hobbyist like you or me.
I'll be re-using the heads from my last engine, though they have been
completely rebuilt for this new project, i.e. new seats, guides, valves,
and so on. When I got my heads back from the shop, the first things were
sealing surfaces, specifically cylinder to head and valve to seat. Checking
the cylinder sealing surface is pretty easy. Apply a little machinists
bluing to the head and gently lap a cylinder into the head. With the cylinder
removed again, you're looking for an uneven contact pattern, indicating
high and low spots. Ideally you'll have a perfect ring appear if the surface
is true. Good thing I checked because mine weren't. Heads that have some
hard miles on them like mine are prone to warping over time, in my case
it happened as a result of welding up a few small cracks during the rebuild
process. Luckily it proved to be minor and cleaned up with a .004"
Next you're going to use that same bluing to check the valve margins,
i.e. the contact patch from valve to seat. Apply the bluing to each of
the seats, and use a hand-lapping tool to spin each valve on its seat.
Make sure to label each valve and it's location so they don't get mixed
up if you remove them. The width of the contact patch is called the valve
margin. Factory spec for this is .050"-.100", though the expert
head guys have favorite numbers they shoot for. Nonetheless, it's a safe
assumption that if your margin measures larger than the top end of spec,
the heads probably need attention. You're also looking for high and low
spots again making sure that the valves make even contact with the seats.
Even though the heads just received a fresh valve job, it's always a good
idea to check this for yourself, and always, always check new heads. Usually
a little time spent lapping the valves in will clean up any imperfections
and assure a good seal.
With all that squared away it's time to measure each head chamber for
volume. You'll use this number not only to calculate/set compression ratio,
but you also want to do what you can to match all four chambers. If you
search online you'll find head CC'ing kits available consisting of a syringe
for measuring liquid, and a piece of plexiglas to seal the chamber. The
problem with using a syringe is that they usually measure to the nearest
whole CC, personally I'd like more resolution than that. This is why I
use a 100mL burette with a .2cc resolution, .1 if you interpolate. Burettes
can be found at most well equipped scientific supply stores. Good ones
are a little pricey, but if you were to pay someone to do the job they
allow you to do, the cost would be much greater. Then I just bought a
sheet of plexiglas from the local hardware store and cut it to match my
Install a set of spark plugs into your heads to seal them up. Take your
piece of plexiglas and set it in the head. Use a felt pen to make a mark
near the edge of the chamber that you can pour liquid through then drill
a hole there. Set up the head so that the chamber is level and apply a
light film of grease to the piece of plexiglas so that it will seal against
the head and set it in place. Now fill your burette up with some water
that you've added a drop of soap to. The soap will help break the surface
tension, which will help keep bubbles out. I also like to add a drop of
food coloring to make the water easier to see. Record the volume of water
in the burette for your starting point then fill the chamber up through
the hole in the plexiglas. Record the volume of water left it the burette
and the difference is your chamber volume. It's best to repeat this procedure
for each chamber at least one or two times and average your numbers. Repeatability
is key here, if you end up with more than a .5cc difference each time
then there's a problem. Air bubbles can easily stick to the spark plug
electrode and throw your numbers by nearly 1cc.
Any difference between chambers of more than 1cc should be dealt with.
If you're a perfectionist like me, then you'll go straight for the resolution
of your burette, .2ccs in my case. Since you can't remove volume from
an individual chamber, you have to add volume to those chambers that are
smaller than the one that is the largest. The best place to remove material
from is the chamber walls around the valves, this is called unshrouding.
Try to keep the general shape of all the chambers the same, after all
if you paid big bucks for a professional porting job, you don't want to
go and mess things up. You may want to set a cylinder in the head and
use a marker to outline the inner dimension of the cylinder wall. This
will ensure you don't go too far.
Once the chambers are all squared away, we can turn our attention to the
other side of the head, the valve springs. If you're going to install
dual springs like I am, you need to have the guide bosses cut down to
make room for the inner spring if it hasn't been done already. Make sure
your shop doesn't cut them more than needed to clear the spring. Many
shops will just use a pre-set cutter and take too much material off. You'll
also want to check for retainer to guide interference. This is done with
the spring/s removed. Set the retainer on the valve, and hold it in place
against the keepers with your hand. Use a dial indicator to open the valve
to the same height your cam will. Now make sure you have at least .120"
of extra clearance. If things are too tight, you'll have to take the heads
to your shop and have the guides cut down to add clearance. Really, you
just want a little more clearance here than clearance you have until coil
bind. Finally check for coil bind and shim as needed to achieve about
.100" of clearance before coil bind. For details on this procedure,
see my previous article found here.
Having checked these things on your heads, they should now be ready to
install on the engine. Be sure to keep them bagged and clean until you're
ready to install them.
1-The small welds
under each spark plug boss were to repair small cracks under each
of the intake seats. In fact cause for tearing the engine down in
the first place was an intake seat that almost fell out.
seats installed and a fresh three-angle job on all of them. I had
quite a bit of clean up work to do in the ports, notice the lip under
the exhaust seat?
the outer edge of the chamber and you'll see a dark ring in the bluing.
If this ring is continuous and uniform all the way around, then the
surface is true and not warped.
4-Checking the valve
seal is just like checking head seal. You're looking for a uniform
mark in the bluing (see arrow) to show the valve is seating properly.
burette setup and ready to go. Make sure you're extra careful is your
burette is glass like mine.
error is all I can say. This was the first piece of plexiglas I cut
out and didn't think to drill the fill hole near the edge. I never
could get rid of those bubbles.
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