Rebuilding cylinder heads is not particularly difficult
and may be done in the home shop with a lathe, mill, a tool post grinder
and a few “shop made tools.” I have brought several engines
back to life for little cost by doing the work here at the house.
In this case it is a 1989 Nissan Sentra with a 1600 cc over head cam
engine.
On our evening walks around the neighborhood I noticed a car had been
sitting in a neighbor’s yard for several months, maybe a year
or more. One day I happened to see him outside and asked about it.
He said that the car had overheated and despite replacing the waterpump,
radiator and hoses, it still blows all the water out after only a
few minutes running. I suggested that the head may be warped and that
car may be fixed. He said that he had already bought a new car and
only wanted to get the Sentra out of his yard. I told him I needed
a 4 door car, I could probably fix it and asked what he wanted for
the car. He replied $50 to $100. I offered him $75. He said, “sold!”
I brought a portable air tank over, filled up the flat tires and with
a few friends, pushed it home.
PREPARING THE HEAD FOR MACHINING:
Before removing the head, all the connections are labeled. It is also
a good idea to photograph all the hoses and wires if you have a camera.
All the accessories, cables and the valve cover are then removed.
A spot on the timing chain and sprocket are cleaned and marked with
a “Sharpie” pen. The cam sprocket is removed and the chain
is secured, The “tightening – removing” sequence
of the head bolts is found in a Haynes service manual available at
auto parts stores. The head bolts are loosened in steps, about a quarter
turn at a time. When all the bolts are removed, the head is rapped
with a rubber mallet (shot filled dead blow hammer) to loosen it.
The head is lifted off the engine and set on the bench.
Before any judgements are made as to the condition of the head it
must be thoroughly cleaned. The best cleaner for this job is “Purple
Stuff” or the equivalent water –based product sold at
auto parts stores. It REALLY WORKS WELL, much better than the solvent
based products I have used. When the head is clean, dry it with compressed
air and give it a light coat of WD-40 to prevent oxidation. Carefully
scrape the remaining gasket with a gasket scraper being careful not
to nick or scratch the head. Wipe or rinse the head clean again.
I carefully look for cracks, especially in between the valve seats.
The head looks good so far, now I’ll check for warp with a straightedge
and a feeler gage. The head had .010-inch warp around and between
cylinders #2 and #3. Other than that, it looks pretty good. The maximum
total height that may be machined off the head and block combined
is .008 inch, however the specification allows .002 inch warp. Machining
more than this brings the cam closer to the crank, changing the valve
timing, among other things. There are a number of options here. I
can machine off .008 and live with a little bit of warp or I can completely
clean up the head and get a copper shim to restore the original height.
Being cheap (it was a $75 car) I chose to live with .002 warp in the
head.
I remove the valves with a Craftsman valvespring compressor. Because
many valve springs are wound with variable pitch, I make a note of
the coil spacing. I put each valve and all the components for that
valve in a labeled ziplock bag. The valves are cleaned up later, after
I see how well the head machines. I lightly sand blast the carbon
from the combustion chambers and the ports being careful not to get
sand down in the valve guides where it may become embedded in the
bearing surface. It also a good idea to try to blast away as much
of the deposits in the water jacket as you can. This head is warped
because it over heated. Try to eliminate as many of the possible reasons
for the overheating as you can. Clean water passages can’t hurt.
I blow all the sand out with compressed air and take the head to the
mill.
SETTING
UP THE MILL:
The mill tabletop is cleaned with kerosene. Any burrs, if found are
smoothed with a fine wet-stone. The mill table is wiped clean again
and then I run my bare hand over it to detect any remaining foreign
material. The spindle is “trammed” or set so that it is
perfectly perpendicular to the tabletop so that successive surfacing
cuts across the cylinder head will not have a burr between them.
Tramming the spindle is done by placing a set of precision ground
parallels on the table and checking the height of them with a dial
indicator. They should be the same height in all 4 directions (left,
right, in front and behind the spindle). The dial indicator is mounted
in on a tramming bar that is chucked in the spindle. Although a long
bar is best for aligning the spindle, I have had good results with
a Starrett “last word” indicator and the short bar that
comes with it. I am using a very flat and parallel section of a disc
(Photo1).
I happened to come upon in the scrap yard one day. It is pretty easy
to align the spindle with this set up. When tramming the spindle,
don’t forget to clamp the knee down. This is sometimes overlooked
producing inaccurate results. This can be a time consuming job, but
it is well worth all the effort you put into it. Your cuts will be
much smoother afterwards.
CLAMPING
THE HEAD:
No matter what kind of head you are surfacing, the real trick is to
clamp it to the table without distorting it. This particular head
is machined parallel on the top and bottom. I often clamp through
the sparkplug holes when surfacing “flatheads” however
the sparkplug holes are at an angle in this head, so that is not an
option. The cam bearing journals are all machined flat on top and
they are level with the top surface of the head. I choose to insert
a 1 ½-inch diameter bar through the cam journals and clamp
it as seen in (photo2).
Before clamping, I drill two holes in the bar, carefully clean it
and remove any burrs. I want to be sure that no dirt is on the bar
so that it does not become embedded in the cam bearings. Because the
head has a little warp, and because I want a direct clamping load
(not transmitted through the head to another surface on the head)
I insert parallels under the journals. The clamping nuts and bolts
are lubricated and torqued in steps to 20 foot pounds. The trick here
is to securely clamp the head, but not to apply enough force to distort
the head.
SURFACING
THE HEAD:
I am using a 2-inch carbide cutter to surface the head. It is a little
slow because I have to make several passes to cut the whole width
of the head. I start by setting the cutter to height with a sheet
of paper (photo3).
This paper is .003-inches thick so I raise the table until the paper
just catches. From there I remove the paper, raise the table .003-inch
and set the dial on zero. Aluminum may be a little gummy to machine
so I’ll use a cutting fluid. I often use kerosene, but because
I want a really smooth finish, I use “tapmagic.” It produces
an almost mirror finish. (photo4).
Good surface finish is important on aluminum heads because, when at
operating temperature, aluminum expands at about 1.7 time the rate
of cast iron. The expansion and contraction of the head combined with
a rough surface saws into the gasket material leading to early failure.
Cast iron heads do not need to be as smooth because they expand at
the same rate as the block. I make cuts .002 deep, use a spindle speed
of 1340 rpm and a slow table feed to cut the surface. My final passes
are .001 deep. The head cleans up well, only 2 small places are left
un-machined. I can barely get a .001 feeler between a straightedge
and the low spots. I am happy with the work so I loosen the clamping
bolts in steps and remove the head.
MAKING A VALVE SEAT TOOL:
Valve seats are cut with a grinding stone available from autoparts
suppliers. I am using a 1 ½-inch diameter general-purpose gray
stone made by “Goodson.” It is made around a brass center
threaded 11/16ths 16tpi so that it screws onto a small motor. Using
a lathe, I made an arbor to hold the stone (see drawing). When cutting
the seats, I can turn the stone by hand or use a small drill to drive
it. The arbor is cut from a 6-inch length of ¾-inch steel rod.
The shaft is knurled to make it easier to turn by hand.
The really critical part of the arbor is the insert that centers the
tool over the valveseat. The inserts are machined to the same diameter
as the valve stems on one end and the other is machined to a good
fit in the arbor. When inserting the tip in the tool, airpressure
gives some resistance and when removing it makes a popping sound somewhat
like a cork. I get this fit by drilling and reaming the arbor to final
size (5/16-inch), this give a smooth finish. I then turn the inserts
in the lathe and polish them at high speed with sandpaper until they
have a smooth fit with minimal clearance. Many heads have different
diameter intake and exhaust valve stems. I had to make two inserts
for this head (Photo5).
SETTING THE LATHE UP TO CUT VALVES AND SEATS: The valves are ground
in the lathe by chucking them in a dedicated chuck and cutting the
face with a tool post grinder. I have a set of collets that I use
to chuck the non-metric valves. I use a chuck setup for all my other
nonstandard valves. I am using a ¾-inch capacity ball-bearing
chuck available from ENCO. The chuck is mounted on a 5MT taper adapted
that fits the lathe spindle. I chose this large combination because
I wanted to drill out the center of the taper adapter so that the
valves will fit down into the spindle. My taper adapter is drilled
out ¾-inch diameter and about 4-inches deep. This is easy to
do, just put the adapter in the lathe spindle (without the chuck)
and drill it using a drill-bit clamped in the tailstock chuck. The
large chuck and adapter were fairly inexpensive. Realistically you
can get by with a smaller chuck and drill your taper adapter out to
3/8-inch diameter because most valve stems are 5/16-inch diameter
or smaller.
Valves
usually are ground to 45-degrees. I have had some valves that are
ground to 30-degrees, you will find this on old flathead Continental
engines that are used in old Jeeps and fork-lifts. To improve the
flow, some valve seats have 3 angles, 30, 45 and 60 degrees. If you
want to cut all three angles you will have to have three stones. Set
the compound rest to whatever angle that you need to cut. Both the
valve and the seat cutting stone should be ground without changing
the setting of the compound rest of the lathe, so that the seats and
the valves have the identical angle.
Carefully clean the lathe spindle and adapter to be sure there are
no particles on either one. Any dirt here will destroy the accuracy
of the valve grinding operation. Mount the tool post grinder on the
compound rest and true up the stone with a diamond wheel dresser.
(Be sure to cover the ways of the lathe with cloth or paper to protect
them from the grinder dust.) I made a simple dresser holder from a
section of ¾-inch diameter steel rod (photo6).
It is drilled at 90 and 45 degrees and also has a set-screw on each
side to hold the dresser in place. The dresser may be chucked in the
head or tailstock for cutting the wheel. In this situation it happens
to be in the headstock (photo7).
I have the spindle set in a low gear so that it does not rotate when
I cut the wheel. For the best accuracy, the bearings in the tool post
grinder have to warm up so I turn it on and let it run for 10 to 15
minutes before making any cuts.
Chuck
up an old valve as close to the head as you can as shown in (Photo8).
Set the lathe to run about 300 rpm in the reverse direction. Make
a practice or “test” cut using the compound rest to feed
the grinder across the valve. I have a good supply of old valves and
springs I got from the scrap yard. I took my spring compressor down
there and stripped several heads. Steel was selling for 5-cents a
pound so I may have a dollar or two in all of them.
When you are satisfied with your grinding set up, start working on
your valves. All these valves cleaned up with .0015-inch to .003-inch
infeed. The whole process may take 15 minutes for all twelve valves.
The
valve seats in this head are hardened. They are dirty and only slightly
pitted so I’ll clean them up by hand (Photo9).
Although It may take me 45 minutes to an hour to clean up all 12 seats,
I feel like I have more control over light cuts when I do not use
the drill to drive the stone. The intake valve seats (Photo10)
are cleaning up nicely as may be seen when comparing them to the intake
seats in the photo of the sparkplug tap.
The intake and exhaust valve seats are different diameters. In order
to clear the combustion chamber walls, I must cut the diameter of
the stone down before cutting the exhaust valve seats. I return the
tool to the lathe and cut the diameter down .100-inch using the diamond
dresser in the tool post as seen in (Photo11).
Clean out the sparkplug hole threads by running a M14 – 1.25
tap through them (Photo12).
Cleaning out all the trash helps keep the new plugs from cross threading
and tearing up the head. Because aluminum (head) and steel (sparkplugs)
are dissimilar metals, I always use “Never Seize” on the
sparkplug threads to minimize any corrosion problems. The plugs come
out easily when it is time to change them.
When
all the machining is done, I scrape any remaining gasket material
from around the ports and thoroughly clean the head again. The valvestem
seals are given a coat of oil and pushed into place. The valve stems
are also given a coat of oil and pushed into the guides. The springs
are compressed and the valve keepers are given a coat of grease and
put into position. The grease keeps them in place until the springs
are released. Be sure to pay attention to the springs. Some springs
are wound with a variable pitch. The coils that are wound closer together
close up and come together when compressed in use. This raises the
natural frequency of the spring to prevent surging and resonance.
The springs on the Nissan engine are wound with the tight coils at
the bottom of the spring.
I clean up the camshaft with the “Purple Stuff” cleaner
and blow it dry with compressed air. I chuck the cam in the lathe
with live center in the tailstock. I spin the cam at about 300 rpm
and use a fine (approximately 400 grit) sandpaper strip coated with
oil to polish it up (Photo13).
After I have gone over all the lobes, I reverse the direction of the
lathe rotation and polish them again. This cleans both sides of the
lobes. I thoroughly rinse the cam and give it a light coat of oil
to prevent rust. The journals in the head are oiled up and the clean
cam is inserted.
The head is ready to install. Now a few notes about bolts and tightening
the newly machined head.
TIGHTENING THE HEAD BOLTS:
The head bolts, the head and the engine block are in effect stiff
springs. They deflect when loaded and relax when the load is removed.
They store potential energy and create a clamping force when they
maintain a certain amount of energy. Typically about 90% of the energy
put into tightening a bolt is converted to heat. About 50% is lost
due to the friction at the head of the bolt and 40% is lost due to
the friction in the threads. This leaves only 10% of the input work
as preload or bolt tension. If there is a 10% increase friction because
of dry threads or trash in the threads, then the input work is reduced
an extra 4%. This leaves only 6% of the total input torque as preload
in the bolted joint. A 10% increase in friction results in a 40% loss
in bolt preload. It is very important to clean the threads on the
bolts (or use new ones) and to clean the threads in the block by running
a tap through them. (Some engines, usually very high compression engines,
have rounded tips on the threads to reduce the tendency to crack at
the threads. These threads require a special tap.) After the threads
are clean, you should use a thread lubricant such as Never –
Seize or a Fel-pro product.
Make a final inspection of the cooling system before starting up your
new head. I traced the original failure in this engine back to a weak
radiator cap. The cap let the coolant boil out and when it was low
enough the head ran hot and warped. A $3.65 part caused the original
failure.
This engine starts right up and runs very well. I thought it would
run, but I am surprised how well it runs. With a little troubleshooting,
the airconditioner is brought back to life and blows ice cold (very
important in Florida). It is time for a paint job and new tires. I
make a 320-mile round trip drive to Orlando several times per week
and the car is running great! I was very lucky to find such a car
for $75 and I am very happy with the end result. If you want to try
cutting a head or two, go get one from the scrap yard and practice
on it before you tackle your wife’s car. You can probably get
an aluminum one for 50-cents a pound. Once you have done it, some
of those other restoration or repair jobs are not so intimidating.
Good luck!
If you are interested in more information on the statistics of bolted
joints, then see “An Introduction to the Design and Behavior
of Bolted Joints” by John Bickford. Try a school library or
request that your local library purchase one.
Affordable machine tools may be found at Enco. Phone number 1 800
USE ENCO. Web address: http://use-enco.com
