Your Heads

[Mike Wattam <101714.1343@domain.elided>]

Hi Tom

POLISHING ALUMINIUM
Most aluminium alloys will not polish well, because of the 
materials used in the aluminium.  This is mainly because
aluminium itself oxidises (forms a coating of oxide-like rust)
within a few seconds of cleaning, but a few types of 
aluminium can be polished to a mirror finish, if the surface
is very flat and free from scratches.  So, look very carefully 
at the material you are going to polish and make a small 
sample area flat and then polish it, to see what happens.  

One of the best aluminium polishes to use is called 'Solvol
 Autosol'.  This comes in a tube (like Toothpaste) and is abrasive.

VALVE SHIMS
Feeler guages are very difficult to use, and to make an accurate
measurement needs a lot of experience.  With a very thin feeler 
this is particularly difficult.  The principle is that you should be able 
to insert and remove a feeler with very light finger pressure, feeling 
the resistance from the surface of the two components without any 
forces being exerted.

The correct clearances for the valves are
inlet   0.006" to 0.008"
exhaust 0.016" to 0.018"

So there is a tolerance of 0.002" on each valve or 2 thou (= 2 
thousand parts of an inch).

The shims are made in steps of 0.001" or 1 thou, between 0.090" 
and 0.114".  This should be enough for every valve to be correctly 
adjusted to the correct clearances shown above.

However, if the valve or valve seat has been ground back a lot, 
the 0.090" shim may still be too thick.  If this is the case, many 
Stag engine repairers will grind down the shim with a surface 
grinder to make the correct valve clearance possible.  If this is 
done, it is very important also to make sure the top valve spring
collar does not hit the underside of the tappet bucket so the 
clearance is still wrong.

The valve clearances should be quite easy to correctly set, if you 
have a micrometer to accurately measure the thickness of shims 
fitted.  Then if you follow the procedure shown in workshop manual 
page 12.29.48, everything should work out fine.

Some of the measurements you show in your e-mail are in 
millimeters, and this could explain the difference you have noted.

Also, when fitting new valve shims, be sure to check that the new 
shims you are fitting do not have their edges chamfered back, but 
are sharp-edged.  Chamfered shims can jump out of the top valve 
spring collar and jam up the valves.

CAMSHAFT
If you suspect the camshaft is running tight in its bearings, you 
need to do 2 things.

1. with the cylinder head on the bench, take out all valves and refit 
the camshaft.  The camshaft should rotate freely.

2. fit the cylinder head to the cylinder block and tighten it to the
correct 
torque - a cylinder head gasket is not required for this.  Now turn the 
camshaft, and it should still be free.

If 1 and 2 are completed successfully, no action is required.  But if 
you have a problem, see below.

If the camshaft is tight in the cylinder head while the cylinder head is 
still on the bench, but it then turns freely when the cylinder head is 
fitted to the cylinder block, no action is needed.

If the camshaft is tight both on the bench and on the cylinder block, 
the camshaft bore must be re-machined.  This can be done with the 
cylinder head fitted to the cylinder block, or on its own.

If the camshaft is free on the bench, but tight when on the cylinder 
block, the cylinder head is probably distorted. 
 ---     If the surface of the head is good, try to find a machine shop 
which will re-machine the cylinder head while it is still fitted to the 
cylinder block.
 ---    if the cylinder head surface is not good, have it machined 
TAKING AWAY THE ABSOLUTE MINIMUM AMOUNT OF MATERIAL, 
then refit it to the cylinder block, to make sure it will now turn freely.


Tom, I hope the above helps you, if not we'll sort it out at the Autumn 
meeting in the Black Forest!

Mike Wattam
Trimph Stag Register