tech38

Need a Rebore Job?
by Lynn Bennett

The old Brit bike is smoking a bit more than usual, the powers down a
little, and the mileage since the last re-build is getting up there. It
must be time for a new (set of ) piston(s). What should you know about
the process and the theory of pistons operating in a bore? Well, here
goes on all that I know.

As probably all are aware the internal combustion engine uses a piston
reciprocating in a bore to make power: a rod connecting it to the
crankshaft on one end of the piston and burning fuel/air mixture on the
other end pushing it. Pistons on most Brit bikes are aluminum while the
cylinder material that the piston moves in is cast iron. The cylinder
might be solid cast iron or an aluminum barrel lined with a thin cast
iron liner held in by a tight fit and the head bolts. But since the
materials of the piston and the barrel (or barrel liner) have different
expansion rates (aluminum, as in the pistons, expands more rapidly with
temperature increases than does cast iron, as in the barrel or liner)
fitting of the piston to the bore while all is cold must take into
account what the final sizes will be at the operating temperature of
the motor. To add to that, the alloying of the aluminum piston with
compounds like silicon and the manufacturing technique of the piston,
like cast verses forged, change the piston expansion rate. The fact is
that forged pistons expand more than cast pistons, for whatever reason,
so more clearance between the cold piston and bore can be necessary.
The expansion rate of the pistons is controlled by the piston
manufacturer so the required piston to bore clearance should be his
call and not some arbitrary rule of thumb number. But some prefer to
use rule of thumbs as a check and for cast pistons they should be fit
with 0.001 inches of clearance for every inch of bore diameter. For
example, a 75 mm diameter piston (3 inches, approximately) needs 0.003
inches of clearance cold, minimum.

The way piston and bore wear is supposed to work is that the piston
should have minimal wear while all the wear should be restricted to the
cast iron bore. As the bore wears the piston becomes a looser and
looser fit into the bore and this, if the wear is excessive, can cause
the piston to rock in that bore. Continued rocking under the pressure
of combustion can cause the skirt of the piston to crack and break off,
falling into the crankcase and causing un-imaginable amounts of damage
there.

Most Brit bikes only allow the bore to wear to 0.006 inches over its
nominal fitted size. For example, for our 75 mm piston (3 inches) we
start with a piston to bore clearance of 0.003 inches. We declare that
a new bore and piston is needed when that clearance grows to 0.006
inches. That means we have 0.003 inches of wear allowed (0.006 total
minus 0.003 fitted). But, to avoid any possibility of seizure (where
the piston grows due to temperature to the point where there is no
clearance at all and stops abruptly in the bore) we could choose to fit
the pistons to 0.004 inches cold. We have only removed 0.001 inches
from the available amount of bore wear or about 30%. There is no
adverse effect, other than a reduced wear cycle, for using a looser
initial cold fit. In fact most Brit twins are fit in that 0.004 to
0.0045 inch range and if fit tighter can seize (Please don’t ask me how
I know that!).

How do we measure that clearance? First we must know how round pistons
are made. But….., they are not round! The piston skirts that support
the piston front to back are wider than the measurement across the area
of the skirt that supports the piston pin. Additionally, the top of the
piston is smaller than at the bottom of the skirts. So pistons are
elliptical when viewed from the top with the larger dimension across
the thrust faces of the skirts (front to back) and tapered when viewed
from the side with the top being narrower than the bottom at the
skirts. But this is at cold temperatures. When hot the piston grows
asymmetrically to a round cylinder (so says the books). To measure the
bore to piston clearances we must compare the diameter of the piston
skirts, about a half to a quarter inch above the bottom, to the
diameter of the bore. The easiest way with a new bore and a new piston
is to insert the piston upside down in the bore and insert feeler
gauges in the bore to piston clearance area. The largest feeler gauge
that can be slid in is within about 0.0005 inches of the actual
clearance. This is close enough of a measurement to verify the
machinist did his job right (they do make mistakes as I found out years
ago).

How do we check a worn bore? That is much harder. The bore does not
wear uniformly around its’ diameter or up and down in the bore. The
thrust surfaces, usually front to back wear more than the piston pin
surfaces. The bore will wear differently at the top than the bottom due
to the high pressures of combustion when the piston is at the top of
the bore. Manufactures usually give you guidelines as to how out of
round and how much taper the bore can get before it is time for a
rebore. Any good rebore shop will measure your cylinder for you to help
you determine if the bore is worn out.

When the bore is determined to be out of spec, we need to rebore the
cylinder and put in the next size bigger piston. Pistons come in
increasing larger diameters usually in 0.010 inch increments from
nominal to maybe 0.080 inches. Some manufactures only provide 0.020
inch increments. Some machinist are so poor at setting up their
machines that they insist on going up two steps instead of one and
sometimes the bore is so crooked from wear that it requires to go up
two steps just to get it straight again or to take out gouges in the
surface. Taking extra steps increases the likely hood of running out of
available piston sizes. When you can no longer get the next size you
have to replace the barrel or the liner, both expensive items. Some
barrels or liners can only be rebored a rather limited number of times
as the structural rigidity becomes an issue if taken too far.

Now that we have a fresh bore and new piston, all must be cleaned, as
the honing process (the final stage of a rebore job) leaves minute
pieces of honing stone in the surface of the bore. Such particles are
extremely erosive and if left, will wear out the piston instantly.
Solvent or soap and water will not remove them. The barrel
bore(s)should be wiped out after the solvent and soap and water
cleaning with a paper towel lightly saturated with oil. The oil
captures the honing stone material into the paper towel so multiple
wipe downs is necessary until the paper towel comes out clean. Do not
flood the bore with copious amounts of oil or the rings will take
forever to seat. Just wipe down the bore with enough oil to keep it
from rusting. Don’t inundate the piston either. A light coating as if
you were protecting it from rust is adequate. The same goes for the
rings. This is considered “Dry Fitting” and assures that the rings seat
nearly instantaneously at startup.

Talking of the rings, be sure to measure the ring gap, verifying it is
within the manufactures specs (Piston and ring manufacture’s spec!) and
if not, filing the ends of the rings to get it correct. Measure the end
gap by putting each ring individually into the new bore and squaring it
in that bore with the top of the piston pushed into the bore after the
ring. Use feeler gauges in the end gap to do the measuring. If set too
tight the rings can expand, bind and break. If too loose their
efficiency as seals will be diminished. The fit of the rings to the
grooves on the piston is commonly given in manuals (ring side
clearances, I believe most manual call this measurement). It does not
hurt to verify that yours are within spec for if they are not and too
loose they may not seal correctly or if too tight may seize in the
lands and not seal correctly.