When an engine is rebuilt, the cylinders usually need attention. Wear tends to create taper in the upper part
of the cylinder that can reduce ring sealing and increase blowby and oil consumption if not removed.
The cylinder may also be out of round, scored or have other damage that requires correcting before a new set
of rings will seal properly.
The main objective when refinishing the cylinders is to make the walls as straight as possible (no taper),
the bores as round as possible (minimal distortion, which is especially important with low tension rings),
to have the right amount of crosshatch for good oil retention and ring support, and to produce a surface
finish that meets the requirements of the rings. This is done by boring and/or honing the cylinders in
one or several steps with various types of abrasives (vitrified or diamond).
After honing, the cylinders need to be cleaned remove residual abrasive and metallic debris that is left in
the bores. Washing and scrubbing with warm soapy water will remove most of the unwanted material. But washing
alone does not loosen or remove surface "swarf" such as torn or folded metal that can wear rings and delay
ring seating. The only way to get rid of this material and smooth the bores is to "polish" the bores after
honing with some type of flexible abrasive brush.
Brushing after honing not only helps clean the bores, but can also plateau the surface depending on the
characteristics of the abrasive used. Brushing sweeps away the torn and folded metal as well as the sharp,
jagged peaks leaving a much smoother surface. The result is a better bore finish with little extra effort.
Another way to plateau the surface is to use very fine #600 grit stones or cork to polish the bores after
honing. One of the advantages of a plateau bore finish is that it preconditions or breaks-in the cylinders.
This reduces the time it takes to seat a new set of rings as well as initial ring wear, blowby and oil
consumption. The engine delivers good compression right away, there is no blue smoke in the exhaust,
emissions and oil consumption are reduced, and the rings last longer because they have not had to wear to
conform to the bores. A plateau bore surface also provides increased bearing area to support the rings
while retaining enough depth in the crosshatch for good oil retention and lubrication. That is why the
original equipment engine manufacturers (OEMs) favor this type of bore finish and use it in many new engines
(see sidebar on how to plateau hone a Ford 4.6L V8).
CYLINDER HONING CHALLENGES
One of the concerns expressed by OEMs who have engine reman programs is that many aftermarket engine
rebuilders may not have the know-how or right kind of honing equipment to reproduce an OEM type of cylinder
bore finish. With emissions testing a fact of life for many motorists in many parts of the country, the
worry is that a rebuilt engine with cylinders honed the "usual way" may not pass an emissions test. The
challenge here is to develop procedures that allow aftermarket engine rebuilders to duplicate an OEM bore
finish.
Ring manufacturers are concerned that some engine rebuilders may not be using the proper honing procedures or
stones for their rings. Too rough a bore finish will produce a lot of scrubbing when the engine is initially
fired up. With prelapped rings, this is not good because it creates unnecessary wear. The challenge here is
to use honing procedures that produce the best possible bore finish for a given set of rings.
RECOMMENDED CYLINDER BORE FINISH
Most ring manufacturers specify a #220 grit honing abrasive for finishing the bores when using cast iron or
chrome rings because the recommended bore finish for these rings is 28 to 35 RA (roughness average in
microinches). A #280 grit stone is generally recommended for moly rings because moly rings like a somewhat
smoother finish of 16 to 23 RA. But these recommendations are for conventional vitrified abrasives, not
diamond. Diamond cuts differently from a vitrified stone, so higher numbers are generally required for an
equivalent finish. A #325 to #550 grit diamond stone may be required for the final honing step to achieve an
RA finish in the desired range. One manufacturer says a 500 to 550 grit diamond honing stone will produce a
surface finish in the 13 to 15 RA range.
To add to the confusion over which honing stones may be required to produce a certain kind of finish, some
vitrified honing stones with identical grit ratings will produce different finishes that may not always agree
with the reference charts.
For example, one #220 grit vitrified stone may produce a surface finish of 28 to 35 RA while another may
leave a much rougher finish of 35 to 60 RA, which is too rough for most prelapped rings. The difference in
actual surface finish is due to the bonding agents and fillers that are used to hold the abrasive particles
together. The type and quality of lubricant used during the honing process can also make a difference, too.
The third challenge is profitability. Cylinder bore refinishing is a time-consuming and expensive step in the
engine rebuilding process. So anything that can be done to reduce honing costs and streamline the procedure
while also improving the bore finish is a step in the right direction.
CONVENTIONAL HONING ABRASIVES VS. DIAMOND HONING STONES
Many shops bore or rough hone cylinders to within .003 inch of final oversize (.010 to .030 in. depending on
the application), then finish hone the last .003 in. of the bore with #220 or #280 grit vitrified abrasives.
Most shops do not have a profilometer to measure surface finish parameters such as RA, RK (core roughness),
RPK (average peak height) and RVK (average valley depth), bearing area and so on, so they rely on stone grits
along with the right honing pressure, head speed, stroke rate and lubricant to achieve the desired bore
finish. Consequently, there�s no way to know if the bore finish actually meets the requirements of the ring
manufacturer or the OEM, unless someone complains about excessive ring wear, blowby or oil consumption. But
even if you have not experienced any ring problems, it does not mean the cylinders are as good as they could
be or should be.
One of the limitations of vitrified abrasives is that they wear rapidly. Depending on the grade of stones and
the hardness of the block, a set of vitrified honing stones might do three V8 blocks (24 cylinder bores)
before they are worn out and have to be replaced. And with each cylinder that is honed, the operator or
equipment must compensate for stone wear to keep the bores straight. If you fail to compensate, you can end
up with taper in the bores.
By comparison, metal bond diamond honing stones wear very little. A set of diamond honing stones might do 300
V8 engine blocks (2400 cylinder bores) before they have to be replaced. The slower wear rate means the stones
tend to cut straighter (less taper) than vitrified stones.
The slower wear rate of diamond versus vitrified abrasives helps to more than offset the much higher initial
cost of diamond stones. Using the above figures, a set of $14 vitrified honing stones cost about 58 cents per
hole if they do 24 cylinder bores. A $300 set of diamond honing stones cost about twelve and a half cents per
hole if they set does 2400 cylinder bores.
For these reasons, many production engine rebuilders (PERs) have switched to diamond honing. Diamond lowers
their honing costs per cylinder, saves labor (fewer stone changes), and gives better overall bore geometry
(straighter and less distortion).