RE: Honing cylinder liners - good or bad??

["John Fielding" <johnf@domain.elided>]

Hi Folks,

All this talk of honing cylinder liners so you can fit new rings onto a possibly worn
piston has me a bit jittery.

Honing is a process used by original manufacturers and cylinder block restorers to remove
the final tiny amount of metal to bring the cylinder to final size and completely round,
with a fine criss-cross series of surface scratches which retain oil and allow the rings
to bed in satisfactorily.  For those who aren't aware, when reboring a cylinder, to say
30thou (mil) oversize, the boring machine will remove sufficient metal to allow honing to
final size, normally a cut of 26 thou (mil).  In many cases this will be about 4thou
(mils) left to hone off.  The important thing to understand is that the boring operation
makes the cylinder truly round, perpendicular to the block face and removes any taper.
In a worn cylinder the bore will  be least worn at the top (because the top ring cannot
reach this far up the bore) and the cylinder will be oval due to the wear which occurs on
the thrust side of the bore from the piston, ie the worn bore will be both oval and
tapered along its length.

Honing a liner or block is normally done with an automatic honing machine, it
can be done by hand with a drill and a suitable set of honing stones but it requires a
definite skill only learnt through many years of practice, and one can presume a number of
poorly honed cylinders!.  To be able to "feel" the slight tightness as the honing stones
are run up and down the bore, because the bore is slightly tapered and oval, is something
I haven't mastered yet!

The bottom line is this:  If the liner has minimal wear and hence no perceptible ridge at
the top of the bore, better yet you can actually see with a naked eye the original honing
marks on the liner all the way down and around the bore, then the liner is probably usable
without honing.  To start honing a liner without at least measuring the bore ovality and
taper is totally stupid.  How on earth can you tell how much metal and from where in the
bore needs to be removed to bring it back into true roundness and free from taper??

Free-hand honing will not normally remove a taper or ovality without a conscious effort on
the operator to do so as the hone will naturally follow the average bore diametre and
taper, and he needs to know where in the bore the problem areas are to address this
problem.  IMHO honing a liner in situ is not a correct method.  The liner will rock in the
block even though it appears to be solidly mounted in the ali bottom of the block.  The
best solution is to remove the liners and have a competent machine shop perform the task
on an automatic honing machine were the stone pressure and taper can be acurately measured
and controlled.  I had this done on the Alfetta motor built for racing, here an extra bore
clearance was needed to suit the pistons and rings being used.

Fitting new rings onto an old piston is also a bit of  a Russian roulette game.  If the
ring grooves are within spec all well and good.  But the continual up & down movement of
the piston causes the top & bottom of the ring grooves to wear, the rings are much harder
than the ali piston and the point may come where the ring groove is no longer small enough
to control the compression rings correctly, thus causing the rings to cock lengthways in
the bore and excessive friction and wear result.  Without accurately measuring the ring
groove wear you cannot assume the piston is able to accept new rings.  One can in certain
instances buy over size thickness rings to counter this problem but they are difficult to
locate in many cases.  And before these can be fitted the ring grooves need to be machined
to create the correct clearance, not a cheap solution.

As far as knurling pistons is concerned, well this is a cheap and dirty fix.  Consider
that in a few hundred kilometers the high spots caused by the knurling will have been worn
away and the tiny pieces of ali will descend into the oil pan causing untold havoc to the
the rest of the bearing surfaces!  The piston will now have more clearance than before the
knurling took place, not a very wise decision in my book!  In any case pistons are not
made to be round, they are machined with either a barrrel-shaped exterior or an oval
section depending on the manufacturer.  Knurling will tend to make the piston round and
this upsets the fine
clearances desired, hence the heavy wear on the thrust side of the piston when the motor
is first run.

just my 2cents worth.

John
Durban
South Africa
Alfetta 1.8L turbo
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