R60/2, etc:
Pistons
(including swapping heads-cylinders-pistons), rings, pins, clips,
keepers, de-carboning
for your BMW Airhead Motorcycle
© Copyright, 2011, R. Fleischer
pistons.htm
Article 60, sub-section 4
De-carboning:
Decarboning of the top end, heads, pistons,
etc. on a BMW Airhead Motorcycle, is best done with the pistons and rings removed and
separated from each other, and the head disassembled.
However, I don't really suggest that you take the top end down
that far just to de-carbon it. Removing the head and
cylinder enables the head to be soaked in something like Gunk
(brand) Hydroseal; or Simple Green. The piston with rings
still in the grooves can be removed from the rod, and similarly
soaked, rotating the rings now and then. Removing the
piston pin rearwards, after removing the rear piston pin clip, is
usually fairly easy, if the piston is heated with a
hair-dryer. I always replace any clip that was removed.
DO
NOT scratch the pin bore area in the slightest. If need-be,
use a draw-bar method, if the pin is too tight, even with the
piston quite hot.
DO NOT let the rod fall and nick the
crankcase!!!! Some will leave the pistons/rings/rod-to-crank all intact,
just removing the head and cylinder, and use a heavily soaked rag on the piston,
with the liquid being Simple Green.
Do not mess up the piston
circlip if you intend to reuse it. I replace
them every time. When removing or installing circlips, do
not do more than just the amount of pressure and movement needed.
Wrist pin circlips have a sharp side, and a rounded
side; the rounded side goes INWARDS.
Swapping heads, cylinders, pistons on your BMW Airhead Motorcycles:
MANY questions have arisen on swapping cylinder heads and cylinders and pistons between the various models of Airheads. I am NOT going to discuss every possibility. If your question is not answered HERE, ask on the Airheads LIST.
a. In 1976 BMW changed the crankcase to
accommodate the 99 mm base cylinders, previously 97 mm. Thus
later
cylinders don't
necessarily fit earlier cases without knowing
more. While some earlier 99 mm base cylinders were made
without grooves for
the large O-ring, all later ones had the very large O-ring,
having that machined groove for it on the cylinder base. It is
possible to install
Nikasil cylinders on some pre-1981 engines. You have to trim a
small amount of material at the bottom of the cylinder, inboard
of the
O-ring
groove. You need the cylinder in a lathe to do this. This
needs explanation, as there are two styles of Nikasil
cylinders. The first
versions,
used between 1981 and 1984 were the same as the earlier
cylinders, that is, they do not have a small step above the
O-ring
groove, and fit
the earlier cases directly with no problems. LATER than
1984 Nikasil cylinders do not directly fit the pre-1981 cases,
and
you have to
machine-off the step. You COULD use
without the O-ring. You can fit Nikasil cylinders and
pistons from a R80 into a
R75/7. The
squish band is different on R80 pistons, but the clearance is
OK. Problems are with the R100 engines with HIGH
COMPRESSION
pistons.
b. 1000 cc
cylinders won't fit R90S heads; and there is a head gasket
difference from the 1000 and all the smaller displacement
sizes. You
COULD use 1000 heads and cylinders and pistons.
c. The high
compression heads and pistons were on U.S. 77-78-79 models of the
1000.
d. It is
possible to, expensively, combine the European high compression
ratio pistons and heads and Nikasil cylinders.
e. Almost
always, using large displacement on a given engine that came with
smaller displacement, will result in more vibration...or power
pulse
feeling.
f.
Do not mess up the piston circlip if you intend to reuse
it....I do not, I replace them every time. When removing or
installing circlips, do not do more than just the amount of
pressure and movement needed. BTW....the wrist pin
circlips have a
sharp side, and a rounded side; the rounded side goes INWARDS.
Old
style pistons with internal pin clips are interchangeable
with
the later external clip types. Airhead pistons up
through EARLY 1977 used a thin circular INside wire piston pin clip.
The later
1977 and perhaps some very early 1978 had a THICKER version of that clip.. The
piston pin clip on most if not all 1978 and
later
had the EXterior type.
g. Note that high
compression pistons may have
problems with the head. Later pistons had the dome
extending to the edge, and the later
1970's types with the squish
band were designed to
work with a matching cylinder head, and might have to be
machined. This might
occur with some modification you
are attempting, such as
later pistons fitting 1977 heads. The squish pistons
used on such as the 1977
R100RS are hard to find. The 1977
year was different regarding the pistons. The 1978+
had a different shape for the
pistons...at
the edge as an example. The 1977 style pistons
are probably no longer available. Perhaps some
PRE-emissions 1978 pistons
will work, but you would have to order them and look at
them. The 1977 R100 pistons were special, only used
in that year and
model, to
be a bit more precise here. If you have a
1977 R100 and need pistons, you could consider using the 9.5:1 1978
pistons with the outside circlip.
h. You can NOT
trust the latest parts information at the dealerships.
i. To remove the inside
type of piston pin clips, use a hardened tip sharp tool with an L shaped tip.
Insert the L shaped tip into the tiny
recess,
getting the tool behind the clip, and move it towards the center of the grooved
hole, so as to remove it. To remove the OUTSIDE
type of clip,
use a conventional circlip pliers. DO NOT mess up the
groove! The SLIGHTEST burr on the groove MUST be fixed,
otherwise the pin will not fit smoothly.
Piston
ring rotation; orientation; cylinder crosshatch:
Piston
rings DO rotate. They are designed to rotate; only a few
engine types do not have rotating rings. Those non-rotating ring engines
are mostly the 2-stroke type with ports on the cylinder walls, that you would
not want the ring ends to grab onto. There ARE some 4 stroke exceptions. The roundness of
the cylinder has an effect, and there are some smaller
effects.
Rings should be set roughly 120 degrees from each other 'IF' you removed the piston assembly from the cylinder. I HIGHLY disagree with those that say it does not matter if the rings are lined up any which way. Rings do move but mostly only with fresh crosshatched bores and initial few miles of break-in, then they tend to be stable. You should not put the oil ring end gap at the bottom. The reason is a possible accumulation of oil...particularly when the bike is on the side-stand.
Note that genuine BMW rings usually have a top and bottom, with the top marked, sometimes very faintly. Due to wear patterns, besides ring design, it is BEST to ALWAYS return the rings to the same orientation and do not install a ring upside down. If you remove the rings, pay attention to the top/bottom orientation of the rings.
Some rings are marked TOP.
BMW piston rings are usually marked TOP near the ring gap. Failure to observe the
orientation of the ring will result in high oil
usage.
Don't assume that pistons, like new ones from a dealer, have the rings
on correctly. The older style of oil rings may or may not be
marked TOP. If put in upside down, very easy to do, your cylinder
will burn oil.
TOP RING, also called the compression ring, the ring closest to
the top of the piston: Use a magnifying glass, the
inner portion of the ring has a tapered
edge that faces the head (or top of the piston), that is, outwards.
The 2nd ring (ring in middle) has a notched outer edge.
The notch faces the engine case. On the 1981+ style 2nd
ring, that ring is marked for the top, the bevel does face the
crankcase.
From the earliest /5 to about
1980, the bevel of the OIL ring was towards the valve
cover.
Old
style oil rings have a taper on one edge, it is not easy to see
sometimes. If you have that type of ring, that taper goes OUTward,
towards the piston top. The wider rim of ring skirt was towards the
crankcase...that is, the wider part of the taper was towards the
crankcase. This has caused confusion with the U
channel ring. There are two chamfers on that ring.
The chamfer on the EDGE faces the top of the piston. The
chamber on the INSIDE of the U faces the engine.
From 1981, a new, symmetrical design was used, there is NO UP NO
DOWN! (for the OIL ring, that is), and there is a radial spring;
Nikasil cylinder bikes oil rings will have a spring expander
behind it.
Early oil rings had
a sub-groove, to allow gases to force the ring against the
sleeve.
Pay attention from which groove
the rings came, and don't break them, they are VERY
brittle. Removing the rings is a job to be done very
carefully, avoiding expanding the ring diameter, use the least
possible amount. Rings are very brittle, and
slipping them off a piston has caused folks some problems...use a
few THIN feeler gauges to help ease them off. Do NOT put much
pressure on the rings, they WILL break
Because of these regards, I sometimes advise folks that are not
changing rings, to leave the existing rings where they are, ON
THE PISTON.
LIGHTLY, EVER SO LIGHTLY, OIL the piston and rings when
re-assembling; this is NOT the place to drip oil or use the oil
can. A bit of oil on your hands is all that is
needed.
Here is the proper
orientation of the ring end gaps, based on the common analog
clock, as you face that side:
LEFT SIDE: outer, compression ring: 4:30
2nd ring: 10:30
innermost (oil ring): 1:30
RIGHT SIDE: outer, compression ring: 7:30
2nd ring: 1:30
innermost (oil ring): 10:30
Rings tend to initially rotate SOME during use, especially break-in, but just assemble
them to the above orientation, they will be fine once the engine
is in operation.
The Pistons have a front/rear and you can cause yourself some
SERIOUS problems if you do not assemble the piston to the rod
with the small stamped arrow, located on the top of the piston
near the center, pointing forward. There
will be the word VORN (German for FRONT) next to the arrow. It
may be covered with carbon.
NEVER do both cylinders at the same time, unless you positively
know which is left and right. If
you leave the pistons in the cylinders, and reverse the cylinders
left to right, you have now reversed that arrow on BOTH
cylinders! DO NOT DO THIS! This caution applies to
everything...pushrods, heads, covers, cylinders, pistons,
rings...etc. for various reasons.
NOTE: Pistons seem to be the same for Nikasil and iron bore cylinders. The rings are NOT the same.
Most airheads are specified at APPROXIMATELY a
maximum cylinder ovality of 0.0004" and a maximum cylinder
taper of 0.0008". The piston clearance is typically
0.001" on the smaller bores, to 0.002" for R100
engines. The Nikasil/Galnikal cylinders use a piston
clearance of approximately 0.0015". The ring end
gaps, prior to
Nikasil/Galnikal, were top ring at 0.012-0.020";
Nikasil/Galnikal
use about 0.020" for the top and second ring; and
0.013" approximately for the scraper ring.
These values I give here are in brief, the factory manuals give
the exact gaps for each ring for all the various models.
The information here, however, is OK. NOTE that one
can use oversize rings to fit worn bores on the PRE-Nikasil
cylinders; adjust the ring gap by filing. DO NOT try that
with the Nikasil cylinders. The RINGS are also different
material between pistons used on the old iron cylinders, and the
1981+. Rings for Nikasil and Galnikal are NOT the
same rings as for the old iron cylinders!
There are plenty of styles of rings. Here
is a copy of Clymers page 146:
R60/2, etc:
Pre-Nikasil piston and rings. The Nikasil rings have a spring expander, and the two bevels are opposed. Photos show order of installation:
Here are two additional photos, note the type of ring construction on this particular set of rings:
This style is used, R60/2, and, see to left.
NOTE how the oil ring bevels are different on this type of oil
ring, from the Clymers manual sketch type of symmetrical ring.
Note that some types of rings use a ring-expander behind the oil ring. Nikasil rings for example.
Oversize pistons from BMW are NOT available for the 1981+ cylinders. Selected graded pistons to fit the bore are available.
AFTERMARKET pistons ARE available.
The following information on AFTERMARKET pistons is believed
accurate, but has not been checked in great detail by me:
San Jose BMW (CC products) sold some pistons for a 1050 cc kit
that gives 1038 cc. These pistons were forged types, by
Venolia. The diameter was 3.810" (96.774 mm).
Alloy 2618, a low silicon, was used. This means these
pistons expand at a goodly rate when they get hot, so MORE piston
clearance is needed. Try 0.0035"
minimum. Alloy 4032 would wear better, and expand
less, but 2618 is stronger. If a crack occurs in
2618, the crack may well find an area of lower stress and
stop....not so the 4032. So, 2618 is a GOOD choice for
piston material, and holds its shape better under high stress use
too.
Mahle makes a big bore kit using 97 mm pistons, giving 1043 cc.
Pistons are purposely tapered to be bigger at the skirt end, so
be careful when measuring things.
See the following websites:
www.jepistons.com/pdf/piston_instrc2618.pdf
http://www.sjbmw.com/
www.venolia.com/main.html
You can also check with German websites, etc.,....pistons/cylinders/rings are available from Europe that are not available in the USA.
PISTONS AND RINGS ARE TO BE INSTALLED ""LIGHTLY"" OILED...not dry. Lightly means the faintest amount of oil that can be used as an oily film, it does NOT mean squirting with an oil can! A few drops, spread with fingers. Perhaps a FAINTLY oily rag on the cylinder walls. I use a straight mineral oil.
If a groove that the piston rings fits into is
heavily worn, the ring will rock with respect to the piston far
more than needed, and things can get bad enough to break rings
and pistons. Pistons DO wear out.
Cylinder
TAPER:
You do NOT need an expensive tool to measure cylinder
taper. You won't probably bother to measure the taper on a
Nikasil cylinder anyway, unless quite anal. But, for the earlier
cylinders, why not do it easily? Install a
ring, squarely, and it need not be an original ring off your
piston. Use your feeler gauge set to measure the
ring end gap, and do this at a point just inboard a bit from the
top of cylinder ridge wear area; and, redo the measurement near
the bottom. Divide each, separately, by pi. Compare the
two. Simple, wasn't
it!
Piston clearances generally, unless there is real damage, don't go much over a few thousandths...certainly not the 4.5 specification limit. It is perfectly OK to use oversize (usually next oversize) rings, adjusting the end gaps, when you have a bike with a good bore. If you do this, we shall assume the bore has low taper, but the rings are perhaps on the edge of specifications. You can purchase an oversize set of rings and adjust the end gaps to specifications, but when doing this, use the smallest measured part of the bore....usually near bottom, where the diameter of the bore in the lowest area that the rings normally contact, is the smallest.....adjust the new oversize ring gaps for that diameter.
MISCL:
The stock cylinder base SHIM, during 1970-1975, was 1/2 mm (about .020"), and are not to be used for 1976 and later. For 1976+, TWO SHIMS were available, and these are ONLY for 1976 and later....the so-called Step 1 compression lowering gasket was .7 mm (.028"); and the very low compression gasket was 1.2 mm (.047"). BMW HAS used the word GASKET for these SHIMS. The original part numbers were 11-11-1-257-092 for the /5, for lowering octane requirements. Used on /5 and /6 was the 11-11-1-255-001. Use of the 11-11-1-335-650 base gasket will lower a 9.2 CR to about 8.6.
rev.:
released 01/26/2008
01/11/2009: updated, mainly to clarify some details, such as piston pin
clips
06/15/2009: clarified a few details.
08/29/2011: clarified ring orientation, as folks were NOT
reading all the information.
08/31/2011: More ring clarifications, photos.
© Copyright, 2011, R. Fleischer