The whole CB92-CA95 (US models) line up was a lightning bolt
into the hearts of other manufacturers during the late 1950s and early 1960s.
These tiny electric-start 124-154cc twins would spin up to 10,000 rpms, when
pressed or just doddle along at traffic speeds without complaint, in most
The engines were 360 degree firing parallel twins with a
short 41mm stroke, two-valve cylinder head design operated by an OHC camshaft.
The camchain ran off of the left end of the crankshaft sprocket, over a guide
roller and through an adjustable camchain tensioner. The 4 speed transmission
was a normal return change design, operated by a 5-plate light-pull clutch
assembly. The back of the clutch basket was machined with an eccentric that
connected a flat steel arm which worked a push-pull plunger inside a cast steel
oil pump body. The whole design was elegantly simple, but built with precision
and quality materials.
A weakness in the design of the crankshaft became evident
early-on which required several different crankshaft changes. The main
differences started with the 1959-90 crankshafts which had a larger center main
bearing than the two on each end. Initially, all three bearings were located in
the engine cases with the “half a piston ring” retainer.
In 1961, the center main bearing was reduced in size to
match the other two and the bearing was located with a dowel pin to keep the
oil feed holes in alignment. The end bearings were still retained with half
rings, which allowed them to rotate in the cases at very high rpms.
The engine cases were
also slotted to help facilitate oil moving over to the crankshaft in one
design. On early CB92s the engine case slot (at the rear of the cylinder base) lead
to a low cut window in the back of the left cylinder liner to help lubricate
the piston skirt, which apparently was not getting enough oil. This lack of
lubrication caused piston seizures and subsequent crankshaft failures when the
connecting rods were brought to a halt suddenly.
The 1962 crankshafts used pinned bearings all the way
across. The rotor side main bearing of the crankshaft was a caged-roller style
with a thrust washer, while the transmission drive gear end used a ball
bearing. Some versions had a larger diameter end bearing, as well.
The current repair bike in the shop was a low-miles, mostly
original, 1961 CA95 Benly with engine number #59. The owner had owned it for a
few years and put less than 1,000 miles on it but the engine began to smoke a
lot from one side, so the top end was freshened up with .50 pistons/rings and a
valve job. Right away, the owner reported that the bike lacked the snap of his
other Benly 150 and wouldn’t go faster than 45 mph in top gear. In the middle
of a ride, the engine slowed down, seized up and began to make some unpleasant
knocking/rattling noises when restarted.
Having owned a handful of CB92s, plus a 150 Benly or two, I
was fairly comfortable doing a forensic exam report on the engine after it was
transferred to my custody at a VJMC event held in Solvang, CA in Sept
2017. Once it was on the bench the
engine was removed and the teardown/inspection process proceeded. The drained
oil had a lot of bright shiny particles in it, which is never a good sign. The
ignition timing was checked to eliminate that as a source of the piston
seizures, due to detonation from too much spark advance. The top cylinder head
cover was removed to view the valve train which all seemed to be in good
condition, but still flecked with brass-colored particles here and there.
The cylinder head was lifted off and set aside for further
inspection. The cylinder bores seemed to be relatively unharmed, apart from
some superficial wear/scratching. Once the cylinder block was lifted off, the
serious damage began to be revealed. There were a couple of missing pieces from
the bottom of the left piston skirt and both pistons showed signs of repeated
piston seizures in the past. When the left side connecting rod was checked,
there was noticeable up/down play which shouldn’t be there at all. A few pieces of the piston skirt were lying
just below the pistons on the crankcase bosses and more of that shiny brass
particle debris became evident.
The camchain, stator and left side engine covers were all
removed to facilitate engine case separation. When the clutch cover was removed
on the right side, a stream of contaminated oil gushed out of the hidden nooks
and crannies all laden with large amounts of brass filings and particles. There
seemed to be more brass residue than was possible from just one rod bearing
failing, but that was about all that could be seen as a source. The entire
engine was contaminated with metal particles, including the oil filter. The
clutch was removed, along with the oil pump which is driven off the back of the
clutch basket eccentric.
The engine cases split without too much difficulty and more
metal debris poured out onto the work bench. The transmission gears seemed to
be in good condition; however one of the locating dowels for the transmission
bushings was half bent to one side, due to a shearing force on the transmission
shaft. There were more questions than answers, but one thing was for sure; the
crankshaft and pistons would have to be replaced!
A lot of cleaning has taken place to expunge all of the
little brass particles that were circulating around inside the engine before it
finally expired. The oil filter, which spins on the end of the crankshaft, was
packed with colored bits of course. Top and bottom cases were cleaned
thoroughly as were all the clutch parts and transmission gearsets. Rather than
replacing the hollow dowel pins on the transmission shaft bushings, a couple of
6mm crankshaft roller bearings were ground down to a matching length and
secured into the top case half, which was lying on its back awaiting newly
The crankshaft replacement really only had used parts
options, as new crankshafts are NLA in much of the world. A $135 (delivered)
used crankshaft from an eBay seller came with the rotor and left cover as the
seller was unable to pop the rotor off the end of the crankshaft during
teardown. After measuring all the
crankshaft main bearings, it was discovered that Honda had increased the
bearing size on one end. The old crankshaft mains were in good condition so the
end main bearings were switched from the old crankshaft to the new(er) 1964
part. That swap was successful and the updated crankshaft was placed into the cases.
Although the engine had been freshened up within the past couple of years, new
seals were ordered along with a fresh gasket kit. The oil pump was cleaned and inspected for damage, then the
small ball spring replaced, along with a new o-ring for the pump’s side plate.
Fortunately, the cylinder bores (bored to .25 oversize)
cleaned up to like-new condition, so fresh pistons and rings will have a nice
new home to work within. A correct slotted base gasket was discovered for sale
on eBay, so the oil channel will have a maximum flow rate to the center main
bearing. The original base gasket was the later model flat version, normally
used with engines which do not have the oil channel, so there might have been
some flow restriction at that point.
The cylinder head had little debris, apart from the inside
of the camshaft oil feed hole. The cam lobes and rocker arms were unharmed.
Spray cleaner was used to check for any valve leakage, but none was found, so
the valves, which had been replaced earlier, were left undisturbed.
Still looking for the “smoking gun” that caused this
disaster, the carburetor was inspected for jet selection. The main jet had been
increased one jet size; however this carburetor has “power jet” enrichening
circuits which are controlled by two tiny air/fuel correction jets. With
information gained from the Facebook Vintage Japanese Bike Club subgroup, which
is focused upon the CB92 (Super sport version of the CA95 Benly), a member from
Sweden offered up a specification page from an ancient manual showing that the
jets for the power jet system were #130 in both locations. When the fuel jet
was removed from the carb body, its size was not marked at all. The air
corrector jet at the top rear of the carburetor body was stamped #130. Using a
metric drill bit set, I determined that the inner jet was a #150 size.
Honda seems to have determined that these jets were not to
be altered, so no references to the part number or sizes can be found on
current online microfiches or even in my 1966 CB92 parts manual. Digging deeply
into arcane parts references books, a part number was discovered that might
have been correct for these jets. The ones listed in the book were not #130
size, according to the part number suffix, but substituting the end numbers with
130 got me closer to a source for them. While an eBay search for “air jets”
revealed similar jets which fit CR250 motocrossers and GL1000 Gold wings, they
are all ISO thread pitch jets. A 1961 CA95 Benly is all JIS thread pitch built.
Doing a web search for the desired part number revealed that
it had been superseded and used in G30 Honda generators and early Honda
360-600cc twin cylinder cars.
Fortunately, I live nearby a Honda 600 car guru,
who has tons of used parts. I ran the part numbers and descriptions past him
and he came up with a #120 air jet from an AZ600 carburetor! The jets matched
up dimensionally and on the thread pitch, so I gratefully brought it home and
reamed it out to #130 size using tapered jet reamers. Right or wrong, at least
the carburetor was now close to specs for that power jet system.
The previous mechanic had contacted me, asking about my
findings, as well as giving an overall view of what he had done and found
previously in this engine. He mentioned that the carburetor needed some
massaging to get the slide to stop sticking and I noticed that the carb flange
was somewhat warped, even now. Obviously any air leaks at the intake flange are
potential causes for piston seizures due to lean mixtures causing piston
Honda’s specifications for spark advance are puzzling as
they show a range from 34 to 47 degrees before top dead center. In comparison,
a CA160 has a 40 degree maximum spark advance range. I chose to squeeze the
stopper ends of the advancer closer together in a vise, so that the maximum
spark advance will be somewhat reduced. Over-advancing spark causes piston
overheating, usually resulting in piston seizures, particularly when the piston
clearance is only about one and a half thousandths of an inch.
Newly installed pistons and rings on a fresh rebore need
some “break-in” time, which I suspect didn’t happen previously. Honda usually
specs about 500-600 miles of easy riding before extended full-throttle is used.
The engine’s last repair work didn’t include case splitting, only a bath in a
solvent tank to remove thick layers of sludge from the bottom of the cases. When
the cases were split, the sealant wasn’t familiar looking to me and someone had
painted the number 78 inside the case halves! One can only guess as to whether
the 78 marked the year when it was taken apart or some mechanic’s ID number or
something totally different!
Having looked at each and every part of this engine
assembly, I have confidence that it can become fully functional as designed,
given a proper break-in period and judicious checking of the oil levels,
ignition timing, carb jetting and use of the correct heat range spark plugs.
The old spark plugs were not present in the engine, as a pair of C6HA plugs
were loosely screwed into the cylinder head. Honda specs for the small 10mm
plugs, used on the 1960-61 engines is a C7HA plug, which is a heat range cooler
than what was supplied. I will fire the
bike up and ride it gently on the C6HS plugs then switch to C7HSA plugs for
extended riding during initial break-in runs.
I’m not sure how long the bike was down since the blow-up,
but the battery fluid levels were near the tops of the cells and measured
voltage was 1.4v. An overnight charge brought it up to 6.66v initially, but
started to fall to 6.4v later in the day. With only 6 volts available to power
these bikes, you need everyone you can corral.
RUNNING, but not ready yet
I was able to get the bike started
up and ran it around briefly. Lingering problems exist in the fuel delivery and
clutch. The engine ran well at low speeds
but started to break up as throttle was increased. One issue was a bad spark
plug cap connection on the right side. I tried jetting the bike down once to
see if the oversized-main jet was too rich, but it got worse and
responded to adding choke under power. I then took a jet reamer to the #100
main jet and opened it up to about 105 size. The bike has stock mufflers, but
no baffles installed so jetting must be compensated for those changes, as well.
The bike began to run much better and then the aftermarket clutch cable broke.
Both the clutch cable and throttle cables were
too long and are old Dixie/Superior branded parts of less than wonderful
quality. So, I needed to track down some correct cables for the bike, which
were found on eBay; real ones this time, not reproductions. eBay sellers came
up with actual OEM correct gray cables with grease fittings installed, plus the
little tensioner springs were included.
The clutch drag continued, even with correct adjustment.
I have to do research on whether the clutch pack wound up being too tall and
that changed the angle of the clutch lifter arm and/or the clutch adjuster
threads are worn out and we are losing pushrod travel because of that.
I had to replace the oil filter with a later
model version due to the change in crankshaft designs. So far the engine sounds
great, doesn't smoke and isn't making any unusual noises or leak oil anywhere.
More work and more research and good results
The OEM Honda clutch and throttle cables came in quickly and were
slowly replaced. Working on a Benly requires a bit of watchmaker and puzzle
solver in order to fit all the small bits back together again. The cables must
be routed through the pressed-steel chassis holes and meet up with their end
With the clutch cable installed, the clutch adjuster was checked
for excessive play, but nothing out of the ordinary was observed. The throttle
cable required the same feed through the chassis holes and then the cable end
connected to the carburetor top components. The main jet was removed and opened
up further, followed by another test ride. While the engine continued to show
more power the clutch continued to show signs of dragging, despite all the
cable adjustments being done correctly.
All that was left was to drain the oil and remove the clutch cover
(after the exhaust system) and take a look at the clutch release function at
the end point. Pulling the clutch lever in, at the handlebars, showed a
restricted clutch release travel at the plate set, when the assembly was all
together. The feeling at the lever was
that perhaps the clutch springs were coil-binding at full travel. Honda didn’t
design that kind of problem into this model and this one shouldn’t be coming to
a hard stop at the handlebar lever.
With the springs and spring plate removed the clutch travel seemed
to increase. All the clutch plates were checked for thickness as one odd one
was in the pack. Most of the correct plates measured out around 3.4mm, but one
was at 3.0mm and a different type with thinner friction material. The PLATE A
was a thick alloy piece, which was changed in later models to a different
design along with a different center hub.
David Silver Spares came up with a correct friction plate, steel Plate A
and outer pressure plate. The pressure plate showed some warping when the
clutch pack was spun by hand. There were two low spots on the plate surface, as
The clutch spring retainer plate has a central hole, which is a
stamped steel piece which is punched through creating an offset hole that
matches the one on the outer clutch pressure plate. Putting the plate on with
the offset outwards appears to cause some unnecessary binding of the plates
when they meet in the middle. Turning the plate around so the offset nestles
down next to the pressure plate’s center allows a more full travel of the plate
set. With a correct set of friction plates, pressure plate and Plate A, the
clutch should resume normal function and travel once again. A rough reassembly using
the old parts showed increased travel/plate spread when the outer support plate
was installed correctly. I discovered some Barnett Kevlar clutch friction
plates in a box which fit the clutch hub on this bike, so it is getting treated
to a hopefully bullet-proof clutch pack to carry it into the future.
When received, the steering head bearings felt notched and
self-centering; so the bike just wanted to go straight ahead all the time.
Removing the handlebars and mount allowed access to the top plate stem nut
which was tightened with a great amount of force. Backing off the nut allowed
the stem bearing nut to be loosened slightly and suddenly the steering felt
transformed back to a normal feel once again.
The drained oil showed minute traces of stray flecks of brass and
some fine powdery grains of steel being loosened and held in suspension by the
Honda GN4 oil. The second cleaning of the filter showed almost no brass
particles now. It appears that the updated, used crankshaft transplant has been
successful, but not a completely easy task. Still, with the inventories of new
crankshafts exhausted, selective use of the newer versions seems to be a viable
The final test rides showed a normal pull clutch lever feel, good power through the rpm range and a very slick feeling transmission function. The owner was very pleased with the results in the end.
This has been a thought-provoking and time-intensive project from
start to finish. Some $600 worth of replacement parts have been purchased to
date, along with a great deal of research time online for parts and hands-on
labor to R&R the engine, tear it down, inspect and clean all the parts and
order the correct replacement items to bring the bike back towards the original
running condition. To paraphrase, “You
learn the greatest lessons on a Honda!”