Tuesday, November 6, 2018

It’s raining CB92s this month…..

My primary focus on vintage Hondas has been the variations of the 250-305cc models for more than a decade.  Having owned, repaired and overhauled numerous vintage big twins, I am perfectly at home with those models.

Honda’s CB92 Super Sport machines are extremely rare in the US, having only received about 1,000 machines in the 1960-62 era and many of those were supposedly the racing versions. I owned CB92 #24 about 30 years ago, which was a bike originally sold here in San Diego. In the years since about half a dozen more have come my way and were resold without taking on full restorations. Obviously, after 58 years, the parts sources are very thin. Additionally, the changes made during production, complicates the picture even further.

Honda began production in 1959 with both a 125cc CB92 and a companion 150cc version CB95. Bear in mind that these were newly designed OHC twins which revved to 10k rpms, but only carried a little over one quart of oil in the crankcases. Early bikes suffered crankshaft failures and left side piston seizures when run hard, especially of the oil level dropped below the safe mark.

From 1959 through 1962, there were three different crankshafts and a matching number of crankcase changes to follow. The first generation crankshafts had a 1mm larger center main bearing than the end bearings, all located in the cases with split half ring retainers. At high rpms the bearings could actually rotate in the cases and not enough oil was passing through the crankshaft to feed the left side crankpin.  Honda did an unusual stopgap modification by machining an oil passage in the base of the cylinder block that lead from the oil feed stud hole across the back and into a window that was machined in the rear of the left side cylinder bore.  A special gasket was created to match the oil channel work.  The early cases featured a “rear breather” design that collected and separated the oil solids from the crankcase vapors at high speeds and allowed just the separated air to escape.

The second generation featured main bearings of all the same diameter, but the center main was locked in place with a locating pin to ensure that it was getting a constant oil feed from above. The end bearings continued to be of the split ring design.  The crankcases were changed to make the breather system function back inside the top cylinder head cover with a vent/drain line coming off of a fitting.

Finally, in 1962, they pinned all three bearings, so the cases were changed again to reflect those modifications. Obviously, the crankshafts and crankcases all have different part numbers and design features which preclude mixing and matching parts from other years.

A good friend in AZ had purchased a couple of less than perfect CB92s and dropped off an engine for me to rebuild. It had suffered a severe piston seizure early in its life and was parked for decades. The engine required a full cleaning and vapor blasting to recover the original finish, plus all the fasteners needed to be re-zinc plated. Fresh pistons and rings were installed back into the original STD bores which were not damaged. All the rocker arms were replaced, but the camshaft appeared to be in serviceable condition. All new valves were installed due to cupping of the seats and valve faces. After a few weeks of running parts back and forth to machine shops, the engine was reassembled and stood waiting for pickup.

As time went on, the VJMC West Coast Rally was announced in Prescott, AZ, so I saved John a 600 mile round trip by dropping off the rebuilt engine at his house. During the interim he pulled another CB92 engine apart for rebuild and had already cleaned and re-plated all the parts. The engine cases had suffered a bizarre damage situation to one of the carburetor cover mounting posts. Apparently one of the mounting screws was seized into the mount and probable use of an impact driver had broken the whole mount off of the case, taking a large chunk of the top case with it.

John sent the case off to Colorado where a skilled welder had managed to re-weld the chunk back into the case, blending the repairs so well that it was almost invisible on the outside.  Good save as that is the half that contains the serial numbers.

When the parts were unpacked, I noticed that the 1960 engine cases had a non-matching 1961 crankshaft installed! Apparently the engine was run like this with the center main bearing free to spin around inside the cases as there was no contact with the crankcase bores.  So, the hunt was on for a new crankshaft with the larger center main bearing. After almost 60 years, finding NOS CB92 parts has become problematical, but within a week, I managed to flush out three different correct crankshafts for sale. Two were out of the country (Thailand and the UK), but the last one came from a stash in WA state, where a long time owner/racer of CB92s had some leftovers buried in his garage.
Had the whole units not been discovered, the Plan B was to buy one of the center main bearings and have a machine shop rebuild the crankshaft with a new main bearing in the middle. These crankshafts are small and difficult to press apart and back together again, so I am glad to have a whole new correct unit to use as a replacement part.

In the meantime, my friend John Stein, in LA, wanted me to revive a 1960 CB92 which he had owned for the past 30 years. I agreed to take on the revival (not restoration) work, so that made three 1960 CB92 bikes or engines that have come to the shop in the past month. That bike showed up dirty and sad looking with a mismatched paint color on the headlight bucket, incorrect tail light, wrong brake cable and other woes, but some restoration work had been done to the hubs and wheels. The bike must have been raced at sometime as there were some crash damage scuffs remaining, a racing rear tire on the back and an all aluminum cylinder block installed.  Plus there were no mufflers on the bike, just a set of loud factory megaphones from the race kit. The bike featured a YB racing seat, as well, plus a tachometer in place of the speedometer.

While all of this was happening, I received a message from a CB92 owner in Los Angeles area who was moving to AZ soon and wanted to sell his “project” 1960 CB92. At the moment, my bike inventory is pretty much at max levels, including a 2013 CB1100, 1988 CBR250R (250 four cylinder), 1988 Hawk NT650 with 1,000 original miles and a newly acquired 1990 NC30 VFR400R. With one 1960 CB92 on the bike stand and another 1960 engine on the work bench, things are in overflow status.

The LA bike was disassembled, had been repainted metallic green, had a chopped off front fender and the kneepad was disintegrating. The engine had 150 cylinders, but the head was still stock 125 parts. The bike had been stored in poor conditions, so there is lots of rust and patina on the parts visible in photos.  The price was at the upper level for what was there, but it had been the seller’s bike since 1964 and he was aware of the value of the bike, even as a parts source. 

Reality check…
After much consideration and parts availability checking, I decided not to pursue the LA CB92 project bike. The 1960 bikes seem to be prone to needing new crankshafts and based upon recent searches, their availability and cost are budget busters. Some of the rare chassis parts are available from a source in Japan, but the seller doesn’t ship to the US, so they would need to be relayed from one of my Japanese based contacts out here to California, increasing costs dramatically.
The current 1960 CB92 engine was held up for a new crankshaft, which arrived recently, but without end bearings or the special thrust washers. I ordered the thrust washers from DSS in the UK, who has to nab them from their partner resource in Holland, so delivery time can be a couple of weeks in the end.  I asked about the spare C92 engine crankshaft still in AZ and they were intact. So, with a little persuasion, the end of the crankshaft was cleaned off and washers shipped out with the set of CA95 cylinders which need to be re-bored before assembly.

Once the cylinders were bored, much of the reassembly went smoothly. The oil filter needed was the shorter version and not serviced. The small 4mm screws tend to get stuck and often the heads are damaged when removal is attempted. I did manage to get them loose and the unit cleaned. Some allen head screws were supplied as replacements, but the thread pitch is slightly different between JIS and ISO so the holes were rethreaded. Eventually, all the bits fell into place and the engine was picked up in late Oct.

Top end tear down…
The 1960 #517 CB92 on the bike lift finally got to the point where I could try the electric starter to spin the engine over in preparation for a startup after 30 years. The starter solenoid just buzzed a little bit, probably due to dirty internal contacts. After a stint on the battery charger, finally the engine spun over with the electric starter, but not very briskly even with the spark plugs removed. A compression check revealed 80 psi on the right cylinder and about 85 psi on the right side, which is way under the suggested 130 psi in the manuals.

The motor was eased down onto the work table and the top end removed. Within the alloy YB racing cylinder block were two STD bore YB pistons, both of which were carboned up on top, but showed little wear on the piston skirts or within the bores. The top ring was removed and checked in the bore for end gap width, but appeared to be on the minimum end of specs.

The head was checked for compression losses and both exhaust valves had odd pits on the valve seat faces, so will be replaced. The camshaft was stamped YB in 2 places. All the cam lobes and rocker arms appeared to be fully serviceable as-is.  Intake valves were de-carboned and re-installed as their seats were still nice and shiny thin rings. Exhaust seats were lightly cleaned and should be a good match for new valves. The ends of the valve stems appeared to be nearly unmarked, so the engine run time must have been minimal on new parts.

The low compression was probably due to the exhaust valves leaking, but the extended valve timing of the camshaft can actually reduce the measured compression readings because of the YB cam timing. The combustion chambers and piston crowns were pretty coked up with burned oil deposits, but the exact cause of excess oil consumption is somewhat mysterious given the condition of the parts inspected so far.

Once some fresh valves are installed, we’ll see what the readings come up to afterwards. The old gaskets were very difficult to remove and more than an hour and a half of labor was expended in just getting the parts cleaned and free of leftover gasket residues. This is always the least pleasant portion of engine rebuilds on vintage Hondas, as far as I am concerned.

Amazingly new correct 205-coded stainless exhaust valves are still available from eBay sellers. The seats were cleaned up and new valves lapped in. New head gasket, sealing rings and a few oil seals were installed and the engine eventually reinstalled. The engine has to go back in with the head first, which is awkward to do with a single jack. I eventually used 2 small floor jacks to get it stuffed back into place. I can do an engine swap on a CB77 in half the time as these little Benly engines.
I managed to track down a new starter solenoid to replace the one whose mounting ears had broken off and were replaced with strap metal. This was another rare part that was found with an eBay seller who had several in stock and took a “best offer” of about 30% less than “retail” for it. CB92 parts like that are probably slow movers in the US, as there were so few bikes sold here and fewer remaining to repair or restore.

Start up and wrap up…
Initially, the bike engine didn’t want to fire up, but after about 15 good kicks, it sputtered to life and kept running with little or no choke, even when cold. The bike stutters just off-idle, then cleans up and pulls through the gears. An open-megaphone CB92 is not something that you want to spend a lot of time on, particularly in a residential neighborhood. I made a couple of mid-day passes up and down the streets, right around home. It pulled past redline going down a long hill, but struggled on the way back up. The 4 speed gearbox has somewhat wide ratios, so it falls off the powerband when you upshift from 2nd to 3rd gears, under load.

I suspect that it needs a larger main jet for pulling high speeds, but also needs a slide with a bigger cutaway or a needle with a fatter taper to help lean out the low end. Despite the hi-dome YB pistons, the D8HA spark plugs screw all the way down and haven’t had their electrodes hammered shut yet.  I think the longer duration YB camshaft profile hurts the bottom end power, causing metering problems for the carburetor, as well as lowering the compression readings at idle.
The clutch was initially “stuck” but leaving it in 3rd gear at 30 mph with the clutch lever pulled in, finally released the pack and the clutch began to work normally. A “best practice” would be to disassemble the clutch pack and clean up the steel plates, but this is to be an occasional bike for running around the block up in LA, so probably isn’t a significant factor in the grand scheme of things.

The task was to get it up and running reliably and make sure that nothing falls off of it during short jaunts. The steering is “twitchy” when changing directions, as if the steering stem is a little bent. There were some signs of at least one lay-down incident, but nothing significant was seen as far as chassis damage. The tires are over 30 years old and the rear one is an old Yokohama road race tire. The front is probably the original OEM 1960 rib tire, so there is a lack of flexibility in the tire combo. The bikes ride like buckboards anyway, with little suspension compliance on either end. A few quick rides did nothing to endear these little Benly buzzers to me and more than in the past. They are an iconic design, but with technology from the 1950s, it leaves a lot to be desired.