The wave of various “opportunities” keep coming this year. Recently, a call from my new friend Doug, who was referred to me by my other buddy Bill, made a different than usual request. This time, Doug wanted me to disassemble a SL350 engine completely, so he could vapor blast all the external surfaces, then bring all the bits back to me for re-assembly.
I had a little lull in the action in the garage, so offered to take on the challenge. Doug brought the engine down on a Friday morning and we twisted, hammered, and dismantled the big lump in about 2.5 hours. Doug had already removed the clutch cover, top cylinder head cover and clutch internals, plus the dyno cover side stuff.
This was basically a “virgin” engine which had not been dismantled previously, so the screws, bolts and nuts were all sitting cozy together in the past 55 years. I laid out some demolition tools and we began the work. Doug is a big, strong man, despite having his own set of motorcycle related past injuries to his wrist and back. Having him hold the engine core steady while I was pulling, pushing, pounding and twisting the bits was very helpful!
The first task was to remove the two cam sprocket mounting bolts. NOTICE! The two bolts are NOT the same. One has full threads and the other one has an unthreaded shoulder. IF you accidentally swap the bolts to install, the end of the camshaft mounts break off and the cam is ruined.
Starting with the two cam holders, held together with 4 Phillips screws, I started by inserting the impact driver #2 bits into each screw head and pounded the bit a few times with a brass hammer. I followed this by inserting the tool bit back into the driver body and started whacking at each screw. The first round was 2 for 4 successes. I upgraded the hammer to a 3 lb sledge hammer, but with no further progress. Plan B for this situation is to use a long chisel to catch the corner of the screw head on an angle and start hammering again. After another 30 seconds per screw, all four were released and removed on the tach drive side.
Turning the engine to the point plate side, the plate screws were removed, more easily because they had been loosened before for ignition timing adjustments. Once the point plate was out of the way, those 4 screws were clear to attack. This side mirrored the first side…. 2 for 4. Another round of chiseling the heads loose gave way to parts removal, at last.
The camchain tensioner was unbolted from the back of the cylinders to release the camchain tension. This allows the camsprocket to drop down on the shoulder of the camshaft allowing the cam to be wriggled out carefully through the right side of the cylinder head. The camchain is endless and the space allowed on both sides of the sprocket with the chain installed is insufficient to just drop the chain down off the sprocket. Conversely, the chain and sprocket will have to be mated together before the cylinder head and is installed.
Once the camshaft is removed, the cylinder head will come off once the two small 6mm bolts are removed next to the spark plug holes. A step that I missed earlier was to loosen the rotor bolt to allow the rotor to be removed with the correct 16 x 1.5mm removal tool. Using the engine’s compression and the internal friction of the pistons and cams, helps keep the rotor from turning easily while removing the bolt. My battery powered impact tool decided to take a nap, so we finally had to put a long screwdriver through the rod ends, after the pistons were removed and a foot long 1/2” drive breaker bar with 14mm, 6 point impact socket finally released the rotor bolt. An air impact would have done the job more easily, but was not available.
Once the head was removed and checked for valve condition and carbon buildup, it was set aside while the cylinders were lifted up to examine the bores and the pistons, plus checking the end gap of the rings. The engine was remarkably clean inside, with little wear shown on the piston skirts. While the cylinder bores didn’t show any ridges or scoring, wiping them down revealed some rust rings on both cylinders in almost the same location for both sides. The marks were about 2” down from the top, indicating that the bike sat for a prolonged period of time with the pistons sitting perfectly still at an almost even place.
The ring gap came out about. .020’ using the top ring inserted just below the top of the bore. This is pretty wide considering the new readings would have been about .006-.013”. The oil ring would have had only .004” to start life with. Clearly, we are at a crossroads about the next steps with the cylinders and pistons. The cylinders will require a good wet-hone treatment to clean up the bores, set up cross-hatching for the rings to seat in and hopefully minimize the rust ring damage. Interestingly enough, the bike had been running well and the spark plugs came out with a nice clean burn appearance.
With pistons removed, it was time to split the cases, after removing the oil pump and clutch basket, then pushing out the shift shaft, which snagged a bit inside the cases. Apparently the drive chain had been run loose and put some little notches in the shaft causing removal difficulties. Once the shift shaft was removed, the shift drum return hardware was lifted away. The kickstarter shaft is held inside the cases, so that had to wait until the cases were completely split.
More screws, 6mm bolts and 8mm nuts were wrenched loose, usually with a snap/crack sound as they gave up their years’ long grip on the engine cases. A few whacks with a plastic dead-blow hammer broke the case sealer bond and allowed the cases to be separated. The transmission counter shaft seal seemed to have been glued to the engine cases, so one shaft was stuck in the top case and the other one in the bottom case. Eventually it all came apart and the stout transmission components were set aside.
The shift forks are retained in the shift drum with little steel pins, held in place by small springs. Fishing the pins out required a strong magnet and screwdriver tip. The shift drum was slid out of the upper case and the crankshaft holders were left to remove once the long bolts were broken loose and set aside.
BIG NOTICE! In the bottom case, there is a small but critical plastic round ball which is inserted before the engine cases are reassembled. The ball is situated just about the oil pump outlet passages. IF the ball is not replaced, the engine will never see oil to the top end and the camshaft will seize in the cam bearings.
Doug hauled the boxes of parts away for cleaning, while I gathered up parts for the rebuild. Sidebar story to include was as Doug drained the oil and began to remove the dyno cover, a small steel curved piece of metal appeared. The engine was described as being a bit noisy with a growling noise coming from the bottom end. The metal piece was actually a 1/2” of the main bearing ball retainer. I’ve never seen this kind of bearing failure before and it could only be some kind of manufacturing failure from the looks of it. Finding a new bearing was a challenge as the two listed on eBay were $80 and $100 plus shipping. A full web search didn’t come up with any other findings. The bearing is not modified with a locating groove or pin hole, so I am surprised that a standard replacement bearing can’t be found.
What we discovered is that the listed 6206 bearing on current Honda microfiche listings is INCORRECT. Who knows why Honda chose an obscure one-off crankshaft bearing. From checking the CB350K0, I see that they started out using a roller bearing and cage, somewhat like those used on the 250-305s. Apparently that design wasn’t up to the task and they chose an obscure sized bearing to take up the spaces. The listed 6206, as we discovered was about a 1mm too small in the OD, so rattled around in the engine cases. Honda superseded the original roller bearing with a 91001-312-014 part number which is now NLA, world-wide. Doug did some digging and found the correct bearings in ITALY for $26 each, plus $50 shipping. He bought the two shown as available and now we sit in waiting for the correct part to reassemble his engine. With so many engines having been produced, the supply of crankshaft bearings over the past 56+ years has gone to zero.
Honda, reportedly produced about 600,000 250-350 engines. Parts lists show the early roller setup into about 1971. The rotors on the 350s are a solid one-piece design vs the layered and open type used on the 205-305s, which are the same diameter. Perhaps they were either heavier or had stronger magnetism which put more strain on the crankshaft bearing. Rather than redesign the crankcase bearing bores to fit something conventional, they seem to have chosen to have the bearing company design a specific bearing just for the 350s. Being that my focus is on the 250-305 engines, we both learned some new and unsettling discoveries about the 250-350 engines. My riding experience with 350s is that they vibrate heavily, causing fasteners to loosen up and the mufflers to suffer failures internally, if they haven’t rotted out by now due to condensation damage. Perhaps the additional engine vibration generated forces that damaged the original bearing designs.
A few weeks later….
Doug had all the parts sparking clean and ready for reassembly, as we began to assemble the bottom end, we discovered that the “CB/CL/SL350” seal kit from my usually reliable 4into1.com came up short with the big, thick, triple row transmission shaft seal. Going back to the site, the seal was listed separately, but with no real notes about the fact that it needed to be included with the rest of the seal kit.
This brought the reassembly to a halt, while we decided what to do next.
"Next" was to put the cases loosely together and assemble the top end. You have to put a little wire on the endless camchain, as it gets drawn upwards through the engine top case, the cylinders and eventually to the cylinder head. The part that has made me uneasy in the past was the goofy lower end of the camchain tensioner with two rollers. The bottom end of the tensioner arm is held, somewhat loosely with a hollow pin with the ends half cut away and two tiny little rubber keepers that fit into the ends of the pin and then the whole thing sits down inside a groove in the top case. There is nothing to keep the parts from moving around or exiting their locations until the cylinder base gasket is installed. And even then, there is not a lot to keep things secure there.
We fitted the pistons back onto the rods, after the rings were installed. The replacement ring set had a 3 piece oil ring design. The top compression ring is shiny on the edges and the second ring black and seems to be flat on both sides instead of having an undercut around the bottom edges for scraping oil off the cylinders. We fitted the cylinders down on top of the pistons and the rings went into the bottom cylinder bevels fairly easily. Unfortunately, we overlooked that he camchain tensioner roller assembly hadn’t made it inside the cylinder cavity. So, back off the pistons to position the tensioner inside the central cylinder block cavity. The rings seemed to have decided to hang somewhat on the right side, even though they appeared to be well inside the cylinder bores. We double checked the three piece ring set as the central expander ends must just butt up against each other and not overlap. We dismantled the ring set to verify that the expander and rails were all properly installed in the ring lands and finally it gave up and let the cylinders fall fully down on the upper crankcase. All of this movement back and forth of the tensioner had me concerned about whether the little end bits were staying in place. I actually put a dab of sealer on the ends to see if they would stay put, but once the cylinders are down on the base gasket, there is no way to determine if everything is still in place. It’s a horrible design in my opinion.
The next challenge was to loop the camchain over the camsprocket and then feed them both up through the narrow opening in the cylinder head. We wrestled with the camchain and sprocket as the camshaft was inserted, but Doug finally noticed that the head was not all the way down on the head gasket and dowel pins. I put a small socket on the end of the cylinder studs with a nut to hold it all in place while we wrestled with the camchain and sprocket. The chain kept skipping off of the crankshaft sprocket causing the camshaft timing to be incorrect. I withdrew the camshaft, pulled up on the chain until it felt like it was engaging with the crankshaft sprocket and positioned the cam sprocket once more while we slipped the end cam bearing on one side to hold the camshaft in place. We fed the rocker arms and adjuster shafts into the cam carrier and then fiddled with the rocker arm position by moving the crankshaft back and forth just enough to get the rocker arms off the cam lobes enough to take tension off of the camshaft and finally install the cam bearings with the gaskets in place.
It’s a very tedious task overall. I haven’t wrenched on a 350 for several years and ever time it was a struggle to get it all right. I’ll gladly stay in my lane with the known 250-305s, thank you.
Doug ordered the special seal and had it delivered to his home, so he could complete the assembly of the lower end and finally button it all up for the last time. I’m available to consult on the final touches of setting the points and ignition timing, plus helping advise the setup of the carburetors as it finally goes back together for the last time. As Doug left with the remaining parts in hand, I reminded him not to forget the little plastic ball in the engine cases!
Whew! I’m going to pass on any further 350 twin engine work in the future. I’m still puzzling over the odd crankshaft bearing issue that is bound to arise for other 350 engine builders. It seemed like Doug found the last two bearings on the planet. What happens to the next guy who needs a crankshaft bearing?
Bill Silver
No comments:
Post a Comment