Z50 Engine building part three. The top end.

Once the primary drive is assembled on the other side of the engine, it is flipped over and the case screws put in. A single bolt holds the chain tensioner arm in place. It should move freely when tightened. The mainshaft and shift shaft seals can be pressed in place with a socket and a hammer.

The special washer and bolt that hold the transmission shift drum in place are installed.

The timing chain tensioner roller is simply dropped in place. It is held in place by the stator plate. The plunger and spring are placed in the bore till they contact the chain tensioner arm. There is a flat side on this particular tensioner plunger, where the locking screw contacts with it. The hole down and to the left of the shifter shaft is where the locking bolt and nut go.

Two of the three chain tensioner plunger styles. The lower one is the earlier one, and the upper one came after, followed by another steel plunger with holes perforating it.

There are two lengths of engine head studs. The longer ones go on the left side of the engine, with the engine dowels at their base. The studs can be snugged up using two nuts tightened against one another. These are some homemade levers with the bolts welded on to avoid having to use a pair of wrenches.

With the studs and locator dowels in place, the paper cylinder base gasket goes on along with the flat o-ring that fits in the round opening on the right of the picture.

The piston rings need to be put on. The shiny one on the right goes at the top of the piston, the dark one in the middle, and the three-piece oil ring goes on the bottom. They are put on starting with the wavy oil ring followed by the skinny rings that go on each side of it. The middle ring goes on next, and the top shiny ring last. The gaps at the end of the rings should be staggered so they do not all lie in the same location.

Once the rings are on, the piston can be joined to the crankshaft. It is easier to place the wrist pin clip into one side of the piston before putting the wrist pin in. Be sure to place the piston with the intake side toward the top of the engine. There is usually a stamp with “IN” marked on the intake side, or an arrow is sometimes used to point it out. If neither of those marks are there, the side with the larger valve recess is the intake side.

There is a small flat o-ring that goes around the lower right engine stud and fits into the head gasket. There is a second larger flat o-ring that goes in the larger opening seen at the upper right of the picture.

To set the valve timing, the flywheel needs to be placed on the shaft and the crank rotated until the “T” mark lines up with the notch at the top of the engine case.

Using a screwdriver or round rod, the timing chain is pulled tight, keeping it between the studs. The place where it comes to a peak is the pin that will be lined up with the mark on the cam sprocket. The cam sprocket is placed on the chain and fed through the head to line up with the cam. The cam should be in the top dead center position. You can tell it is in the right position if the cam turns freely about a quarter turn or more. If it is in the wrong position, the valves will be engaged, and the cam will be difficult to turn.

The small mark on the cam sprocket should line up with the mark on the head, making sure the “T” mark on the flywheel is still aligned.

Z50 engine building part two. Assembling the primary drive.

The clutch, primary drive gears, and gear selector assembly can be installed after the main engine cases are together. None of the engine case screws have been installed yet, and will be put in from the opposite side once the primary drive is done.

The kickstart return spring assembly should be put on first. The shaft should be turned fully clockwise till it stops, then the splined cog is slid partially down the shaft with the tip of the cog contacting the engine case as in the above picture. Using a screwdriver or a hook, the return spring is pulled so the end loops around the engine case boss. Once the spring is there, the cog can be pushed down fully till the groove for the C-clip can be seen on the kick shaft.

The high volume oil pump used for this build has a collar that must be fitted in the drive shaft hole to support the drive shaft. Don’t forget to use a new gasket.

The oil pump is installed being careful to make sure the shaft and pump mesh. The drive sprocket can be turned to get them to mesh correctly. The three larger screws are the ones securing the pump. The gear selector assembly goes on next, with the shaft dropping in the hole, the end of the arm contacting the gear selector star pins, being sure the spring that goes on the shaft is contacting the engine stud correctly. The extended ends of the spring should be on opposite sides of the shaft. The shift star can be put on now, or before placing the selector arm in the case. Lastly, the small arm with the roller on it (the shift detent arm) can be put on, making sure it moves freely once its securing bolt has been tightened. Notice that the C-clip for the kick shaft assembly is in place, and the return spring around the engine case boss.

If your crank is new, you will need to put the spacer and bronze bushing on before the clutch and drive gear are put on. Use plenty of oil assembling these parts.

The clutch drive gear drops in place, and the larger primary gear slips over the shaft, secured by a C-clip. The oil filter screen can be put in place.

The clutch drops onto its shaft, making sure the gear beneath it meshes with the backside of the clutch, the large primary gear, and the clutch/crankshaft splines. It will be necessary to turn the clutch and large gear while wiggling the clutch to get it all to mesh correctly. The clutch nut is put on after the locking tab and special domed washer.

This picture was from the disassembly, but using it to show the clutch cover in place.

This is the auto clutch actuator assembly in place. Usually I will wait to put these parts on till after the opposite side of the engine is done. The reason is that once these parts are on, the outer engine side cover has to be put on to keep them in place. Having the cover in place makes it harder to work on the stator side of the engine because of the roundness of the cover. This engine stand secures the engine even with the cover in place, but will contact the cover, marring a newly painted cover. The actuator assembly just floats until the cover is on, and centering it so the cover contacts it correctly is much more difficult if the engine is upright.

This is the clutch adjuster screw assembly. It fits on the inside of the cover with the screw passing through the center hole, and the post in the blind hole. On the opposite side, an O-ring, washer and nut will be placed to secure and seal it.

The Festival of Engines 2020. Honda Z50 and CT70 6 Volt engine rebuilding Part One

Once again it is time to do engine building around the shop. It is a lot faster to do multiples, so six rebuilds were undertaken. A number of people have asked for info, so I am going to put out a light guide for getting an early (1968-1979) engine rebuilt. Not meant to be a definitive guide, you would still want to get one of the Clymer or Haynes manuals for the details.

Everything gets rebuilt, replaced, or re-plated. All seals , gaskets, and bearings get replaced. All hardware is re-plated or new. There are four 108cc stroker engines, two with 4 speed trannys, one with a rolling rocker head. All four were given new heavy duty clutches. The two stock rebuilds have new pistons and rings, clutch plates and bearings. The stators and ignition components are restored, and any stripped screws are repaired (although in a rare case, there were no stripped holes on any of the engines.) All the cases were finished as close to the original finish as possible, refinishing any scratches or damage to the surfaces.

This engine case looks like it was hit by a wire wheel at some point, leaving deep scratches that were removed by hand wet sanding. There are not a lot of shortcuts in metal finishing if you are going to restore a smooth finish removing as little material as possible.

The clutch covers are painted with Cloud Silver paint from Honda. There are new kick and gear shift shafts used on all these engines.

Soon these bikes will be going into some of the bikes in previous posts, and some yet to be seen!

Some of the tools needed. In addition to these, a 6mm and 8mm tap, variable speed drill, an air compressor, and some JES screw drivers are needed. Tools: clutch nut tool, flywheel puller, 17, 14, 10, 9mm sockets, #2 and #3 phillips impact bits, impact driver slip jaw pliers, crescent wrench, needle nose pliers, ring pliers, brass drift, rubber hammer, seal puller.

Somewhere around 60 Honda horizontal engine rebuilds done in the shop, most of them on this stand. You don’t need anything too fancy, but a way to elevate the engine off the bench to clear the shift shaft and kick shaft is needed. A couple pieces of 4x4 lumber will work, or a box shape made of 2x4’s and screws that fits under the outer edges of the engine cases. The main reason this stand was built was to hold the engine firmly while extracting case screws. Because the screws are Phillips type screws usually frozen in the cases, the only reliable method I’ve found to get them out without damage is to use an impact driver, #3 Phillips impact bit, and a considerable amount of downward force. Without a solid base, the irregular surfaces on both sides make this a challenge. The engine will sometimes spin or even fall off the blocks during this process. The stand keeps this from happening and is easy to use, requiring no tools to mount the engine on it.

The actual tear-down is pretty fast and easy. This assembly guide can be done in reverse to disassemble. The time consuming part is cleaning grime and gaskets from the parts. The cases are cleaned first with citrus degreaser, scrapers, towels, and compressed air, removing as much oil and grime as possible. They are washed and scrubbed in kerosene and all gaskets removed. They are then wiped down, blown out, and washed in warm soapy water. They are bead blasted after that, and then blown again, scuffed with fine Scotch-Brite pads, rinsed, and blown. Along the way, any stubborn deposits are scraped off with a razor or pick. Finally, the screw holes are all chased with a tap, and then blown out agian. Obviously, a lot of blowing going on, and some would argue not to do so much in between the steps, but degreasing is tricky in the sense that some processes don’t work as well in different situations. A common example would be thick oil and dirt deposits inside and outside the engine., that are not easily removed by any method, can trap glass bead from blasting, and interfere with trying to chase your threads. In that case, mechanical scraping and wiping are the only sure-fire low tech ways (you can always drop some coin for a large ultrasonic cleaning tank.) The other principle is that each stage in cleaning should bring the minimum amount of contaminants to the next, and blowing does this effectively. A word of caution: eye, face, hand, and ear protection are absolutely necessary for all the blowing, especially if you are using high pressure. Small particles of grime can fly out when blowing and hit you like a sand blaster to the eyes. The ear protection is for the ultra high-pitched whistling that is produced blowing across screw holes.

Once the cases are clean, the oil orifice is drilled out with a 1/16” bit. You can go as high as 2mm in diameter, and as small as 1.5mm. Blow out after drilling.

The transmission bearings need to be pressed in first. The cases can be heated with a small propane torch or heat gun to expand the hole size slightly, and then the bearing can be tapped into place easily with a drift and hammer, making sure only to strike the outer ring of the bearing.

Next, the oil drive shaft and sprocket have to be put into the cases. The fat shaft is the original, the skinny one has been turned to clear the stroker crank, and came as part of a high-volume oil pump kit.

The screw end of the shaft has to pass though the case and into the sprocket. A small rod or screwdriver and a wrench are used to snug up the shaft. The center of either style shaft can be grabbed with vise grips to screw or unscrew it. It does not need to be super tight, the direction the pump rotates keeps it from unscrewing.

The three speed stock tranny with the last parts of the output shaft laid out in order, and the shift drum as it mates up with the shafts. Notice the small thin washer on the left end of the upper shaft, and laying on the far right of the picture. Those special washers are sometimes overlooked, often staying stuck to the engine cases and even lost during disassembly.

The transmission ready for installation. The output shaft on the right of the picture fits into the bearing pressed into the case, the smaller end of the other shaft goes in the bronze bush in the case half, and the shift drum shaft through the top hole in the case.

A small amount of oil needs to be squirted in the bronze bush in the engine case, and the transmission dropped in. If too much oil is used, it will keep the shaft from seating fully. The three main parts of the tranny need to be held together carefully, making sure the thin washer is fitted between the shaft and the bronze bush.

To keep the shifting tight, the shift drum guide pins should be replaced. There is a small hairpin-like clip that holds them in that can be pulled out easily with a metal pick. The pin sometimes gets stuck in the hole, and a strong magnet while wiggling the shift fork will usually do the trick.

Once the tranny is in, the kickstart shaft is placed in the oiled case hole, making sure the arm goes into the groove on the side of the case.

Dropping the crank in requires no tools, and should fit in easily with a little hand pressure and wiggling. The engine case dowels (always use fresh ones, trust me) and center case gasket are the last thing needed before closing the cases back up. Leave the piece of gasket paper that passes across the piston rod until after the case screws are in place.

The right side case should fit on the assembled left side easily, with only slight taps of a rubber mallet needed to coax it together. Look out for the oil drive shaft, transmission shafts and the shift drum to be aligned. They may need slight adjusting to hit their holes right. The remainder of this guide will be in installments, focusing on individual sections of the build for easier future reference. If anyone has questions, please ask on this site so others can perhaps benefit from the Q and A.