EricGorr
Super Power AssClown
- Aug 24, 2000
- 708
- 1
CR450F First Top End Rebuild
Reading the Honda manual made me paranoid that if I didn’t perform a top end rebuild on my friend’s CR450F in the prescribed 15 hour interval, that I’d have a pile of molted shattered metal on my hands. I’m here to tell you that Honda has build quite a reliable little motorcycle. The rider, Tyler Smulders (The New Prospect) is an amateur expert level mx rider from Wisconsin who rides all year in mx and ice racing. The bike had 50 hours of running when we tore it down for the rebuild. Although I had the owners manual on the workbench, I tried to resist the temptation to use it to do the top end rebuild until absolutely necessary. I think that a top end rebuild procedure should be obvious to the mechanic, if an engine is designed for easy maintenance. Overall I’d say that the CRF is an easier bike to work on then the YZF, especially when it comes to installing the cam correctly. However the Honda manual skips over some obvious points where the average guy is bound to get in trouble. Honda’s manual outlines the basics of what parts need to be removed to gain access to the top end. Obvious stuff like removing the fuel tank, sub-frame, hoses and wires. Some of the bolts are a bit hard to reach so do yourself a favor and get a deep-well socket set, short and long extensions, and a spark plug socket with a rubber sleeve to hold the plug, and a full selection of Allen-head sockets. Forget about T-handles because they’ll only get you as far as the plastic on this bike!
The valve cover has 3 bolts with rubber sleeves, don’t loose the sleeves! Whoever designed the two front bolts deserves to be demoted to head toilet scrubber! Those two bolts will take you between 15 seconds and 10 hours to re-install. I’m not joking, it’s the stupidest thing I’ve ever seen on a Honda! The bolts resemble air-filter retaining bolts, in that they are long and have a double shoulder. After an hour of frustration I finally drilled and tapped the cover bolt heads to accept a 5m x .8 bolt that enabled me to align the cover bolts into their recessed threaded holes. Another possible alternative is to buy a 10mm nut-driver with a magnet imbedded inside the socket. Some aftermarket company needs to make bolts that enable quick installation.
Next remove the Allen covers on the left and right side of the engine that is positioned near the crank centerline. The cover on the right side gains access to an Allen bolt that enables you to turn the crankshaft, turn it in a clockwise direction (normal engine rotation) until the piston comes up to TDC (compression stroke), there are two marks visible through the left side Allen cover and an alignment mark on the case. Before you remove the cam, check the valve clearance and record the dimensions. The intake and exhaust valve clearance can only be adjusted with different shims.
Remove the cam chain tensioner cap screw and then the tensioner. Now you can remove the two Allen bolts that retain the cam gear. In order to remove those bolts you need to hold the Allen bolt on the crank (right side). Do yourself a favor and pack rags around the gear so the bolts don’t fall into the bottom of the crankcases. The gear slides out of the way and the UNICAM can be removed easily as a cartridge after the four retaining bolts are removed. Be extremely careful not to loose or mix up the valve shims. The UNICAM cartridge houses the intake shim cups, the shims may or may not stay on the top of the valves. If they stick to the inside of the cups, remove them and mark them for left and right. The exhaust valve shims seem to stick well to the top of the valve spring retainer, that is unless you turn the head upside-down to scrape the remains of the head gasket! Best to remove and mark them too. Use a magnet to remove them rather than a pick that might scratch or mare the surface finish. I really like the design of the UNICAM cartridge. Most of the new style thumpers run the cam on the head bare, with no Babbitt bearings. So if you loose oil pressure the cams will score the head and valve caps, which require you to replace the $700+ cylinder head. With the CRF, if you loose oil pressure, the damage will be isolated to the relatively inexpensive UNICAM cartridge, rather than the expensive cylinder head. One minor complaint, Honda had a golden opportunity to enable mechanics to wire the cam gear to the chain so the head and cylinder could be removed without disturbing the timing of the cam gear, chain, and crankshaft. If the rectangular slot of the cam chain pocket was 5mm bigger, the head and cylinder could easily be removed; unfortunately you have to remove the gear from the chain in order to remove the head and cylinder. Its no big deal for an experienced mechanic to align the cam gear, chain, and crank position, but the average guy trying to follow the vague owners manual is bound to get in trouble.
With the cam cartridge out of the way, the head can be removed by removing the four head nuts. Selecting the right tools for this job is important. There’s very little swing space for a ratchet and the nuts are very tight and easy to strip. I used a 6-point short impact socket and a ratchet with a pivoting head and a banana handle, available from Craftsmen. Take care weaving the ratchet in the tight space available and take short strokes to remove the nuts. If you strip the head nuts, which are made from a relatively soft material, you’ll have to cut them off with a die grinder and that is messy! Honda should’ve used Allen or TORX fasteners for the head nuts. Maybe the aftermarket will step-up to the plate and make these fasteners along with valve cover bolts that are user friendly. Remove the cam chain adjuster from the back left side of the cylinder and the plastic guide block from the front of the cam chain before trying to remove the head. Use a plastic mallet to tap on the right and left sides of the head to rock it off of the dowel pins. Same thing goes for the cylinder. Don’t be too concerned about the cam chain. Unlike the YZF the chain can’t bunch up under the crank sprocket because there isn’t much space. On the YZF if the chain falls off the sprocket it can be a frustrating task to align it properly. Honda took that into account on the CRF.
I removed the piston and cylinder, pulled the compression ring off the piston and measured the ring end gap, piston skirt diameter, bore size, and piston to cylinder wall clearance. After 50 hours of teenage abuse, all the dimensions fell within Honda specs as listed in the factory service manual. I’m guessing that the average guy will get 100 engine hours between top end rebuilds. For this particular bike, I replaced the parts anyways. I checked the ring end gap of the new ring and it was right where Honda prescribes so no filing was needed. The ring is marked for the up facing position. One note about ring opening alignment, four-stroke engines don’t use alignment pins like two-strokes so you have to position the gaps 90 degrees from each other to prevent leakage. The compression ring should be positioned either left or right and not in the direction of crank rotation for longest wear. The one thing that distinguishes the Honda cylinder from other popular thumpers is the half-round cutaways in the skirt. This enables easy installation of the cylinder onto the piston assembly. You squeeze the rings just like on a two-stroke. The skirt is also tapered to prevent ring breakage when the piston is inserted into the cylinder. I tried installing the piston assembly in to ways; installing the piston into the cylinder then lowering it onto the rod, and installing the piston on the rod first and lowering the cylinder onto it. Both are fairly easy, the first way is preferable because you don’t have to install the clip in a narrow space. The base gasket is made of stainless steel so there are no worries of it squeezing out like a paper gasket.
Once you get the cylinder on and install the front chain guide, the head is ready for installation. The biggest concern is how to torque the head nuts. Because of the narrow space and the bulkiness of most torque wrenches it requires a long extension. Whenever you use an extension between a socket and torque wrench there is going to be problems with offsetting the load and aligning the torque wrench to get a proper setting. Just use common sense and be careful.
Tips for installing the UNICAM cartridge include installing the shims in the valve spring retainers first, wrap a piece of wire around the cam chain to pull it up, set the crank position so the two slash marks on the left side of the crank center on the stator mark on the case. The UNICAM will naturally install in the correct position because of the spring tension on the valves. Now the tricky part, loop the sprocket on the chain and try to align the boltholes with the cam. It may take several attempts at rotating the gear to different chain positions in order to get it to align properly. It’s still way easier than the same task on an YZF. Before you install the cam gear bolts, clean the bolts and threaded holes with LOCTITE Clean & Prime or brake cleaner and allow them to air dry before applying Blue LOCTITE. Remember if those bolts come loose they will shred that expensive engine! Torquing the bolts on the cam is easy if your arms are 4 feet long, for the rest of us you’ll need a helper to hold the crank tight via the right side Allen bolt so you can tighten the cam gear bolts. Honda’s wisdom of putting the crank alignment marks on one side of the engine and the bolt to rotate the crank on the other is questionable. From a mechanic’s perspective its better to have them on the same side like the YZF. The best method of aligning and holding the crank in position is to use a special alignment bolt like KTM LC-4 engines use. The cam chain tensioner must be retracted by inserting a short flat-blade screwdriver into the back of the tensioner and turning it clockwise. Hold it in place and install the tensioner. Release the tensioner screw by turning it counterclockwise until it takes up the slack in the cam chain.
Overall I’d say that Honda has made a great engine that is reliable and easy to service. Hopefully someone from Honda will read this thread and include some of the tips in next year’s owner’s manual.
(Poster's note: Dumbell me revised this because I later found the section of the owners manual addressing the cam chain tensioner.)
Reading the Honda manual made me paranoid that if I didn’t perform a top end rebuild on my friend’s CR450F in the prescribed 15 hour interval, that I’d have a pile of molted shattered metal on my hands. I’m here to tell you that Honda has build quite a reliable little motorcycle. The rider, Tyler Smulders (The New Prospect) is an amateur expert level mx rider from Wisconsin who rides all year in mx and ice racing. The bike had 50 hours of running when we tore it down for the rebuild. Although I had the owners manual on the workbench, I tried to resist the temptation to use it to do the top end rebuild until absolutely necessary. I think that a top end rebuild procedure should be obvious to the mechanic, if an engine is designed for easy maintenance. Overall I’d say that the CRF is an easier bike to work on then the YZF, especially when it comes to installing the cam correctly. However the Honda manual skips over some obvious points where the average guy is bound to get in trouble. Honda’s manual outlines the basics of what parts need to be removed to gain access to the top end. Obvious stuff like removing the fuel tank, sub-frame, hoses and wires. Some of the bolts are a bit hard to reach so do yourself a favor and get a deep-well socket set, short and long extensions, and a spark plug socket with a rubber sleeve to hold the plug, and a full selection of Allen-head sockets. Forget about T-handles because they’ll only get you as far as the plastic on this bike!
The valve cover has 3 bolts with rubber sleeves, don’t loose the sleeves! Whoever designed the two front bolts deserves to be demoted to head toilet scrubber! Those two bolts will take you between 15 seconds and 10 hours to re-install. I’m not joking, it’s the stupidest thing I’ve ever seen on a Honda! The bolts resemble air-filter retaining bolts, in that they are long and have a double shoulder. After an hour of frustration I finally drilled and tapped the cover bolt heads to accept a 5m x .8 bolt that enabled me to align the cover bolts into their recessed threaded holes. Another possible alternative is to buy a 10mm nut-driver with a magnet imbedded inside the socket. Some aftermarket company needs to make bolts that enable quick installation.
Next remove the Allen covers on the left and right side of the engine that is positioned near the crank centerline. The cover on the right side gains access to an Allen bolt that enables you to turn the crankshaft, turn it in a clockwise direction (normal engine rotation) until the piston comes up to TDC (compression stroke), there are two marks visible through the left side Allen cover and an alignment mark on the case. Before you remove the cam, check the valve clearance and record the dimensions. The intake and exhaust valve clearance can only be adjusted with different shims.
Remove the cam chain tensioner cap screw and then the tensioner. Now you can remove the two Allen bolts that retain the cam gear. In order to remove those bolts you need to hold the Allen bolt on the crank (right side). Do yourself a favor and pack rags around the gear so the bolts don’t fall into the bottom of the crankcases. The gear slides out of the way and the UNICAM can be removed easily as a cartridge after the four retaining bolts are removed. Be extremely careful not to loose or mix up the valve shims. The UNICAM cartridge houses the intake shim cups, the shims may or may not stay on the top of the valves. If they stick to the inside of the cups, remove them and mark them for left and right. The exhaust valve shims seem to stick well to the top of the valve spring retainer, that is unless you turn the head upside-down to scrape the remains of the head gasket! Best to remove and mark them too. Use a magnet to remove them rather than a pick that might scratch or mare the surface finish. I really like the design of the UNICAM cartridge. Most of the new style thumpers run the cam on the head bare, with no Babbitt bearings. So if you loose oil pressure the cams will score the head and valve caps, which require you to replace the $700+ cylinder head. With the CRF, if you loose oil pressure, the damage will be isolated to the relatively inexpensive UNICAM cartridge, rather than the expensive cylinder head. One minor complaint, Honda had a golden opportunity to enable mechanics to wire the cam gear to the chain so the head and cylinder could be removed without disturbing the timing of the cam gear, chain, and crankshaft. If the rectangular slot of the cam chain pocket was 5mm bigger, the head and cylinder could easily be removed; unfortunately you have to remove the gear from the chain in order to remove the head and cylinder. Its no big deal for an experienced mechanic to align the cam gear, chain, and crank position, but the average guy trying to follow the vague owners manual is bound to get in trouble.
With the cam cartridge out of the way, the head can be removed by removing the four head nuts. Selecting the right tools for this job is important. There’s very little swing space for a ratchet and the nuts are very tight and easy to strip. I used a 6-point short impact socket and a ratchet with a pivoting head and a banana handle, available from Craftsmen. Take care weaving the ratchet in the tight space available and take short strokes to remove the nuts. If you strip the head nuts, which are made from a relatively soft material, you’ll have to cut them off with a die grinder and that is messy! Honda should’ve used Allen or TORX fasteners for the head nuts. Maybe the aftermarket will step-up to the plate and make these fasteners along with valve cover bolts that are user friendly. Remove the cam chain adjuster from the back left side of the cylinder and the plastic guide block from the front of the cam chain before trying to remove the head. Use a plastic mallet to tap on the right and left sides of the head to rock it off of the dowel pins. Same thing goes for the cylinder. Don’t be too concerned about the cam chain. Unlike the YZF the chain can’t bunch up under the crank sprocket because there isn’t much space. On the YZF if the chain falls off the sprocket it can be a frustrating task to align it properly. Honda took that into account on the CRF.
I removed the piston and cylinder, pulled the compression ring off the piston and measured the ring end gap, piston skirt diameter, bore size, and piston to cylinder wall clearance. After 50 hours of teenage abuse, all the dimensions fell within Honda specs as listed in the factory service manual. I’m guessing that the average guy will get 100 engine hours between top end rebuilds. For this particular bike, I replaced the parts anyways. I checked the ring end gap of the new ring and it was right where Honda prescribes so no filing was needed. The ring is marked for the up facing position. One note about ring opening alignment, four-stroke engines don’t use alignment pins like two-strokes so you have to position the gaps 90 degrees from each other to prevent leakage. The compression ring should be positioned either left or right and not in the direction of crank rotation for longest wear. The one thing that distinguishes the Honda cylinder from other popular thumpers is the half-round cutaways in the skirt. This enables easy installation of the cylinder onto the piston assembly. You squeeze the rings just like on a two-stroke. The skirt is also tapered to prevent ring breakage when the piston is inserted into the cylinder. I tried installing the piston assembly in to ways; installing the piston into the cylinder then lowering it onto the rod, and installing the piston on the rod first and lowering the cylinder onto it. Both are fairly easy, the first way is preferable because you don’t have to install the clip in a narrow space. The base gasket is made of stainless steel so there are no worries of it squeezing out like a paper gasket.
Once you get the cylinder on and install the front chain guide, the head is ready for installation. The biggest concern is how to torque the head nuts. Because of the narrow space and the bulkiness of most torque wrenches it requires a long extension. Whenever you use an extension between a socket and torque wrench there is going to be problems with offsetting the load and aligning the torque wrench to get a proper setting. Just use common sense and be careful.
Tips for installing the UNICAM cartridge include installing the shims in the valve spring retainers first, wrap a piece of wire around the cam chain to pull it up, set the crank position so the two slash marks on the left side of the crank center on the stator mark on the case. The UNICAM will naturally install in the correct position because of the spring tension on the valves. Now the tricky part, loop the sprocket on the chain and try to align the boltholes with the cam. It may take several attempts at rotating the gear to different chain positions in order to get it to align properly. It’s still way easier than the same task on an YZF. Before you install the cam gear bolts, clean the bolts and threaded holes with LOCTITE Clean & Prime or brake cleaner and allow them to air dry before applying Blue LOCTITE. Remember if those bolts come loose they will shred that expensive engine! Torquing the bolts on the cam is easy if your arms are 4 feet long, for the rest of us you’ll need a helper to hold the crank tight via the right side Allen bolt so you can tighten the cam gear bolts. Honda’s wisdom of putting the crank alignment marks on one side of the engine and the bolt to rotate the crank on the other is questionable. From a mechanic’s perspective its better to have them on the same side like the YZF. The best method of aligning and holding the crank in position is to use a special alignment bolt like KTM LC-4 engines use. The cam chain tensioner must be retracted by inserting a short flat-blade screwdriver into the back of the tensioner and turning it clockwise. Hold it in place and install the tensioner. Release the tensioner screw by turning it counterclockwise until it takes up the slack in the cam chain.
Overall I’d say that Honda has made a great engine that is reliable and easy to service. Hopefully someone from Honda will read this thread and include some of the tips in next year’s owner’s manual.
(Poster's note: Dumbell me revised this because I later found the section of the owners manual addressing the cam chain tensioner.)
