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1 HONDA CB160/175 RACERS Michael A. Moore Copyright HONDA CB160/175 RACERS STATEMENT OF NON-LIABILITY: Racing a motorcycle is dangerous, and modifying engines can result not only in damaged
1 HONDA CB160/175 RACERS Michael A. Moore Copyright 2 HONDA CB160/175 RACERS STATEMENT OF NON-LIABILITY: Racing a motorcycle is dangerous, and modifying engines can result not only in damaged parts, but damaged people as well. All the information presented here is based on my own experiences - it may not work for you. Any changes you make to your vehicle are at your own risk, and the result of your own decision to make the change. I disclaim any responsibility for your actions, or your accident. INTRODUCTION I had a lot of Ducati singles before I switched to my first little Honda twin. I enjoyed riding the Ducks (and was sponsored by a friend on a very nice 350 for a season), but I grew to loathe some of the design features that often kept my bikes from running for any appreciable length of time. My friend Henry Deaton had been roadracing a sloper CB175 (all future references to 175s are about the CB160-style sloper, not the late 1969 (1970 model year) vertical cylinder 175) with AFM, and I found myself intrigued by the styling and construction. Horizontally split crankcases, real electrics, and no bevel gears - you could even get parts from your friendly local dealer! It didn't hurt that I already had a 750 Laverda twin, which looked a lot like a Super Hawk, heavy on the steroids/pasta. After discussing the potential of the engine with Craig Hanson, my favorite race-tuning maven, I bought a 175 rolling chassis with engine from Henry. As the 175 was not terribly effective against full 250s, we decided that we would shoot for building two identical engines that would be competitive in the vintage 250GP class. Henry had Craig modify a 160 chassis for his pumped up motor, and I built a frame to house mine. When I got my engine assembled and into Henry's modified frame I rushed down to the AMA/AHRMA national at Laguna Seca. I was pleased with the performance of the very roughly tuned engine. 3 DYNOMOMETER TUNING My engine received a long evening of tuning on Craig's dyno after the Laguna Seca race. I can not stress too heavily how important dyno tuning is to a race project. When it was first run on the dyno my engine put out the grand amount of 12 bhp at the rear wheel. An evening of dyno tuning which included changes only to jetting, timing, and velocity stack length bumped the horsepower up to just over 20 12,000 rpm. Power was still increasing, but we didn't run any higher on the dyno, although I later used an occasional redline of 13,000 on the track. I might have been able to get half of that increase on my own, but since many of the changes resulted in increases of one-half to one horsepower I would have never realized the full amount. Dyno time is the cheapest horsepower you will ever buy! My Honda is competitive with most 250 singles. In fact, the next year at Laguna Seca (the first AMA national using the extended track) I spent the whole race dicing with (and beating) a 250 Bultaco, which should be one of the faster 250s around. ENGINE CYLINDER HEAD The 175 cylinder head is a much better item than the 160 part. It has bigger ports, valves and finning. It also doesn't use a cast iron skull in the combustion chamber. 1. Porting A good port job is needed, and is worth paying a professional to do. However, beware the sewer pipe style port - bigger is not always better! At minimum, the area around the exhaust valve guide is very constricted and should be opened up, especially in the 160. Weld should be added to raise the floor of the exhaust port as the stock port has too tight of a radius after the valve seat. It is interesting to note how an exhaust port being tested on a flow bench improves in flow as the bottom of the port is filled in with clay - making the port smaller! In stock configuration the lower half of the port doesn't do anything except promote unwanted turbulence. The intake port is better but will benefit from judicious porting. Metal should be removed from the roof of the intake port until it matches the intake manifold. The face of the intake flange can be milled at an angle, tipping the carburetor so that the port and carburetor have a common axis. We milled an additional 15 degrees off the carburetor flange. Check your carburetor first before doing this, as not all carburetors will work at a 45 degree angle (30 degrees in the cylinder tilt plus an extra 15 degrees at the flange). 4 2. Valves Be sure to use real Honda valves or racing quality aftermarket valves. I had some very inexpensive replacement valves from Dixie International that had a valve head break off after several hours of running. You will not appreciate what happens then. OEM 175 Honda stock valves work fine, but the 160 valves are much too small. The rough area where the head is welded to the stem is a stress-raiser and must be polished so that there is no abrupt change in diameter. This area on the intake valves can be lightened by reducing the diameter of the part of the stem that is exposed below the guide when on the seat to the same diameter as the collet groove. Standard valves were used during the dyno testing. After the cheap valve broke shortened XL250 valves were used on both sides as they are slightly bigger. The hassle of modifying the XL valves makes me recommend the stock valves, since the valves should probably be replaced periodically. We later discovered that race quality stainless steel valves are not a lot more expensive than valves from the Honda dealer - Honda parts can be pricey. We didn't redyno after changing the valves size so I can't definitely say that the bigger valves make any significant difference in performance. 3. Springs Racing quality PM valve springs can be obtained, and work with the stock steel retainers which are strong and reasonably light (as well as cheap). Use a rpm red line, and plan on changing the valve springs at least at the beginning of each season. I've found that a rpm limit can be used if you really need it to squeeze by someone at the finish line, but continued use of it will result in greatly increased valve train wear. If the engine starts to misfire at the red line (and a fresh battery doesn't cure it) valve float is setting in. Stop the engine and change the springs immediately. Once the springs have weakened they can fail even if the rpm is reduced. 4. Camshaft Use the Megacycle grind cam (they will do the 180 degree cams for another $25-30). Coating the cam lobes and rocker arm pad with KalGard Gear Kote is recommended (as are all of their fine products). I have converted the cam to needle bearings. This isn't that hard, but you can't run the bearings directly on the cast-iron cam. You must turn down the journals, press on an oversize bearing race, and then have it ground back to the stock 20mm dimension. By using the stock journal size the cam can be used, if needed, in the standard endcaps. The outer bearing caps must be bored out to take the needle bearing. This reduces friction (and heat), and stabilizes the cam giving more accurate valve (and ignition if run off the cam) timing. As with the wristpins, the rocker arm spindles should be nitrided to reduce wear (there are no bushings in the rocker arms). The rockers can be lightened by grinding away the end part of the pad that never wipes on the cam. Be sure to carefully smooth all edges after grinding to remove potential stress raising notches. 5 5. Camchain Stock cam chains are OK but the master-links aren't. At high rpm the master-link clip is subjected to a very high sideways loading as it goes over the crankshaft sprocket. Eventually the outer leg of the clip will break off, and the master link will fall out. What I have been doing is epoxying the clip to the link plate. This is bothersome, but I have not yet found a rivet-type link for the stock chain. The 450 chain is the same pitch but the pins are about.003 undersize, which makes for a rather wobbly master-link. I intend to go with a 450 cam chain when I replace the stock item, so that I can use a rivet-up master link. CYLINDER/PISTONS/CAMCHAIN TENSIONER 1. Cylinder and Pistons There are still some 160/175 oversize piston kits available that will give a displacement around cc. However the piston crown is usually fairly high. For 200GP I would use either 55mm pistons from the Z500, a 500cc version of the KZ550 that was exported to England, or 108cc big-bore kit pistons for the Honda S90/ATC90 engines. CB200 pistons will not be suitable because of the large dome on top and a bigger wristpin than the 175. The 90 pistons are readily available, cheap, and have the same deck height as stock. The 90 pistons do have a substantial dome that may interfere with flame propogation in the combustion chamber, so I would prefer the Kawasaki pistons if available. This would give a displacement just under 200cc, and the oversizes would be within the 200cc plus.060 overbore rule used by AHRMA. I haven't checked to see if these pistons would bore into the 175 liners, or if replacement liners will be needed. (SEE ADDENDA) To get the 216cc displacement I used standard size pistons (58mm) from a Kawasaki KZ550. They have the right size of wrist pin, and after minor machining of the top to clear the head they have a fairly flat top that will not restrict combustion-flame propagation. They are also readily available. Remove just enough from the top to clear the head - this creates a squish band and leaves the rest of the piston to take up space in the combustion chamber. Cylinder liners from a CB200 must be installed. They require the cylinder block to be bored out, and the liners must be shortened and the bottom OD reduced. The upper crankcase must also be bored to accept the new liners. Wiseco makes 61mm pistons for the KZ550 which would give 240cc. I know that some people have taken the cylinders out this far. I haven't, and it looks a bit questionable to me. Proceed at your own risk. After installing the sleeves have them bored to fit the pistons. I made and recommend using a set of torque plates, which clamp the cylinder assembly and simulate the stresses caused by the cylinder bolts during the cylinder boring. This will ensure that the bores are round when they are installed, not round while on the workbench. The CB200 liners normally take a 55.5mm piston, so undersize liners or oversized pistons might needed. The deck height of the pistons must be reduced so that they are even with the top of the cylinder at top dead center. The squish band will extend in from the edge of the piston to the outer edge of the combustion chamber. The squish clearance is then set by making a copper head gasket of the appropriate thickness i.e. about inches. Remember to check all of the squish and valve clearances with thin rosin core solder or clay after assembling the motor. It is also a good idea to drill holes from the oil ring groove into the wrist pin hole. This helps supply oil to the pin/piston and pin/conrod interfaces. Since the rods do not have bushings at the small end you should have the wrist pins nitrided to harden their surface. When combined with the pin oil holes in the piston pin/conrod wear will not be a problem. 2. Camchain Tensioner The larger liners require that the hex bolts and washers that hold the cam chain tensioner to the case between the cylinders be exchanged for Allen bolts and smaller OD washers. The mounting flange for the tensioner will also have to be narrowed slightly. The rubber cam chain rollers are junk. A new one will chunk after several races. Replace the adjustable (small) roller with the upper camchain tensioner sprocket from a Kawasaki Z1. This will give you a steel sprocket of the correct size (it also runs on a needle bearing). Order it from K&L and you will pay about $25 instead of the $50 your Kawasaki dealer wants. Or you can go to your local salvage yard and maybe get one even cheaper. Get the sprocket that uses the 6mm bolt, not the one that uses a pin. Use a 1/4 bolt instead of a 6mm to get a better fit. The head of the bolt and the nut will have to be narrowed so the assembly fits in the cam chain tunnel. The large roller is not so heavily stressed. Put a new one in to start, and plan on replacing it now and then. The camchain tensioner fixing bolt in the front of the cylinder often strips the soft aluminum threads out of the head. I replaced the 6mm stock bolt with an 8mm rear chain tensioner bolt when the cylinder head finally stripped. 6 7 ENGINE CASES Other than boring the cases for the oversize sleeves no modifications are required. I did remove the part of the case and left side cover that support the electric starter, but this was done mainly for cosmetic reasons. It is also moderately expensive to have plates heliarced on the case in place of the removed electric starter cover. A worthwhile modification would be modifying an oil filler cap to take a breather hose and check-valve. This will help to reduce pumping losses inside the crankcase by allowing air in the crankcases to be pumped out but not sucked back into the engine. The best valve to use is an automotive PCV valve. There are some nice metal valves that are spring-loaded with a rubber seal. These valves are not position sensitive. Be sure to plug the small useless breather in the rocker cover. You can also remove the steel baffle plate in the cover when this is done. Remember to plug the clutch cover if the kickstarter is removed. CRANKSHAFT The crankshaft in the 160/175s is very nice. It has a good rod length/stroke ratio, lots of bearings, and sturdy rods. Rod kits are still available (I think we used C200 rod kits from Rocky Dist.), and other than magnafluxing and shotpeening the rods no reliability modifications were done. On our engines we did rephase the crank to 180 degrees to reduce crankcase pumping. This was a good move theoretically, but may prove to be a lot more work than the benefits gained. Don't forget that the cam and ignition must be redone to match the new crankshaft orientation. The crankshaft was also lightened since it was going to need rebalancing anyway. After cutting the crank down we had to weld metal back on opposite to the crankpins to bring it into balance. It sure does rev quickly. Removal of the alternator and use of a total-loss ignition will also help reduce flywheel effect in the crank assembly. OIL PUMP The stock oil pump is bored out to use a CB200 piston, which increases the pump volume by about 18%. Some braze was added to the flat near the top of the piston bore prior to boring to reinforce the thin section of the pump body. Boring the pump to take the bigger piston also allows setting up a tighter clearance between the piston and the pump body, which will increase the pump's efficiency. Even when running a stock pump you must check the fit of the check valve retaining plug on the side of the pump body. On my pump this plug screwed in (earlier pumps use a push fit with a retainer) far enough to block about 1/2 of the outlet port. Increased pump volume won't help if the oil can't get out of the pump. The oil pump and crankcase oil system can be ported like a cylinder head. Radius all sharp inside corners where two or more oil passages meet. The engine doesn't circulate much oil, so anything that can be done to reduce friction is worthwhile. Remember, oil is much thicker than air. I have been using Kendall GT-1 30wt oil and don't see any reason to go to a synthetic. It has occurred to me that running external oil lines from the cases to the head, instead of feeding oil up the cylinder studs would probably reduce the oil temperature. However, it seems to be another of those good but not required modifications. We have recently fitted up an oil cooler because the 216 tends to run hot, due to the extra power being extracted. We modified the clutch cover to route oil to the cooler before it goes to the oil gallery that feeds the crank and top end. This required a moderate amount of welding. The cooler being used is off of a Kawasaki 900 Ninja since it works well and fell readily to hand. Braided steel hoses are not necessary as the Honda oil pump only makes a couple of pounds of pressure. 8 CLUTCH/TRANSMISSION Several different clutch baskets were used on the CB160/175. Some of them can have the gears drilled to reduce the rotating mass. Do be careful that you do not drill into the cushdrive rubbers. I am using Barnett friction discs with stock springs and steel plates. Slippage is not a problem. However, the Barnett plates tend to stick together between race days, and the clutch will not free up until part way through the first practice lap. Also, the Barnett plates need to be carefully inspected, as sometimes the friction material hangs out over the edge of the plate. This will prevent the clutch from releasing properly. The ratios in the speed transmission are adequately close for racing. The 5 speed can be fitted to the 160 engine cases with some minor machining in a vertical milling machine. 9 CARBURATION When this engine project began the AHRMA carburetor rule allowed updating carburetors as long as the OEM brand was used. 26mm Kei'hin smoothbores were chosen for their historical accuracy as well as their performance and tunability. The carburetors are rubber mounted and use long manifolds and velocity stacks (7.5 inches overall length from head - the longer the intake the more the bhp went up). If 26mm Super Hawk carbs are used the bore can be cleaned up by removing the choke plate and filling the recess with epoxy filler. I haven't checked, but if the Hawk slides and body are big enough it would be possible to bore the carbs out. Make sure that the metal is not removed from the floor, and that the bore is less than the slide diameter. Boring also smooths out the venturi area. Current AHRMA rules prohibit integral float-chamber smoothbore carbs, so we could probably plumb an Amal matchbox remote float-chamber into the CR float bowls to be technically legal. This is a stupid rule! I have been agitating to get this rule changed. There are some specially made float needle/seat sets made for a number of carbs that will do a much better job of sealing at extreme angles than the stock float needle assemblies. The stock CR carburetors do seem to work better at high angles than some other carbs. A 28mm non-smoothbore would be needed to get equivalent flow to a CR. A VM Mikuni or a Kei'hin PE carb could be used. We now think that the engine could likely use a bit more carburetor than now used. EXHAUST Since this engine has a 180 degree crankshaft twin exhausts are needed. A stock 360 degree crankshaft can run with a 2 into 1 exhaust which will save some weight and tuck in better. The 360 degree engine will also run better with the 2-1 system. We tried removing the 2-1 tailpipe on my Laverda 750 and slipping megaphones onto the headpipes while running on the dyno. Power immediately dropped from 72 to 50, and the powerband had multiple flat spots. The header pipes are of 1.0 x.049 tubing (approximately 20.5 in length), and the 4 degree taper megaphones are about 16 long. Do not fall for the common error of sticking giant head pipes on -the smaller size keeps the gas velocity high. The tailpipe and collector assembly on a 2-1 system should be of equal length to the headpipes. If a megaphone is run (recommended) use something similar to the individual pipe dimensions and reduce the length of the tailpipe by one-half the length of the megaphone. Warning: these exhausts are not for street use! When tested with a db meter at a local race the bike registered 136dB at half throttle. My local club (AFM) at Sears Point requires 104db maximum. We'll be fitting up 3.5 Super Trapp discs (and an airbox to reduce intake noise) to try and cut the noise down to acceptable levels. The open pipes sound neat when out on the track - with the high rpm some people thought it was a two stroke. IGNITION The stock points are hopeless. A good ignition should be th
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