Honda CB160 Vintage Trials Bike

Early in 1996 I was browsing through some of my old motorcycle magazines. In the April 1969 issue of Modern Cycle I found an article on a Honda CB160 that was converted for Observed Trials use by Bill Deeter of Southern California. My first response was "This is a neat special". My second response was "I've got all the parts to do one of these in my garage". From such humble beginnings do time-consuming projects grow.

Mr. Deeter had overbored the engine to 180ccs, and fitted a single carb head from a 175 Honda. The compression was lowered (method not specified), a milder cam from a Honda CA was inserted, and a brass flywheel fitted to the back of the alternator rotor where the electric starter sprag clutch would normally go. He also built a 2-1 high pipe with fairly short head pipes and a very long tail pipe. The bike was lightened with an aluminum fuel tank from a Van Tech frame kit and Akront alloy rims (19" front/18" rear). A small hub from a Honda S90 was used in the front wheel, and attached to forks from a CL160. The bike was reported to weigh 215 pounds, and Mr. Deeter figured he had a year of part-time labor and $1000 invested in the project.

Mr. Deeter's Honda CB160 trials special 

A poor-quality Polaroid photo of the first mockup of my Honda CB160 trials special As my garage is very crowded and I couldn't back any farther away from the bike, I had to take the picture aligning the bike along the diagonal of the photo.

This material reflects progress on the bike as of early December 1996. I'll post more info/pictures as additional progress is made.

The project can be divided into two major components - engine and chassis.


An Observed Trials bike needs very good low end and midrange power, a fair amount of flywheel mass to help it keep trickling along at near idle RPM, and smooth throttle response. 10-20 rear wheel horsepower is generally adequate for the majority of trials sections.

My first task was to call my friend and tuner Craig Hanson. Besides building race winning 1000cc four cylinder engines and killer dirt thumpers, Craig has also done a number of Honda TL125/250 engines. I described the project, and Craig decided we should be able to get the engine to perform properly for trials. I packed up a cylinder head with the intake manifolds and carbs and shipped it to him for analysis/modification.

Small cylinder head ports would be needed to keep the gas speed up at low RPM. Craig achieved this by welding up the exhaust port and using a high temperature epoxy filler in the intake port and intake manifolds. Curiously enough, similar changes have proven to increase flow when used on small Honda twin road racers (see the pictures of my CR216 race bike on the graphics page of the web site). So these modifications should provide a power boost as well as improved low speed running on the trials engine.

The modified intake and exhaust ports
Craig's dyno experience (especially on my Laverda 750 road racer) has convinced him of the value of a 2-1 exhaust on a 360 degree crank parallel twin engine. He applied his formulae to the Honda specifications and came up with dimensions for an exhaust that would maximize power around 5000 RPM (as opposed to the stock power peak of 9-10000 RPM). He sent me a box with an assortment of .875" and 1.25" tubing bends, and some exhaust rings with the .875" hole offset to match the raised exhaust ports.

Several nights were spent holding tubing and a tape measure up to the bike to figure out how to run the exhaust. Of course, for trials the exhaust needs to be high mounted to preserve it from bashing into obstacles. It also needs to not heat up the carbs or burn the rider, and should fit within the confines of the motorcycle. This was a bit of a problem because the low power peak required a very long exhaust system - both head pipes and tail pipe should be 37" long. This means fitting 74" of exhaust on a bike where the exhaust ports are only 45" in front of the back of the rear tire. Hmmmm....

I didn't quite make it - the tail pipe is about 6 inches too short. If not running a muffler I could have picked up the extra space, but that would have been a bit loud. In actuality, the slight reduction in the tail pipe length has less effect than a similar reduction in the head pipes, and the length of the muffler would help to make up the difference. The completed exhaust (less muffler) weighs 4.75 pounds. The head pipes run down the right hand side of the engine and duck in between the frame back bone and the rear tire to meet up with the tail pipe. The tail pipe then continues out the left side of the bike, loops around the front of the frame back bone back to the right hand side, and then towards the back of the bike, tucking inside the rear shock. I welded 21 short sections of tubing (excluding the tail pipe collector) together to build the pipe, and it definitely took more than 1 afternoon to complete.

The custom exhaust for my bike
Trials bikes usually have fairly low compression, as this makes them smoother at low RPM. The 160 pistons have a fairly high dome, so I had to find suitable pistons to use with a lower dome. Some extra displacement would be nice too. I could have use Kawasaki KZ550 pistons as I do in my road racer, but the low compression dome would be offset by the increased cylinder volume (108cc vs 80cc stock). I knew that many of the little Honda engines have very similar piston dimensions as far as wrist pin diameter and deck height, so the next avenue was to see what I could find. Unfortunately, all the 90cc single and 175cc twin pistons still had a noticeable dome to them. I remembered then that Powroll in Oregon had been modifying little Honda engines since the 1960s so I gave them a call to pick their brains. I was passed between several people who were bemused by the information I was seeking. I finally was connected to a gentleman in the R&D department (Jack I think his name was) who had been there for years. He advised me to look at the pistons from an XR100 Honda, as he said they were pretty flat on the top, and not a great amount larger in diameter than the 160 pistons. I took his advice, ordered the pistons, and they were almost exactly what was needed. Thanks Jack! They became exactly what was needed when Craig dished the top of the pistons with his lathe. The new displacement is about 180cc. Craig was able to leave a ring of untouched aluminum around the circumference of the piston, essentially the area between the stock bore size and the OD of the piston. This will give a squish band that will increase turbulence in the combustion chamber and very much reduce any chance of detonation.

The modified XR100 pistons
That takes care of the top end, and should result in the smooth low end power I'm working toward.

Now it is time to increase the flywheel mass. The stock crank isn't particularly light for a small twin cylinder bike (the road racer has a much lighter version of the crank), but it isn't very big in diameter. It is better to add a little weight at a good distance from the crank center than more weight on a smaller radius. Even better is a lot of weight at a good distance. I was going to make a flywheel to mount on the back of the small stock alternator rotor (1.75 pounds and 3.1" OD) as did Mr. Deeter. The drawback is that the extra flywheel couldn't be very thick if it was to fit between the engine case and alternator stator. While I was fiddling with this stuff it occurred to me that I had seen a larger flywheel rotor in a box of parts for a Kawasaki EX250 engine I had recently purchased (see the EX250 + 125GP = ? article). I dug it out and compared it to the stock rotor. It was 2" larger in OD and weighed 1 pound more. Even better, the extra weight was all at the periphery of the rotor (the EX stator fits inside of the rotor, where the CB160 rotor fits inside of the stator), greatly increasing the effective mass of the flywheel. The widths were nearly the same, and the OD of the EX250 rotor just cleared the engine cases. Of course, there was a small problem of the EX250 rotor being rather loose on the smaller OD 160 crankshaft. I've got some ideas on how to make an adapter, so I think this should be resolved soon. I do find myself wondering why Kawasaki put a big dirt-bike rotor on a 14000 RPM sport bike engine.

Of course, I'll be building a Lucas RITA electronic ignition for the bike. A plus is that I'll be able to sell some replicas of it to folks road racing the 160/175 Hondas in AHRMA 200GP


Typical Trials bike wheels are 21" front and 18" rear. I presume that Mr. Deeter used a 19" front wheel because of travel/length limitations in the CL160 forks he used - there just wasn't room. I dug out a set of Betor forks from a 250 OSSA that Craig had given me and fixed the wheel size limitations. I already had a Yamaha AT1 front wheel with a 21" rim and a rummage in the garage supplied a WM3 Akront (flanged) that fits the 160 rear hub. I had several new pairs of 14.5" Fox trials dampers so that determined the rear suspension. The Honda uses a clevis shock mount on the swingarm so I made new top mounts for the Fox trials dampers (they are the top mounts on the dampers, but the damper is mounted upside down).

The clevis mount for the Fox dampers
One drawback to the longer travel/length suspension and bigger wheels/tires was too much ground clearance. A Trials bike needs plenty of ground clearance, but beyond about 10-11" you start to suffer more from an excessively high center of gravity than you gain from the extra clearance. Fixing this would require some serious chopping of the frame, and since the AHRMA Premier Lightweight class specifies only "Period Modifications" I don't want to drastically change the frame lest someone snivel about it. Even though all the English factories made major changes to their "same as stock" works bikes, this is a HONDA, and some people in the vintage crowd still harbor some prejuidices against Japanese bikes. It will be best to stay on the safe side of the modification fence to avoid ruffling more feathers than absolutely needed as there is enough sniveling about completely legal Japanese vintage bikes. I will point out that most of the AHRMA folks are quite happy looking at old Japanese bikes, and have generally been supportive of this project.

I did de-rake the steering head a couple/three degrees to quicken the steering somewhat. This brings the exhaust pipe and front fender banging together when the forks bottom, so I'll build some sheet metal triple clamps (see pictures of the clamps I made for the Laverda on the graphics page) with a bit more offset. This will reduce the trail a bit too, so that will help make the steering more trials-ish.

To get the wheelbase right I'll need to run the rear axle near the back of the adjustment slot - I guess the chain tensioner will have to take care of any chain slack I can't adjust out.

I borrowed a TL125 tank from a friend to try on the bike. I like the lines, but there are severe frame/petcock interference problems. It looks like I get to make an aluminum gas tank, along with an aluminum air box to go under the vestigial seat. I'll use Acerbis white plastic trials fenders since aluminum fenders and my dirt riding style don't mix very well.

Mind you, all of these modifications could be done in a well equipped garage shop (like mine) so I don't think I'm going against the spirit of vintage competition. From what I've seen from my collection of American and English motorcycle newspapers from the 1960/70s the buyer of a new G50/Manx/TD1 etc couldn't wait to start modifying the thing to make it better. To stay in the proper spirit, when I deraked the steering head I didn't even use my frame fixture - I just scribed some lines on the backbone tube, cut it, and attached my come-along between the front wheel and center stand and tightend things up. A quick check that all the tubes still seemed to be in the same vertical plane, filing of an appropriate spacer, a bit of welding and, as the English are prone to say, Bob was my uncle.

03 January 1997

Here are two Instamatic snapshots of the CB160T with the exhuast system installed. The light colored TL125 fuel tank and fenders washed out and blend with the white garage door, but you should get a reasonable idea of how things are looking. The bike's suspension is fully topped out in these pictures.

The shot of the complete bike with exhaust 

The detail shot of the bike with exhaust

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1996-2010 Michael Moore, last update for this page 01 June 2010

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