Stripping the frame to the bones

Time to turn our attention to the frame.  Let’s remove the front fork, rear shock and swing arm, as well as the wheels.

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Ready to be stripped down.

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Front fork assembly is removed.

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Rear swing arm is disassembled.  There will be some work to get this element restored and looking like new!

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Rear shock will need a full rebuild.

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Naked frame ready to be media blasted and powder coated.

I’ve decided to keep the red and black color scheme of the original ‘82 XR500R and will have the frame and swing arm powder coated with the original color tones (red and aluminum silver).  The Honda Paint Color for that year is “Tahitian Red”.  Researching information on Honda Paint Colors and Codes led to vsource.org which has assembled quite a reference for Honda Paints.  Tahitian Red’s Paint Code is R-23.

Each officially produced modern motorcycle has a Paint Color and a Paint Code. The Paint Color is the name of the paint color, and can include information such as the type of paint and whether it requires an undercoat.  The Paint Code is the name of the paint scheme used on the frame and other parts. The two are related, but they are not the same. For example, there may be several model years of the same bike that share the same Paint Code, though they don’t look the same. The Paint Code relates to the predominant paint color used in the scheme, which can at times make things a bit confusing. So, in order to correctly identify a particular model’s original paint scheme, you need to know two things: the Paint Code and the bike’s Model Year.

There is a large number of Honda motorcycle Paint Codes and finding the right one for a bike is sometimes not trivial.  Color-Rite is a great source for original motorcycle, ATV, watercraft and snowmobile color paints and touch up kits.  Try its paint wizard to help find the right paint for your bike.

When the parts come back from their powder coating treatment, we’ll start re-assembling the XR500.  Meanwhile there is a long list of parts which need to be either cleaned, fixed or rebuilt before they can be put back on.

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XR500 exhaust system gets a facelift

Last part of the engine that needs some tender loving care is the XR500 exhaust system.  The pipes are rusty, the Supertrapp muffler which came with the Basket Case is covered with exhaust grime and soot and its disks are all clogged with solid carbon deposits.  Not a pretty picture but it’s salvageable.

Let’s start with the header pipes.  Over the years, I’ve experimented with a long list of rust eating chemicals but none deliver on the promises so I don’t have any recommendations.  At the end of the day what works is an old fashion metal brush, sandpaper and the all mighty patience factor.

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Exhaust pipes as they came off the bike.  Interesting amounts of rust.

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A set of metal brushes, sandpaper and lots of patience gives the pipes a new life.

To paint the header pipes, I’m using a very high temperature resistant flat black silica/ceramic VHT header pain.  It can supposedly withstand temperatures of up to 2000F (1093 Celsius).  We’ll find out if this is for real or just yet another claim that only a marketing department can dream up.

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Painted header pipes.

Now on to the Supertrapp performance muffler.  The Supertrapp design has been around for ages (40 years) without major changes.  What differentiates it from other mufflers is how the back pressure can be tuned by changing the number of disks in its diffuser system i.e. by adding or subtracting discs, one can tune the powerband and sound of a motorcycle in just a few minutes and with basic hand tools.

Removing discs decreases the exhaust opening and increases backpressure. This effectively decreases the powerband to create more low-end torque. It also decreases exhaust tone and enriches carburetion.

Adding discs increases the exhaust outlet and decreases back pressure. This widens the powerband at the top end. It also increases exhaust tone and leans out carburetion.

Sound energy in SuperTrapp mufflers is reduced in two ways: it is absorbed through a perforated core inside the body of the muffler, and it is diffused 360º radially as the exhaust passes through the diffuser discs. The number of discs used determines how much total sound energy will be absorbed and diffused.

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Supertrapp’s disk diffuser system.

An interesting tidbit of related information is that the human ear is more sensitive to higher frequencies; this means that we perceive higher frequencies sounds as louder.  So now you know — focus on reducing these higher frequencies to stay on friendly terms with your neighbors!  Or not.

Another tidbit of information and popular misconception: “The more free-flowing an exhaust system is, the more power it will make”.  Not quite.  Some backpressure (2-3 PSI) is necessary to achieve maximum power and best usable powerband.  Removing the muffler altogether thus is not the greatest idea.  Supertrapp suggests to start with 6-8 disks and then add or remove disks according to tuning needs.  Our muffler includes 8 disks.

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Disassembled and cleaned Supertrapp muffler.

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New packing material from FMF is fitted around the center core.

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Exhaust system is overhauled and ready to be mounted.

This is it for the engine parts – everything is ready for prime time.  We’ll now focus our attention on the frame and suspension elements.

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Re-assembling the XR’s engine top end

I let the paint cure for a few days and then took the left and right engine covers off as well as the camshaft cover, cylinder head and cylinder.  Then installed the rotor, stator, front sprocket (15 teeth in replacement of the original 14), and various small parts before screwing both side covers back on, this time for good. 

Now we’re ready to get the top end mounted on the engine.  The work is straight forward as Honda engineered everything rather superbly for the XR500 engine.  Let’s animate the steps through a slide show and see how everything is coming together:

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All main parts are here, we’re ready to get started…

 

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The cylinder is bored 1mm over to fit the 90mm Wiseco piston (see previous post for details on this piston). I’ve ordered a new set of corresponding piston rings — let’s verify that they fit.  The end gap should be between 0.30mm and 0.50mm and not exceed 0.65mm.  The measured end gap is 0.60mm, so we’re borderline.  I inherited this piston and cylinder and unfortunately I don’t know who machined it.  It was bored in the upper limit, but within tolerances.  Crossing my fingers that we’ll not have too much piston rattle (!).

 

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Let’s mount the piston rings.  I’ve had a few unfortunate accidents in the past and now am hyper cautious during this operation and always use the help of a piston ring installer.

 

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Piston is mounted in the engine.  The new Wiseco pin fits too tightly so I have to make a few adjustments to make it slide in properly.  The ring ends are positioned according to the chart provided by Wiseco.

 

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Piston and cylinder are in place.  Sliding the piston in the cylinder while compressing the rings is another operation where I am very cautious not to damage a ring.  I’m breathing again.

 

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The next step is to mount the valves back into the cylinder head.  Over the years I’ve used all sorts of valve compressors which were for the most part designed for large engines and thus were incredibly frustrating when applied to smaller valves and cylinder heads.  This time around though I got myself a new tool, and this one is killer, making the job a real joy (Genius Tools valve spring compressor).

 

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Each of the four valves is mounted back in place.  The Genius valve compressor has me smiling all the way.

 

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Valves are back on, cylinder head is ready to go.

 

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Valve side of the cylinder head.  The camshaft bearing journals in the XR500 engine are known to easily get scored and show signs of seizure.  This cylinder head bearing journals though are in great shape.

 

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Cylinder head is mounted.  Next step is to install the camshaft.

 

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Camshaft is in place.  Here again, Honda engineering designed everything very well and the camshaft timing adjustment is straight forward.

 

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Starting to look like an engine.  Final step is to adjust the cam rocker clearance and then manually rotate the engine to make sure that the 10.5:1 Wiseco piston offers enough clearance for the valves.  Everything is good.

 

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Tadahhh!  The engine is ready to be mounted in the frame.

This was fun!  Next step is to clean and paint the exhaust and then we’ll start focusing on the frame and suspension components.

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Black engine? Oh yeah!

I compared for hours pictures of XR500 engines in their plain aluminum finish (pre-‘82 models) and semi-gloss black finish (‘82 and on) and decided that black it is.  We’re going all black!

And so I painstakingly cleaned and degreased every part which would get painted and wished a hundred times or more that I had a sandblaster (this has to be my next acquisition).  Every chemical under the sky had its turn in helping the effort but none were magical.  A set of metal brushes, patience and elbow grease ended up being the only working solution.

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As clean as it’s going to get – the XR500 parts are ready for assembly and paint.

Because I want a bit of contrast between the black parts and the assembly screws & bolts, I’ll assemble the parts but just temporarily.  After painting is completed, I’ll take the top end and engine side covers off again to finish the assembly with actual gaskets, piston, camshaft and valves.

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The XR500 engine ready to be painted.

The engine paint needs to be high temperature resistant to assure that nothing will peal off while traversing, say, heat-crazy-sun-baking Arizona in mid July!  I’m sold with the Dupli-Color Semi-Gloss Black DE1635 Engine Enamel rattle can — claims heat resistance up to 500°F (260 Celsius) and will handle oil and gas.  It is also easy to apply and doesn’t run.  Maybe it even helps grow hair.  Sounds like perfection in a can?  Well, it better be because I committed, now the engine is Dupli-Color black!

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All black, loving it.

Now let’s let it cure for a few days and then take it apart again to actually complete the engine assembly.  This is probably my favorite part.  Put some Pink Floyd music on, drift away, and enjoy every minute of the final touches.

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Re-assembling the XR’s engine bottom end

Time to clean up and put things back together!  No real surprises here, Honda made it fairly straight forward to re-assemble the bottom end.  The crankcase splits horizontally allowing one to assemble the transmission’s shift drum, forks & gears, balancer shafts, and crankshaft, and lay them  all down in the lower crankcase before closing everything with the upper crankcase.  Then install the clutch and align the front and rear balancers and things start looking promising.  Funny thing is that there is always a short moment of self-doubt were one wonders if every part was really in place and if something was forgotten.  Probably like a doctor counting all his tools after completing a surgery and making sure that he didn’t forget any instrument in the patient!  Nah, we’re good.

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XR500 bottom end, left side.  Note the chain driven front balancer, shift plate and  shift spindle.

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XR500 bottom end, right side.  From left to right: clutch, ignition advancer rotor, camshaft timing chain, front balancer weight.

The intend has been to rebuild the bottom end as an XR500.  I inserted though the neutral (gear) position indicator from the XL just in case I decide to use the XL left engine cover and its corresponding neutral switch sensor.  Then again, who needs a bright neutral light indicator on a dashboard?!

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XL’s neutral gear position indicator.

Now that the bottom end is back together, let’s go crazy with paint!

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XR500 versus XL500 rotor and stator

The rotor on the XR/XL is used to create a variable magnetic field and produce current through the stator coils.  It is also used as a flywheel with a significant moment of inertia to store and release rotational energy and smooth out engine pulses.  The more mass rotating at a given speed and radius, the more moment of inertia, the more stored rotational energy and the more “resistance” to acceleration or deceleration.  In other words, an engine’s smoothness at low RPM for instance (resistance to stalling) can be helped by a flywheel with more inertial mass.  Acceleration is also better controlled (less abrupt).  This thinking is reflected in the choice of rotor/flywheel installed on the XR versus the XL.  The XL model is mainly intended for street use with occasional off highway escapades — it thus has a rotor/flywheel with more inertial mass.  The XR on the other hand is intended for sportier use where engine acceleration is favored over low RPM smoothness and resistance to stalling — its rotor/flywheel thus packs measurably less inertial mass (see picture below).

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XR rotor on the left, XL on the right.  Note the visible difference in flywheel inertial mass.

The XR rotor weights 2.31 kg (5 lbs and 1.75 oz) whereas the XL rotor weights 2.96 kg (6 lbs and 8.75 oz).  The difference of 0.65 kg (22.9 oz) with a rotational radius of about 70 mm (2 3/4 in) is quite significant.  Swapping one rotor for the other will result in an absolutely perceivable change in engine character.  For this project I’m more interested in a very responsive motor than anti-stalling capabilities or smooth delivery of power and thus I’ll install the lighter XR rotor.

On the stator side, the XR and XL differ as well.  The XR is delivered with minimal accessories – no blinkers, no battery, no real world headlight, just an enduro interpretation of a headlight.  Its stator thus is a reflection of this minimal electrical power needs (see picture below).

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XR left side engine cover and stator.  Simple two-wire system, low power output.

The XL electrical system on the other hand includes blinkers, a real headlight and a battery which needs charging.  Its stator thus incorporates beefier coils which provide the necessary current to the bike’s electrical needs (see picture below).

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XL left side engine cover and stator.  Larger coils, higher power output.

I intend to keep the project bike as simple and light and possible and thus don’t plan on mounting blinkers or a battery — I’ll use the XR stator.  And if I change my mind, I’ll just swap the rotors and re-wire!

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Main jet and pilot screw adjustments for altitude and temperature

The official Honda XR500 shop manual offers a handy chart which helps determine the optimal main jet size for altitude and temperature (averages).  Refer to the Honda chart below to find C, the correction factor.

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Altitude and temperature main jet correction factor C chart.

If C is 0.95 or below, lower the jet needle one groove and screw the pilot screw out 1/2 turn.  Jet needle position and pilot screw adjustments are not needed if C is over 0.95.

To determine the main jet size, multiply the standard main jet size (155) by C.

For example, at a temperature of 28C (82F) and an altitude of 1655 m (5430 ft, which happens to be the altitude of Boulder, Colorado where I live), C is 0.95 and carburetor recommendations will be as follows:

  • Main jet, 155 x 0.95 = 147.25 (#148 main jet is the closest, part number 99101-357-148)
  • Jet needle, 3 –1 = 2 (2nd groove)
  • Pilot screw opening, 2 1/4 – 1/2 = 1 3/4 (1 3/4 turns out)

Note that the above is for a stock engine.  In this project we have a few modifications which will likely require a slightly “richer” carburetor setting due to the small increase in displacement and the Supertrapp performance muffler.

Optimal carburation is dependent on several other factors besides altitude and temperature, such as engine temperature, engine tune-up, load, RPM, and throttle position for example.  The following chart helps us visually understand which carburetor component is impacting mixture with respect to throttle position.  Yes, the chart is in French, now hold your colorful comments for a second, I’ll translate:

  • On the horizontal X-axis we have throttle position from idle to fully open.
  • On the vertical Y-axis we have the carburetor mixture-impacting components, showing at which throttle position each one is active.
  • Orange: needle position
  • Green: cylindrical part of the needle
  • Yellow: conical part of the needle
  • Blue: throttle piston chamfer
  • Darker red (left): idle air screw
  • Red (right): main jet
  • Purple: Pilot screw

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Chart indicating which carburetor component is impacting mixture at increasing throttle position.

This chart can help trouble shoot carburation problems and understand which component should be tuned in situations where the engine is not running properly at some throttle position.

Next step, let’s compare XR and XL stator & rotor.

Posted in XR500 Project | 11 Comments