Introduction: Cafe Racer Gas Tank

This Instructable details the restoration and modification of a 1973 Honda CB350 gas tank for use on a 1983 Honda CB650SC Nighthawk Cafe Racer conversion. The '73 CB350 tank does not fit the frame of the '83 CB650SC which required the fabrication of a custom mounting bracket. The original CB350 gas cap was non-serviceable so it was cut out and replaced with a more modern pop-up gas cap typically used on a Harley Davidson custom motorcycle. Using 40 year old parts does mean that they will have acquired lots of "character", AKA dents, rust and potentially leaks. While the old tank earned this character over time, my goal was to restore and modify the tank, so that it could start a new character building journey mounted on the restored and modified 1983 CB650SC Nighthawk.

Having sweated through a ground-up bike build before, I expected a restoration/modification project to be a lot easier. Wrong! The mechanical and electrical challenges were fortunately all surmountable and I had a blast riding the completed project for about 6 months. The project dealt some hefty blows to the wallet, but I was eventually able to sell the bike for a small profit, which has subsequently been eaten up by another cafe racer project. Evidently learning from experience is not within my tool set.

Before we get started, please note that modifying a gas tank can be extremely dangerous. It is imperative that the tank is completely cleaned before you attempt any modifications as a single spark can ignite any left-over fuel vapor causing a deadly explosion. It is not enough to empty the tank - you must wash the insides thoroughly with a product that will completely remove the gasoline. Marine Clean is a good choice.

Also be aware that all the paints and cleaners associated with this project have safety warnings. Please review all of the manufacturer safety information before using any of the products mentioned in this instructable. In general use a good quality painting respirator when sanding and painting, always wear eye protection and use chemical resistant gloves to avoid direct contact with paints, body filler and cleaners.

Step 1: Tools and Things

This is a non prescriptive list of tools and parts that I used to restore and modify the CB350 gas tank. Most of the items are available from Eastwood which is a site that specializes in automotive bodywork and chassis repair products and tools. As always google is your friend - there are multiple alternative sources. Overall, considering all the consumables needed, this is a very expensive project and you can possibly get the work done at a custom motorcycle shop for about the same amount of money that you are going to spend on supplies. In my case I had most consumable and tools from a previous custom chopper project, plus some experience using them on that project. Doing it yourself the first time will result in mistakes that take more money to fix. The donor gas tank will be one of the cheaper components of the process. Keep that in mind!

  • Old gas tank for restoration. The better the condition of the tank, the easier the restoration will be so choose carefully! Ebay is a good source for used tanks.
  • Sandpaper - 80 Grit for body filler, 600, 1000 and 2000 for paint. Use wet-or-dry sandpaper.
  • Pop-up gas cap.
  • POR15 Motorcycle tank repair kit. Includes everything you need to repair and preserve the inside of the gas tank.
  • Paint Respirator. I used the 3M paint respirator and correctly adjusted you won't smell the paint fumes. If you are smelling fumes, then you need to tighten the straps to ensure a proper seal to your face.
  • 1/8" Weld Steel for constructing a mounting bracket
  • Nitrile gloves and painting body suit to protect your skin from paint and airborne paint over-spray
  • Safety glasses
  • Epoxy Direct To Metal Primer and automotive top coat. You will only need a quart of each.
  • Clear Coat. I used SprayMax 2K. Simple, 2 Part spray on clear-coat in a can. Avoids a lot of prep work and gives great coverage and protection of your color coat. Avoids having to purchase more clear-coat than you need. A single can is enough for 3 heavy coats of the tank. Very convenient gasoline resistant catalyst hardened eurothane paint in a rattle can!
  • Mixing cup and Lacquer Thinner. The Lacquer Thinner will remove epoxy primer before it has had a chance to cure, so clean spills early!
  • Preval Paint Sprayer. This will allow you to spray any paint you want without needing a compressor and spray gun. I suggest buy 4 kits. While available at various places online, I found it cheapest in-store at Lowe's and Home Depot in the USA (~$5 each).
  • Evercoat Rage Gold body filler. This stuff is way better than Bondo which is difficult to sand once it sets. The Evercoat Rage products sand easily and stick like crazy. I have successfully filled 1/4" defects which have withstood many years of vibration on a custom chopper. It's expensive for a one-off project so Bondo will do in a pinch....
  • Glazing And Spot Putty is useful for filling in tiny air pockets (pinholes) that sometimes appear in the body filler after sanding, and other minor scratch repairs. Much easier to sand than bondo and hardens quickly. For any defect more than say 1/32" deep, use body filler.
  • Propane torch, solder and acid core flux. Available at most Home Improvement stores. Used to solder the pop-up gas cap in place. Not needed if you keep the original gas cap.
  • Crossbar dent repair kit. Makes it easier for the home hobbyist to pull out small dents without welders, slide hammers etc. The kit from Harbor Freight is roughly $12 and includes everything you will need.
  • Dremel Tool, cutoff disks and flap disks for modification to the fill turret
  • Paint stripper to remove the old paint from the tank. I used Tuff-Strip
  • A pre-painting cleaner and de-greaser. Eastwood Pre works well for me
  • Heat-gun or hair dryer for drying the tank internal surfaces prior to sealing
  • Craft Foam to use as a sanding block. Much cheaper than buying a professional sanding block system and works just as well. If you need a stiffer block, just laminate two pieces together.

Step 2: Cleaning the Tank

As mentioned previously, this is a critical step before you start work on the tank. An errant spark can cause a deadly explosion. I suggest the following steps.

1. Drain the gas tank. If you bought the tank on-line as I did, this step is already done, or at least we hope it was! If you are removing a tank from a donor bike, the gas may be old. A great use for this gas is to mix it with your lawnmower gas. Your lawnmower will run on any old contaminated gas. If you don't have a lawnmower, find a friend who does. Alternately, research the disposal requirements for your local area and comply with them.

2. Wash the insides of the tank thoroughly. I recommend Marine Clean. The product will remove all remnants of gas and varnish from the inside of the tank. The instructions are provided with the product - just follow them. I used duct tape to block off the gas cap hole. The job is done when you can't smell gasoline at all.

3. Leave the tank to dry and do another smell test to ensure that there is no gasoline odor. If there is, repeat the wash cycle.

Step 3: Removing Dents

There are many ways to remove dents. The easiest and least destructive method for small dents in which the metal hasn't been creased is to use an adhesive crossbar dent puller. The puller has 2 supports which are placed either side of the dent and a threaded puller which is glued to the dent. By turning the nut, the threaded puller is pulled backward, pulling the dent out. Or at least that's the promise of the tool.

I found this to be a struggle initially with the threaded puller detaching from the tank without moving the dent at all. Eventually after 3 or 4 tries, the puller stayed attached and worked as advertised. An important step is to make sure the surface to which the puller will be glued is very clean. I used the supplied acetone to wipe down the surface before each attempt. Depending on the size and shape of the dent, you will need to move the puller around to ease out the entire surface. In order to see how you are progressing, you can use a flexible straight rod (I used a 1/4" piece of all-thread) and hold it against the side of the tank applying pressure to the two ends so that it follows the contour of the tank. You will clearly see from the reflection in the paint if the surface is still dented. The last two pictures show a before-and-after so that you can see the effectiveness of the process.

Fortunately the dents in this tank did not crease the metal which made them relatively easy to remove. If you have creases in the metal, the job is much more difficult. It may even require cutting a section of the metal out and welding in a new piece. Be careful when selecting your tank. The better the donor tank, the less work is required.

Step 4: Removing the Old Gas Cap

The old gas cap was dented and pitted, the lock was broken and the seal was perished. Cafe Racer period correct gas caps are expensive so I opted for the sleeker look of a Harley style pop-up gas cap. To integrate the pop-up gas cap, the old gas cap first has to be removed. With the gas cap open, the complete locking mechanism can be removed by removing the long M3 screw that secures it to the tank. The cap is removed by driving out the rivet with a hammer and punch. You may need to use a drill to partially remove some material from the rivet to make this easier.

Step 5: Removing the Filling Turret

Please ensure that you use a sanding dust mask and eye protection for this step. There will be a lot of airborne particulate which may be hazardous to your health. You don't want to breath this into your lungs.

A dremel tool with a 0.050" cut-off disk makes quick work of removing the turret and the bracket for the lock mechanism. After the turret is removed, a Dremel grinding attachment was used to smooth out the cut as well as grind the turret metal from the inner circumference of the tank entry hole. The old brazing material was also removed using the grinding attachment. With a huge amount of luck, the pop-up gas cap fit the opening perfectly without further modification.

Step 6: Soldering the Pop-up Gas Cap

To secure the new gas cap, it was soldered into place. Before soldering, the surface needs to be properly prepared. A Dremel 80 grit flap wheel attachment was used to remove the paint from the area around the filler hole. Then the shiny area and the gas cap were thoroughly cleaned using a degreaser. I used Eastwood Pre for this but any good paint prep degreaser will work. Finally, use the flap disk to roughen the underside of the pop-up gas cap flange to provide a better surface for the solder to stick to.

Water soluble flux was applied to the cleaned area surrounding the hole and to the underside of the gas cap flange. With the gas cap in place, the propane torch was used to solder the gas cap in place. I made that sound way easier than it actually is! I battled to get the right temperature to the metal so that the solder would basically just flow neatly and evenly into the joint between the tank and the gas cap. I had to repeat the process and reflow old solder multiple times until the job was eventually done to my satisfaction. Perseverance is key!

Step 7: Rust Removal and Sealing

Sealing the gas tank is an essential step. The tank is old and rusty inside and its impossible to tell whether there are pin-hole leaks in the tank. These leaks will later ruin your paint job so sealing the tank with an epoxy coating is valuable insurance as far as I am concerned. A sealer will also inhibit future rust. I used POR-15 epoxy sealer for this based on previous experience with the product on a custom chopper build. It does not appear to degrade with the standard ethanol-gas mixture available in our area which is a very good thing.

Please read the safety instructions provided by the manufacturer before using this product.

The first two steps in preparing the tank for POR-15 are to clean and etch the insides. First an application of Marine Clean dissolves all grease and oils on the inside of tank. You can seal the ports in your tank with Duct tape. Follow the instructions -Motor Cycle Fuel Tank Repair Kit

Next, each surface within the tank needs to be exposed to the Metal-Ready for about 20 minutes so you will need to rotate the tank into a new position for each exposure. Metal-Ready is a rust remover that leaves a zinc phosphate coating on the base metal.

I used duct tape to seal the holes in the tank. After Metal-Ready is complete, the tank needs to be emptied and rinsed with hot water a number of times. The manufacturer recommends no longer than 2 hours of exposure to Metal-Ready.

Then the tank needs to be thoroughly dried inside. One way to ensure that the insides are dry is to use a heatgun or hair dryer to force hot air through the tank. Just leaving the tank out in the sun will not be sufficient as a certain amount of water vapor will remain trapped within the tank. Using a heatgun will force this out.

Finally, making sure you have sealed the petock hole and crossover ports, thoroughly mix the sealer and pour it into the tank. Slowly rotate the tank to ensure all internal surfaces are coated. Then pour out the excess sealer through the petcock hole. This stuff sticks like crazy to anything it touches so try not to make a mess. If you do need to clean it off something, laquer thinner can be used while the sealer is still wet. The excess will harden in the can and can then be disposed of according to your local disposal requirements.

Step 8: Stripping the Paint

Tuff-Strip was used to remove the old paint. This is potent stuff so make sure you use heavy duty nitrile gloves, a respirator and eye protection. Please read the safety instructions on the container. Work in a well ventilated area - I set up a table outdoors to minimize harmful vapor build-up.

The process is simple and the results were spectacular. Using a disposable chip brush, brush the gel onto the surface and wait 10-15 minutes. The paint on the CB350 tank literally popped right off the surface requiring minimal effort to yield a paint free surface. A wire brush and electric drill with a wire brush attachment were used to remove stubborn paint areas particularly on the underside of the tank. I am guessing that the heat from the motor causes the paint to bake itself into the metal pores making it harder to remove underneath the tank.

The net result is a paint free tank.... the easy way!

Step 9: Smoothing Out the Surface

With the paint removed, you will notice a lot of minor defects on the surface that where camouflaged to some extent by the paint - that or you just didn't look closely enough! By this stage you will have invested way more hours in a gas tank than you would ever have thought possible and it's not even painted yet.... and now you have a whole bunch of hidden defects to deal with! We call this restoration process fun because we don't understand the definition of the word fun!

To fix these superficial defects, a light coating of body filler is applied. I use Rage Gold because it is very easy to sand, and adheres to the surface like crazy. Previous experience building a chopper that vibrates like a paint shaker shows that it will cling under even the worst conditions if properly applied.

Work in an area with a lot of clean air flow and wear a paint respirator and nitrile gloves when mixing and applying the body filler.Read and comply with all manufacturers warnings.

It all starts with proper surface prep. The surface needs to be abraded with 80 grit sandpaper (wear a sanding mask), and then cleaned with a paint grade degreaser. I used Eastwood Pre to clean the abraded surface. The body filler is a 2 part product which hardens within roughly 10-15 minutes of the catalyst being applied, meaning you have to work quickly. Place about a 2" dollop on a mixing board and add about a 1.5" long strip of hardener over the top. Then work the hardener into the filler. It's best not to use a stirring motion because that will result in air bubbles in the filler which will need more filler to fill-in later on.

Using a spatula, apply a thin layer of filler to the tank and smooth it down as best possible. The more care you apply now, the less work will be required sanding it smooth once it has hardened. You do not need to cover the whole tank, only those areas that have gouges in the metal or small indentations that you could not completely remove in the dent removal step.

Use 80 grit sandpaper to smooth the surface. Wear a sanding mask. The goal is to leave as little filler on the surface as necessary to have a smooth surface for paint. I use a flexible foam pad as a sanding block. If you don't use a block, your fingers will ride into all the surface depressions resulting in a bumpy surface. The sanding block will bridge a depression and only remove the high points which will eventually result in a smooth surface. You will have to repeat the body filler and sanding steps multiple times until you have a perfectly smooth surface. The edges where the meta and body filler meet must not have ridges or these will be highly visible in the paint. The edges will naturally "feather" with block sanding if the metal and filler are on the same plane. If not, add more filler, and repeat the sanding process.

Step 10: Prime and Paint

This topic could be a complete instructable on it's own. The first step is to read and understand the multiple safety warnings associated with the paint that you choose to use.You will need a respirator, eye and skin protection.

The application of paint is a huge amount of work and needs to be done properly because this is the first thing that people see of your competed project. If the paint looks bad, they will assume the rest of the bike is bad. That's not to say that rat bikes don't have their legitimate place in history, but you need to decide at the start of the build process whether you are building a rat bike or a polished bike. For me, it had to be a shiny polished bike.

I used a single stage metallic paint from Eastwood. The reason for choosing single stage paint was because I thought it would be easier to apply. The reality is that single stage paint is an oxymoron where Metallics are concerned. A metallic paint cannot be smoothed after painting with the normal wet-sand and polish process because it disrupts the sheen of the metallic particles causing the paint to appear dull. So if you are doing metallics you may as well do the 2 stage base-coat and clear-coat process. Even if you use single stage metallic, you will need to protect the metal flake with a clear coat so there is no benefit to buying single stage metallic paint

Not being aware of this I bought single stage metallic paint and ended up buying clear coat to protect it. I also learned the hard way that there is a special technique for applying metallics. Again, a lesson I learned the hard way after multiple failures and disappointments. What follows is a description of a process that you can follow to avoid some of the obvious-in-hindsight problems.

You will often read that surface prep is everything. Well not quite:

Surface prep + primer + wet sanding + base coat + wet sanding + clear coat + wet sanding + polishing = Everything.

Any shortcuts will result in a less than ideal paint job.

The primer I used is a Direct-To-Metal 2 part epoxy primer from Eastwood. I had used this primer before with good results so saw no reason to experiment with anything else. There are so many details in building a bike, that you can only finish if you don't turn everything into an experiment. For me, using metallics was enough of an experiment so sticking with a known primer was a good decision.

Safety Warning for Primer and Topcoat

This product was designed for and is intended solely for use by trained professionals. Read all
warning statements and heed all recommended safety precautions before proceeding. DO NOT USE THIS SYSTEM WITHOUT SUFFICIENT VENTILATION. Users must wear appropriate, properly fitted NIOSH-approved activated charcoal cartridge respirator if a forced fresh-air system is not available. Always wear eye and face protection, as well as gloves and protective clothing. Do not use this product, or be exposed to spray mist / vapors if you have respiratory problems. KEEP OUT OF REACH OF CHILDREN AND PETS. This system comprises multiple components. Once mixed, this system will have hazards of all components. Read warnings on all packages before opening.

My paint booth is outdoors. The parts are hung from a temporarily constructed 2x4 frame. While this is great for ventilation, you will need to choose a time of year where airborne particulate and insect concentration is low. Pollen will wreck your paint job, as will insects that settle on the surface before it is dry. I keep a pair of tweezers handy to pluck insects out of the wet paint. However, this can result in surface defects so sometimes it is better to let the paint dry completely and then wet-sand the insect out of the paint.

Before applying primer, use a surface degreaser to ensure that the surface is properly clean. DO NOT TOUCH THE SURFACE WITH YOUR BARE HANDS!!! Only handle the tank with clean gloves. Oils from your fingers will cause poor paint adhesion wherever you touch the tank. Sometimes this defect is immediately visible in the primer, sometimes paint fails at some point in the future.

The primer is a 1:1 mixture of paint and catalyst. Pour the paint into a mixing cup, add the catalyst and mix this thoroughly with a wooden or plastic mixing stick. To apply the paint, I used the Preval compressed air canisters primarily to reduce all the cleanup drudgery associated with using a compressor and paint gun. This also avoids trying to adjust the spray gun for the right pressure and paint thickness. A single Preval can can atomize between 70 and 150 sq.ft of paint so technically you should be able to prime the tank with 2 coats of primer and have air to spare. Practically you will screw up if this is your first time, so keep a spare can of Preval handy. Pour the primer into the Preval paint jar through a paint strainer to remove any particulate. The primer is designed to provide complete coverage in 1 to 2 coats with a paint thickness of 1mil. The second coat can only be applied after the first coat has flashed which takes about 30 minutes. "Flashing" occurs when you see the paint change from a wet texture to a dry texture.

After you have applied the second coat, it's likely that you will notice surface defects in both the paint application as well as the tank sheet metal. While being painful, this is normal. All it means is that you have a lot of work ahead of you. You start getting the idea why a custom single color motorcycle "tins only" paint job from a professional can run you at least $2000. It's a huge amount of manual labor.

After allowing the primer to dry for at least 24 hours, you can wet-sand the defects out of the primer. Use 600 grit sandpaper, plenty of water and a sanding block. I use a flexible foam pad that I cut from 6mm crafting foam. This follows the contours of the tank nicely leaving a smooth surface. Ideally you want to avoid sanding through to metal or filler, but that may well occur depending on how severe the defects you are trying remove are. It is a waste of time assuming that the color coat will hide the defects in the primer..... don't ask me how I know this!

If you do sand through to metal, see images above, you will need to reapply primer. The primer may also have shown areas that require more body filler. If you do need to apply more filler, use 80 grit sand paper to prepare the surface for the filler.

Getting the primer right is critically important. Defects in the primer will show through your color coat because the color coat does not fill surface imperfections as well as primer does. Primer is designed to "build" and cover small surface imperfections whereas top coats are not.

The 600 grit sand paper leaves a good "tooth" for the top coat. Technically, if you were able to apply the primer without requiring any correction of surface defects, you can proceed with the top coat without sanding. The bonding between the primer coat and the top coat is a chemical bond in this case which is better than a mechanical bond. If the primer is allowed to fully cure (roughly 5 days), then the bond will be mechanical.

The top coat is applied using the Preval paint system. Follow the mixing directions supplied with the paint you choose to use. Pour the paint through a paint strainer into the Preval Paint jar and add a small nut or ball bearing to help with mixing the paint in the jar by shaking (don't shake the Preval - always separate the jar from the can before shaking). You will need to apply 2 to 3 coats for complete coverage.

For metallic paint, immediately after applying a coat of paint, apply a complete misting of paint from a distance of 10-12 inches to even out the metal flake in the paint. This will ensure an even sheen. If you don't do this, will you will have a blotchy metallic appearance. Before applying the next coat, it is critically important to wait for the paint to flash. If you apply the second coat to early, "solvent pops" will start appearing as unevaporated solvents from the underlying layer breaks free and tunnels through the new paint layer, leaving millions of pin-holes in the new layer.

TIP - for metallic paints, the flake will tend to settle in the paint sprayer jar between coats. Remove the paint jar from the Preval can and shake up the paint jar to evenly distribute the flake before painting each layer. Do not shake the Preval can. Here's the official Preval Warning: Never shake the Preval Can. Shaking the Can will disrupt the spray and cause it to sputter or cease spraying. If this occurs stop and let the unit rest for 45 seconds.

Do not wet-sand the metallic paint. You will need to lay the paint down almost perfectly with minimal orange peel. If you have to sand the paint to correct a defect, apply another thin coat of paint followed by a mist coat over the entire tank to re-create an even metallic surface sheen after sanding.

Finally, after spending more time than you could have ever thought possible with this simple gas tank, you are not done yet! You need to protect your paint job with a clear coat. It is critical that the clear coat be gasoline resistant. In fact, I chose all the paint layers to be gasoline resistant. You are not done yet. After the clear coat has cured, you now get to spend more laborious hours wet-sanding the surface. Use 1000 grit sandpaper to remove the worst of the orange peel in the surface and work your way up to 2000 grit. Then use a polishing compound to restore shine to the surface. Now you are done painting!

Step 11: Mounting Bracket

The shape of the Nighthawk tank is very different to the CB350 tank. A bracket was required to mount the CB350 tank to the Nighthawk frame. The bracket is a piece of 1/8" weldable steel that has been bent in the shape of a relaxed s-curve. The height was chosen so that the tank undersides cleared the engine by a small amount. There is no exact measurement for this - trial and error with a piece of cardboard got the parts into the right zone and the metal was bent accordingly. I did try heating the metal to make it easier to bend but I wasn't able to get the metal hot enough with the propane torch. In any event, 1/8" metal is "easy" to bend in a vise with appropriate encouragement from a 3lb hammer.

To stabilize the tank horizontally, two pieces where welded either side of the upper mounting point to fill in the gap between the tank mounting tang and the new bracket. The weld was ground clean with an angle grinder and then primed and painted using rattle can paint shown in the pictures.

It is very important that this mounting be secure for safety purposes. If in doubt, please consult with a professional bike builder.

Step 12: Emblems and Conclusion

No tank restoration is complete without emblems. Fortunately the emblems that came with the CB350 were in reasonably good shape. I used Tuff-Strip strip to remove most of the old paint and then lightly abraded the surface with 600 grit sandpaper. This was followed by a primer coat and a couple of coats of the same metallic silver paint used for the tank. This was left to cure for a couple of days. Finally, I hand painted the internals using automotive grade black paint to ensure gasoline resistance. Before the paint was able to dry, I carefully used a solvent (Eastwood Pre) to clean up any paint that had gone out of the lines. Because the silver paint had already cured, the solvent had no effect on it and only affected the semi dry black paint. I used this same trick to color the inlays on the engine, but that is a story for another day!

Modifying and restoring the tank took a huge amount of time. The end result was worth it as the tank completely transformed the look of the motorcycle.

Car and Motorcycle Contest

Second Prize in the
Car and Motorcycle Contest

Epilog Contest VII

Participated in the
Epilog Contest VII