Saturday, February 25, 2012

Part 18 - Mounting the gas tank

The gas tank is mounted by welding two plates to the frame. It is a good idea to isolate the tank from the frame using neoprene rubber or similar wrapped around the backbone.

With the tank in position on the frame, the positions of the mounting plates can be determined. The rear mounting plate needed to be notched to clear the frame motor mount.

After notching, a 1/4" piece of steel was cut to make a mounting tab which was welded to the center of the mounting plate. The welded assembly is then welded to the underside of the frame for the rear tank support. The plates are centered on the frame but they are not neccesarily horizontal. The tank needs to be in place to get the correct angle of the mounting plate. Once you have the angle estimated, remove the gas tank  make a small tack weld to hold the mounting plate to the frame. Then put the gas tank in place and careful adjust the angle of the mounting plate so that the mouting bosses on the tank are perpindicular to the mounting plate. Then carefully remove the gas tank and add another more substantial tack weld. Put the gas tank back on the frame to check, make adjustments as needed. Then remove the gas tank from the frame  and finish the weld.

Rear tank mount welded assembly

For the front tank mounting, a 1" diameter steel tube was cut to approximate size and coped using a file. The angle of the cope is determined by the angle of the mounting plate so that the mounting plate is perpendicular to the mounting bosses on the tank. Then the mounting plate was mounted to the tank with the tank in place on the frame. The length of the steel tube was determined at the intersection of the steel tube and the mounting plate. The tube was marked cut it to length + 1/8". Then it was trial and error test fitting until the length was perfect by grinding off a little at a time. Once the length was correct, the position of the tube on the backbone was marked and the tank removed. The tube was then welded to the center of the mounting plate. The assembly was tack welded to the frame and checked for fit and adjusted as needed after which the welding was completed. I used the Harbor Freight Flux Wire welder for all these welds.

Front tank mounting assembly 

Front and rear tank mounts welded to the frame

One of the problems I ran into is that the tank mounting bosses are not all at the same height on the tank. There is about a 1/4" offset between the left rear and right rear mounting boss. This must have occurred during tank fabrication. Not super critical but I would have preferred it not to be so.

The tank welds are ugly around the mounting bosses as you can see in the picture below. The crossover lines connect to the tank through 1/4" tubes welded to the tank. This causes the fuel crossover tube to be highly visible. I cut the tubes off, drilled the holes to 3/8" diameter and welded 1/8 NPT threaded bungs into the tank. This allows the fuel crossover to lie parallel to the underside of the tank making it less of a visual distraction.

One of the crossover tubes cut off

Both crossover tubes removed and ground flat
Holes enlarged to 3/8"
1/8" NPT bung
At this point, I had borrowed a TIG welder which made welding on the sheet metal a more precise adventure. I destroyed the first set of bungs with heat distortion and a complete weld-through on one of them so had to cut them out. The second set of bungs I purchased from Summit Racing were beefier. That made welding them much easier - that and I now had practice with the TIG on thin sheet metal. 

Lay-flat crossover tube
Note, the original fabricator of the gas tank installed the crossover tubes skew. I only noticed after I had welded both bungs in. Not what I like to see but I have to live with it now. In general, the tank is not the greatest quality.

I'm not a welder, but I do like using the TIG. It is far superior to MIG and Wire Flux welding. You have total control over the weld pool and filler material. Takes practice though.


  1. what do you use the other 1/4" tube connection for? I take it the 3/8 ntp is for the fuel pet cock?

  2. The 1/4" tube connection is the breather tube. The tank cap is sealed so the breather lets the air in to replace the gas that runs out. I have a breather tube that connects to the back of the air cleaner housing. The other port is 22mm for the petcock.