Induction and Exhaust Assemblies

In the best of circumstances, bending copper tubing, let alone stainless steel tubing, is a pain and a real hit or miss affair. 99% of model IC engines use copper tubing, which is best bent with a custom tubing bender of appropriate radius. This can be nothing more advanced than a pair of aluminum hemispherical rollers which keep the tube from kinking. The tube is first heated dull red and quenched in water to anneal (soften) the hard-drawn copper tube.


The 5/16" OD copper tube is inserted into the bender, and the base of the bender is clamped firmly in a bench vise. The tubing is also lighlty greased to prevent galling. The bender consists of 2 aluminum rollers, with a 5/16" OD half-groove turned in each. The initial grooving is done with a form tool, and final polishing work is done with a brass rod turned down to .304" After wrapping with 320 wet-dry paper, this creates a .312" dia abrasive for final contouring. The rollers are pinned to a pair of slab aluminum handles, and with the tubing inserted and clamped....

... the tubing is bent slightly past 90 degrees for this particular application. Properly made, a bender of this sort will NOT kink tubing. It is well worth the time to construct, especially when you need 9 (or more) absolutely identical assemblies.

Each intake and exhaust unit consists of a vertical induction member and a stub exhaust pipe, slightly bent to deflect the exhaust. I really wanted to create a proper exhaust collector ring for this radial, but after much consideration, I realized the layout of the distributor, engine mount, carb, etc., would prevent this.

To silver braze this assembly, the loose pipes and flange are mounted on a mild steel jig which holds everything at the proper angle. 1/32" silver braze wire is then wound on a mandrel so as to look like a spring with a 5/16" ID. Individual ringlets of the braze are then pre-mounted at the joint, and the entire assembly is heated dull red to allow the braze to flow. Try to avoid direct flame on the braze... the entire assemly MUST be brought to brazing temperature for a proper bond. After brazing, the unit is washed and pickled in a Sodium Bisulphate solution which is an excellent copper and brass pickle. This is available at any pool store as a pH reducing agent.
The assembly is wire brushed with a VERY fine wheel, and washed thoroughly with soap and water. A loop is then formed in a 20 guage copper wire, which is inserted into the induction pipe. The pipes are next soaked in hot TSP (TriSodium Phosphate), a powerful degreaser.

To nickel plate the pipes, I ordered a kit from Caswell Plating, an excellent company which caters to home platers and hobbyists. Check them out! A voltage of ~ 4V produced a current of 450 mA, just about right for the pipe assembly. Note the pipes are suspended from a tiny gearhead motor, which rotates the pipes at 30 RPM. This not only forces a more even plate, it gently agitates the solution to prevent hydrogen bubbles from sticking to the fresh plating, which causes pits and unsightly blemishes.

Here is a nice contrast between the brazed copper induction assy, and one that has been nickle plated for one hour. The photo does not do the plate job justice... it looks great! Nickle plating will prevent corrosion of the pipes, looks great, and is more scratch resistant. While not as flashy as chrome, nickle plating is MUCH safer and easier.
HOORAY! Mounted on a cylinder head, and inserted into the crankcase, the finished assembly looks top-notch... much nicer than a copper pipe!

Radial | Home | Links | The Shop