by Roger J Schroeder

Click here for the full plan set.

Editor's Note: These instructions were supplied by Roger with a materials kit that is no longer available. However, nearly all the materials required can be found in the K&S Metals rack at your local hobby store. For part C, you can significantly reduce the amount of effort required by using a short length the next size up brass tube that slides over the 1/2" ID tube used for the cylinders (see the Pachasa Air Engine Page).

A 1928 issue of The Modelmaker magazine featured a construction article entitled, "Design for a Two-Cylinder Compressed Air Engine To Be Used in Flying Model Airplanes" written by Edward T. Pachasa. The engine described had been built by Edward's brother William from sketches prepared by Edward. At the time of writing the engine had not been run because an air tank was not finished.

To my knowledge the Pachasa Air Engine was never produced commercially, but a number have been built by amateur builders. The engine may not be historically notable, but the designer is. Edward Pachasa is better known by the name he later adopted, Ed Packard, the founder of Cleveland Model Supply.

The Pachasa engine first came to my attention in 1968 as a construction kit offered for sale by Jerry Burk of Arlington, Texas. I liked the one I built very much and described the kit and included construction plans in the March-April 1969 issue of The Engine Collectors Journal (Vol. 7, #1). Since a kit is no longer available from Jerry Burk I decided in 1983 to make one again available for home construction.


The first step is to check the materials supplied with this kit. The materials should consist of the following:

1Brass tube 17/32" OD x 1/2" ID x 2-1/4" long
1Brass rod 1/4" OD x 1-7/8" long
3Brass tubes 1/8" OD x 3/32" ID x 3" long
1Brass sheet 1-1/2" x 2-1/2" x 1/32" thick
1Brass hex nut 5-40 thd.
1Steel rod 1/4" OD x 3" long
1Steel rod 3/32" OD x 3/8" long
1Steel rod 1/16" OD x 2" long
1Steel strip 3/4" x 5/16" x 3/32" thick
1Phenolic rod 5/8" OD x 1-1/2" long
1Set of drawings
1Set of Instructions

If there is anything missing please let me know and it will be provided by return mail.

No materials are included for making an air tank, but a copy of typical directions for making an old time tank are included at the end of these instructions.

Study the drawings and instructions and match the materials with the finished parts shown on the drawings. The parts are labeled "A" through "0" on the drawings. The following table will help to match materials with the finished parts:

A2Cylinder Head Brass 1/32 sheet, 1/2 dia.
B2Cylinder Brass 17/32 OD x 1/2 ID x 1-1/16 long
C1Tie Support Brass 1/32 sheet, 1-3/8 x 33/64
D1Crank Web Steel 3/32 strip, 5/16 x 33/64
E2Piston(s) Phenolic 1/2 dia. x 7/16 long
F1Crankshaft Bearing Brass 1/4 OD x 1 5/32 long
G1Crankshaft Collar Brass 1/4 OD x 3/16 long
H2Crankpin Spacers Brass 7/32 OD x 1/64 thick
I1Crankshaft Steel 3/16 OD x 2-5/16 long
J2Propeller Drive Pin(s) Steel 1/16 OD x 17/32 long
K1Connecting Rod Brass 1/32 sheet, 7/32 x 1-1/16
L1Connecting Rod Brass 1/32 sheet, 7/32 x 1-7/64
M2Wrist Pin(s) Steel 1/16 OD x 7/32 long
N1Crank Pin Steel 3/32 OD x 1/4 long
01Prop Drive Plate Brass 1/32 sheet, 3/8 x 1-1/2
P3Air Tubes Brass 1/8 OD x 3/32 ID x 3

Machining The Parts

The drawings for the detailed parts are marked by letters from (A) to (P). Where more than one of each is required, the appropriate number is marked. The following comments are the result of my experience in making the engine parts. They may not always suggest the best way for you to make the particular part, but are the way that I found to be the most convenient. Edward Pachasa specified the cylinders to be made from standard 1/2" bore tubing with a 1/32" wall thickness. In the instructions he suggested the cylinders could be turned down to obtain a thinner wall and reduce the engine weight. The cylinder tubing supplied with this kit already has a thin wall (.0156") and does not need to be turned down. The tubing is, however, easily dented or pushed out of shape. Be careful with the cylinders. Resist the temptation to polish them on a powered buffing wheel. I ruined several cylinders when they caught in the wheel and were snatched out of my hand.

A. Cylinder Head - 2 required

These are cut from 1/32 brass flat stock. Rough cut the OD, drill the 1/8 dia. hole in the center of each and then, using a 5-40 screw through the center, mount in the lathe and finish the OD to size.

B. Cylinder - 2 required

Use 17/32 OD x 1/2 ID brass tube stock and cut the cylinders to length. Use care not to distort the cylinder when cutting off. I used a reamer to finish the bore to size. This was not a good idea as the resulting surface finish was not as good as the stock finish. You can leave the bores stock- and machine the pistons to fit. Another way of finishing the ID is to coat a 1/2" dia. steel bar (or a lap) with fine grinding compound, slip it into the tubing and rotate to get a fine finish. Don't forget the 1/16" dia. hole in one side of each cylinder. To drill the 1/16" dia. hole, first slip a round wood dowel having a diameter a little less than 1/2" through the cylinder. Support the cylinder by the dowel when drilling the hole. Without this support, the drill pressure will dent the area around each hole and make the piston stick in the bore. De-burr the cylinder ID around the hole when finished.

C. Cylinder Support

One of the more difficult parts. Try drilling the 1/4" dia. hole first and then bending the brass sheet to a 19/64" radius. Bend over a round bar with the aid of a wooden hammer. Finish by re-drilling the 1/4" hole because it will be distorted slightly in bending. File any uneven surfaces.

D. Crankshaft Web

Drill the 3/32" dia. and 0.1875" dia. holes first in the piece of 3/32" flat stock. After the holes are drilled, cut and file to the shape specified.

E. Piston (2 required)

Check your finished cylinder diameters before finishing the piston OD. I fit my pistons a little tight and freed them up during the breakin period. Cut the 1/64 wide grooves in the piston OD before finishing the piston to the final size. A good fit is .0005 to .001 less than bore dia. when broken in. Use a sharp tool and fine (.001"-.005") cuts when finishing the piston OD.

F. Crankshaft Bearing

Make this from the length of 1/4 OD brass rod. Drill the inside diameter a little under 3/16" and then use a reamer to bring the diameter up to' size. Note that two of the 1/8" dia. holes (the pair) go through both walls of the tube while one of the holes goes through only one wall.

G. Crankshaft Spacer

This spacer is made from the 1/4" dia. brass bar and soldered to the crankshaft, so the dimensions are not critical.

H. Crankpin Spacers

These spacers are to be a free fit on the crankpin, (N). The outer spacer is soldered to hold the rods in place.

I. Crankshaft

Machine the cranshaft from 1/4" OD steel rod stock. I milled the flats, but a file might be just as easy. Check the fit in the bearing (J) and adjust the OD accordingly. A tight, close fit is necessary to prevent excessive air leakage.

J. Propeller Drive Pin (2 required)

Use the steel rod supplied to make these.

K. & L. Connecting Rod(s)

K and L are the same except for the length between the 1/16 and 3/32 diameter holes. The parts should be cut out and filed to shape after the holes are drilled. The longer rod is bent as shown on the assembly drawing.

M. Wrist Pin (2 required)

Use the steel rod supplied.

N. Crank Pin

Use the steel rod supplied. This must fit into the 3/32 dia. hole in (D).

O. Propeller Drive Plate

Drill the three holes first in the piece of 1/32" brass sheet and then cut to shape. Right now it seems as if the plate should have been made of thicker stock, but this is the way it was originally specified.

P. Air Tubes

Bent from 1/8" OD brass tube. Supplied pre-bent, but if you need to make them, anneal the tube first by heating to red heat and quenching in water.

Engine Assembly

It will be obvious from looking at the assembly drawing that there are many soldering operations required in putting this engine together. There will always be the danger that the heat from one soldering operation will melt the previously soldered joints and that the whole thing will come apart. There is a method I have found to avoid this difficulty. The method is to use two solders with different melting temperatures. Accomplish the basic assembly with the higher temperature solder and finish with the lower melting temperature solder. The following paragraphs detail this procedure.

Silver solder is a very good, high temperature solder for the basic assembly. Start by silver soldering the cylinder heads, Part (A) to the cylinders, (B). Next join the 1/4 round support, (C) to the crankshaft bearing, (F) with silver solder. Note the 63/64 dimension given on the assembly drawing. Silver solder the crankpin, (N) into the crank, (D) but do not attach the crank to the shaft yet. The last silver soldering operating is to put the propeller drive pins, (J) into the propeller drive plate, (0).

Stop at this point and hand polish all the above parts because the heat from silver soldering will have discolored the metal parts. Be sure to clean any remaining soldering flux from the cylinder bores. A little time may be necessary with grinding compound to final fit the pistons. Once this is accomplished, the rods, (K) and (L) are assembled to the pistons with the piston pins, (M). A little epoxy on each end of the piston pins will keep the pins positioned correctly in the pistons. Do not assemble the pistons in the cylinders until all local soldering is completed.

The remaining operations are to be performed with a low temperature melting point, soft solder. These operations will require some patience, but stay with it, the results will be worth all your suffering. Join the cylinders, (B) to the 1/4 round support, (C) and space them according to the 1/2 in. dimension shown on the assembly drawing. The 1/16 dia. holes in the cylinders go to the rear of the engine. If you have not already done so, bend 1/8 in. OD brass tubing to the outlines shown on the assembly drawing and solder the tubing to the engine. Heating the brass and allowing it to cool will make it softer and easier to bend. Make all bends over a round bar.

Assemble the pistons and rods in the cylinders and slip the crankshaft, (I) into the bearing, (F). Slip the crank, (D) over the end of the shaft, but do not solder yet. Slip the connecting rods onto the crank after having put one of the (H) washers on the crank pin.

Engine Timing

The engine air porting is designed so the engine will run either clock- wise or counterclockwise, but not both. Choose the desired direction at this time.

Turn the crank and pistons in the 'direction of desired rotation until one piston is 1/32 in. past top center and the other is 1/32 in. past bottom center. At this point the exhaust valve should be opening for the piston on the upstroke and the inlet valve should be opening for the piston on the down stroke. Turn the shaft, holding the crank still, until this relation is obtained. Remove the connecting rods from the crank pin, then solder the crank to the crankshaft. Now, Put the rods back onto the crank pin. I did not find the timing to be critical.

Solder another washer, (H) to the crankpin, thus retaining the rods on the pin. Warning, be careful not to overheat the pistons when soldering the last washer in place. Slip the spacer, (G) over the threaded end of the shaft then slip the prop drive plate, (0), over the shaft. Arrange for approximately .010 in. fore and aft play in the shaft and solder the drive plate to the shaft and spacer. Use care that solder does not get between the shaft and bearing. I stuck my shaft solid at this point and had to take everything apart and start over again!!

If the assembled engine is tight, clamp the threaded end of the shaft in the chuck of a lathe or drill press and turn the engine over until it is free. A little lapping compound will help here.

The instructions appearing in The Modelmaker say that the Pachasa engine should swing a 14 in. prop of low pitch. I did not have a suitable large prop, so a 7-3 Rite Pitch was fitted to the engine. I hooked the air supply tube to a 10 PSI air supply and the engine took off smoothly. At 60 PSI, my engine turned the 7-3 at 2000 rpm.

Besides being a smooth running piece of machinery, the Pachasa Air Engine is a beautiful display engine when all its brass parts are polished. I believe you will all agree that the effort has been worthwhile.

I would like to hear from everyone who constructs one of the Pachasa engines. Call or write if you have any problems.



  Back to the Model Engine News Home Page

Please submit all questions and comments to