How to replace the Diaphragm on an O.S PD-07 pump

OS claims that the pump is a “Sealed” unit that you can not take apart.  Unless I happened to get the only one that wasn’t sealed, that statement isn’t correct. 

The pump consists of three layers.  Layer 1 is a cup that holds the other two layers, and diaphragm between layer 2 and 3 and a check valve layer between 1 and 2.  Layer 1 also has the inlet outlet port for the fuel lines. The stack is riveted together with two 3.5mm OD x 3.2mm ID tube that are flared on both ends. They go through the two mounting lugs and this rivet is the “seal”.  

 Before taking apart the pump, you need to make sure you need to.  When you dismantle it, you will damage the diaphragm and it will need to be replaced.  First, how the pump works.   There is a port on layer 3 that connects to the crank case nipple on the engine.  The layer 3 and layer two have a small chamber that the diaphragm oscillates back and forth in with each stroke of the engine.  The check valve layer has two valves.  The first allows fuel from the tank to enter the chamber, but not be pushed back to the tank.  The other allows fuel from the chamber to be pushed to the carb, but prevents it from being drawn back into the chamber.  The diaphragm, in addition to providing the changing volume of the chamber, is also the seal that keeps the fuel out of the crank case and keeps air out of the fuel.  If the Diaphragm fails, fuel finds its way into the crank case and from there through the crankcase vent up to the intake manifold.  Added fuel in the manifold makes tuning the engine impossible.

To check for a leaky diaphragm, use a little squeeze bulb like the little 1/2A engines use for a tank filler.  With the pump off the engine, hook the squeeze bulb up to the port on layer 3, the crankcase pressure port, and gently squeeze.  I’ve found that if you lick you upper lip and place the inlet and outlet port pointed at your lip, you can detect even a slight amount of air escaping.  There should be none.  You should be able to squeeze the bulb and then hook it up to the port and the blub should remain collapsed.  If the diaphragm checks OK, stop right here.  There is no need to tear down the pump.  It could be failing, but if the diaphragm is good, that failure can be taken care of externally.  I’ll cover that later.

So, the diaphragm is leaking and it needs to be replaced.  Start by using a fine tapered burr in your Drimmel tool and gently cut the flange off the two brass tubes.  Be very careful to just cut the flange and not the plastic.  DO NOT try to drill it as the drill will grab and may destroy the pump housing.

 Once the rivets are gone, it is time to separate the layer 3 from the body.  In the above photo there are six rectangle indentations around the edge. These are the back side of the clips that hold the pump together once the rivets are removed. 

Getting layer three off the body and the remaining tubing requires you to slide a knife blade between layer 3 and layer 2.  This step will destroy the diaphragm. 

Place the blade between Layer 3 and Layer 2 on one lug.  Make sure you favor the side near the nipple, not layer 2.  An Xacto knife works.  Keep the blade square and lined up with the layer.  You want to just put a little spreading pressure on the lugs.  Now use a fine screwdriver tip and gently pull the cup side away from layer 3.  You will feel it snap as the case releases a tab.  Now go to the one on the bottom and release it.  If you are applying a little tilting pressure to the nipple on Layer 3, at about the third tap, it should release enough the you can finish prying the layer three up on the brass tube rivet just a bit.   Now go the other lug and do  the same. 

Once you have all six tabs loose pull layer 3 from the stack.  The diaphragm should be staring you in the face at this time.  Remove the diaphragm, spring cap, spring and spacer and keep them in order

The hard part is now done.  All you need to do is to put the new diaphragm in place and close it up.  I used layer 3 and pressed it onto the replacement sheet and traced around it with a sharp scribe.  I then used an xacto knife to cut it out.  I used the removed brass tube as punch to punch the holes. 

There is no need to replace the flared brass tubes.  They are simply to hold things together and the two mounting screws will do a good job of that.

Cllick here for a PDF to scale of the diaphragm  NOTE:  the drawing is at the bottom of the page.  Make sure your printer is printing 100% scale.   The stock diaphragm is 0.12mm thick.  It is a two layer piece of what looks like Mylar and supper thin stainless steel. The overall thickness is 0.12mm

 The problem becomes finding a suitable replacement diaphragm material.  Mylar is a good choice as it is very tough and will stand up to the heat from the puff from the engine.  I found some Mylar sheets for artist work on eBay   They are exactly 0.10MM thick and have a slight mat finish on one side.  That should work well.  What I ended up using was some packaging material from a box of Indian food from Trader Joe’s.  Other like packaging will work.  The package is an aluminized Mylar that you place in a pot of boiling water for several minutes to heat the contents.  It is very tough.  I couldn’t tear it until I made a starter slit with a knife.  It take heat without deforming, and the thickness is 0.12MM, same as the original.  This is what I used for my testing.  First, I cut a piece and placed it in a closed jar with some 15% nitro fuel and let it set for a couple days.  No hint of any attack from the fuel.  It performed quite well in my testing.  I would think either the Mylar or this packaging would work well.  The packaging is less expensive and you get to eat some good Indian food in the process.

Prior to this project, I made a replacement pump.  I used 1/32” silicone diaphragm material from McMaster Carr for the diaphragm.  For check valves, I use some Fourmost Check valves.  That pump also worked very well.  If you are interested, I’ll send you a drawing for it. The only critical dimensions are the spacing of the four mounting screws, they have to fit the back plate.  Some porting changes would clear up that overhang quite a bit.   As long as my replacement diaphragm worked, there isn't a need to make a pump.  The Diaphragm replacement is basically a free fix, only labor and landfill material are needed.  The pump below would cost you about $25 to make one, but you would have enough leftovers to make at least a dozen more for just the cost of the check valves.

The reason I mention this second pump is that, if the diaphragm is good in your pump, but the check valves are bad, you can simply add a Fourmost check valve in the line from the stock pump.  To check the check valves, use the squeeze bulb and blow into the intake port.  It should blow through freely to the outlet port.  Now blow into the outlet port and it will block the air flow.  Just add a Fourmost check valve on both ports, you don’t have a way to tell which is bad and you should be good to go.  Just make sure the flow is from the tank to carb and block is from the carb to tank on both valves.