VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 Installing a neoprene diaphragm in the fuel pump

Which is the best method to install a new neoprene diaphragm in the AC 405 fuel pump and other AC “B” type pumps?

Old time “Assembly Instructions“ describes a method which I guess was correct for installing the old type of diaphragms, consisting of three (or four?) layers of coated cloth. They state that “The most important single item in the repair of fuel pumps is to pull sufficient diaphragm cloth inside the pump to prevent stretching of the cloth when pump is placed in operation”. I have seen cross section pictures of this type of pump, showing a distinct wrinkle of the diaphragm, all way around the center protector and the rim (brim?).

The thickness and character of the neoprene diaphragms makes it hard to believe that it is possible to make or maintain a “wrinkle” inside the pump according to the old time instructions. The neoprene diaphragms also feels flexible, so I guess it is not necessary “to pull sufficient diaphragm cloth inside the pump to prevent stretching”.

If so, which is the best method? Is it a good idea to bolt the pump housing, with diaphragm, to the block, turn the crankshaft until the rocker arm is on the top of the cam and then reinstall the pump cover? Or is it better to put the hosing in a vise in a position where the diaphragm is in line with the housing rim/brim, reinstall the pump cover, and then bolt the pump to the block? Or is the neoprene so flexible that it does not matter?

With the first method, with the engine running, I understand the diaphragm will work in a flat position. The tension on the diaphragm occurs only the first 5-10-15 revs on the camshaft. During these revs, when the rocker arm moves to the lowest part of the cam, the diaphragm spring lifts the diaphragm, when the rocker arm then moves to the top the diaphragm moves down and sucks fuel, first to the glass bowl, then the pump and then the float chamber. After that the diaphragm moves only enough to replace the fuel consumed since the latest rev on the camshaft. That is … almost nothing. Correct?

With the second method you can not know for sure if the diaphragm, when the pump is bolted to the block and the engine running, works in the flat position or with some tension. Am I right?

Any experiences or suggestions are welcome!

The old time Instructions also state that “The diaphragm should be dipped in kerosene…“ before installing. I guess that is not necessary with a neoprene diaphragm?

Just if you wonder: I have not equipped my 1922 490 Touring with a fuel pump. And I do not intend to! But my 1931 Opel 1,2 liter Coach has an AC 5982, similar to the 405 except position of inlet and outlet.

here is my Rebuild on my 1929 AC Fuel Pump

Thanks, lovely pictures, everything looks nice. I guess you bolted the assembled pump to the block?

Approximately, how many revs was required to fill the glass bowl, the pump chamber and the float chamber?

I notice that the pot metal goods keeping the diaphragm, both on the housing and on the cover, is much thicker than on my AC pump. Very good, on my pump the areas on both housing and cover were warped, so I had to level them.

HI swedechevy
When I rebuild a fuel pump, I lightly tighten the screws holding the cover to the body for the diaphragm. Then I operate the fuel pump by had a few times, then hold the rocker arm in the applied position while I finis tightening the cover screws. This positions or centers the diaphragm.

I do the exact same thing as well.

This is what i did as well, worked the diaphragm before tightening the case together. also vacuum tested to ensure it sealed !!

Then and Now has a step by step picture guide and is an excellent source for fuel pump parts;

http://www.then-now-auto.com/

Dave

The Repair/Rebuild Kit #FS-908 from TFS has detailed instructions as well. These are what i followed when doing my rebuild.

Marxparts, I’m sorry I’m not sure if I understand the meaning of “the applied position” when you write “then hold the rocker arm in the applied position”. Please describe in other words.

Also, I would be happy if you will take your time to read and hopefully comment my reply to BearsFan315, as he refers to the Assembly Instructions from The Filling Station.

BearsFan315, I bought The Filling Station Repair/Rebuild Kit FS-908 February 2017. I followed the instructions. The first 4-5 short tours, together maybe 50 kilometers (40 miles) the pump seemed to work fine. But then it began leaking. Now my guess is that the pump was a little to warped. I had noticed a little warping before I installed the new neoprene diaphragm, but I didn’t measure and at that time I didn’t realize the risk. So I tried to stop the leaking by tightening the six screws, not understanding that I made the warps and leaking worse and worse. After some time, and mail contact with Stephen Cassis on The Filling Station, I measured the warp more precisely. With the housing and the cover together, without diaphragm, I could insert up to a 0,50 mm (.020”) blade between two screws. I realized that was too much, so I put the pump aside, having a lot of other things to do.

Before I realized that I had to handle the warping, I had really tried to understand how the pump worked, in order to find out if there was something else that caused the leaking. One of the things I thought about was the different character of the neoprene diaphragm compared to the old type of diaphragm that had been in my fuel pump since decades. That’s one of the reasons for my question in my first post.

After reading your posts, and posts from marxparts and Junkyard Dog I was still not convinced, maybe because I was not sure if I understood marxparts correct. And because the Instructions that I got from The Filling station, which you refer to, are copied from instructions that were wrote in the 1940-ies or maybe the 1930-ies, when the diaphragms consisting of three (or four?) layers of coated clothe not were as flexible as the neoprene ones, or maybe more correct: flexible in another way than the neoprene diaphragms.

Today I examined some things more closely than I have done before.

The neoprene diaphragms are quite flexible. I think I can stretch it by hand enough to increase the diameter say at least 2 millimeters (.080”). The distance between the centers of two opposite holes is nearly 70 millimeters (2.75”). The six holes are located almost exactly like the holes in the housing and the cover. This made it easy to insert the six screws, I just needed to press the cover slightly against the diaphragm, and the screws took thread in the housing. Before I installed the cover, the diaphragm was positioned approximately 11-12 millimeters (.43”) above the housing. When I tightened the six screws - lightly, not binding - the diaphragm spring pressed the middle of the diaphragm upwards. As far as I could feel, all the way or almost all the way, say 10 mm (.40”). As a consequence, the diaphragm must have been stretched somewhere, as its edge was fixed by the six screws. It is not possible to see where the diaphragm became stretched, my guess is that most of the stretching was at the six holes. When I pressed the rocker arm, I noticed that the edge of the diaphragm expanded all around. I measured the expansion roughly, I say that the diameter increased more than 1 mm (.040”). And I guess that the six holes expanded say at least 0,5 mm (.020”), maybe up to 1 mm (.040”) considering the middle of the diaphragm was steadily fixed between the upper and lower protectors. The round holes became oval holes.

Now I picked up a NOS diaphragm layer that I happen to have, probably one left from a set of four. The thickness of the layer is 0,55 mm (.022”). I tried to stretch it, but it was impossible. Maybe these layers were a little more flexible when new, but I don’t think so. The distance between the centers of two opposite holes was a little more than 72 millimeters (2.86”), 2 mm more than on the neoprene diaphragm, the housing and the cover. Surprising? I was not surprised. I am quite sure this extra distance and area are needed to form the wrinkle in the space between the middle (the two protectors) and the edge/brim on the housing and cover, a wrinkle which is needed to “prevent stretching of the cloth when pump in placed in operation”, as the old instructions states. Particularly the first revs on the camshaft when the middle of the diaphragm moves maybe 6-7 millimeters up and down. The need for a wrinkle makes it necessary to follow the old instructions, in The Filling Stations version paragraphs #16-17. At least for diaphragms of the old type.

I also measured the used diaphragm that has been in my pump for decades. The distance between two opposite holes was 69-70 millimeters, like the neoprene diaphragm. But this diaphragm has a clearly visible wrinkle between the middle and the part of the diaphragm that had been fixed between the edges/brims of the housing and the cover, while the neoprene diaphragm is quite flat.

My preliminary conclusion, regarding what you, marxparts and Junkyard Dog has stated, is that the neoprene diaphragms are very flexible. Maybe so flexible that it doesn’t matter which method I choose, given that the housing and the cover are not warped.

But I am still doubtful it the method in the old instructions, like the one from The Filling Station, is the best. Tighten the six screws with the neoprene diaphragm in a position where the six holes has become six ovals? That seems too risky to me.

If you think I have missed something important in my testing and thinking about the question, or if I have not made myself clear, please let me know.

If not, please read my first post again: Which method of the two that I describe do you think is the best?

I have rebuilt at least 50 of the AC-405 fuel pumps over the years (maybe even more than that) and I have used various methods to rebuild them, including the method described in the repair manual. I have used a neoprene diaphragm in all of them and to date none of them have leaked.

The top cover and the body of the fuel pump must be flat or leaks are going to develop.

I have also rebuilt a large number of those fuel pumps. Probably not as many as Skipper. Can't remember enough warping between top and bottom to cause leaking though it surely is possible.

To maximize the pumping action the diaphragm must be in line with the the top and bottom joint when the pump arm is in the middle of its stroke. That results in the maximum volume change in the diaphragm cavity and maximum fuel supplied.

The best way to determine that is to mount the bottom part with arm on the engine and rotate the engine one complete revolution while measuring the movement of the diaphragm. I have never done that! Most of the rebuilds were done on others fuel pumps and there was no practical way to know the amount of wear in the cam, fuel pump, etc. Despite that fuel pumps have worked well.

All that being written it is not necessary to maximize the pumping volume for it to be functional. It is necessary to be close enough so the volume is adequate.

Warping of the fuel pump castings is a problem. I have found the section where the glass bowl mounts is often the biggest problem with rebuilds. Minor warping can be overcome with thick cork gaskets. The neoprene gaskets are often too hard to compress enough to prevent air leaks and render the pump useless.

I have found the above to be true as well. When rebuilding an AC 405 fuel pump, I never use the neoprene gasket that is supplied with the rebuild kit for the glass bowl because of the air leak issue. I always use the correct type of cork gasket instead.

Hi Swedechev
What I meant by "Applied Position" is manually pushing the rocker arm as far as you can toward the fuel pump with the top cover screws just tight enough to locate the cover and still let the diaphragm slightly move between the two parts. I do this with the spring cover removed. This simulates the action of the eccentric lobe on the camshaft. This sets the diaphragm at the bottom of the travel of the pull rod in the fuel pump so when the fuel pump is installed the action of the fuel pump diaphragm going down to the bottom of its travel or coming up to the top cover with the spring pressure under the pull rod is not hindered by having to stretch the diaphragm. This should not oblong the holes in the diaphragm. If is does, then the diaphragm you are using is too small or not stamped out correctly. You have to remember that the diaphragm pull rod only moves about 1/4 -3/8 inch travel when the fuel pump is operating.

The diaphragms I put in my fuel pump kits are made in the USA. These diaphragms are made out of chemically resistant Nitrile rubber material with a nylon cord mesh for strength. Between the diaphragms I have sold in my kits, just the bare diaphragms in bulk that we have sold and what I have used myself to rebuild AC Model B fuel pumps total 1500+ at this point.

When I rebuild an AC 405 fuel pump ( or any fuel pump) , the first thing I do after cleaning and inspecting the fuel pump parts is to flat sand by hand in in a circle motion the fuel pump main housing diaphragm face, top cover diaphragm face, mounting flange face, 3 bolt spring cover face and the flange where the spring cover bolts on to. To do this, I use a known flat piece of steel with 60 grit sand paper (3/4 inch sanded plywood or thick glass will work), sand the item in a circular motion keeping even pressure on the housing until all evidence or warping is gone and then finish the face off with 150 grit paper. If it is too warped, then it goes in the scrap pile.

I used to throw away some of the fuel pump covers because the fuel bowl mounting was overly warped from excess tightening of the thumb nut on the bail and I couldn't get a good seal on the fuel bowl gasket. This is what I do to fix this and it is a very, very, very tricky business and you have be really patient and check often on your progress: Install a small 2 jaw puller (battery cable puller works real good) on fuel pump cover with the puller centered over the fuel pump bowl flange and the ears of the puller arms centered on the bowl flange. Of course the fuel bowl, gasket and screen has to be removed. Use a spacer of some sort slightly higher and next to the check valve port so you can center the puller. Apply pressure on the puller just to slightly tighten is up. Use a 1500 watt heat gun on high and warm up the housing for while until you think it is good and hot. Die-cast melts at about 625-650 degrees so a heat gun works the best. Tighten the puller screw a little at a time very slowly while still heating the housing. You will have to feel the resistance one you start tightening the screw to merit how fast you can turn the puller screw. If you go to fast or get aggressive, the housing will crack or snap and you will be looking for a replacement. Only go about 1/4 turn max on the screw while heating the casting. When you get to the end of the turn, take the heat away and leave the puller in place with tension on it until the housing cools down. Then check the flatness of the housing flange face by seeing if the glass fuel bowl will rock back and forth length wise with the housing. If the flange is still not straight, then do the same procedure again. I would not suggest doing this if you are not very mechanically inclined. I have made my own special tools and spacers for doing this.

You can see what you have for travel on the fuel pump diaphragm once the fuel pump is installed. Take or leave out the pressure side fuel pump check valve disc, spring and retainer nut. Have some one turn the engine over by hand or with the starting crank and you will be able to see the top retainer plate for the diaphragm move up and down. Put a piece of 1/8 welding rod in the hole and this will show you your diaphragm travel. AC actually made a tool for doing this.

If you want to see what my fuel pumps look like when they are done, check out our website www.marxparts.com or our facebook page Marx Parts LLC
Regards Bob@marxparts