MEP002a IP plunger broken

[psu927]

I have a military rebuilt unit that only has 15 hours on it. I am pretty consistent on starting it every few months but it had been about 8-10 months since I started it. Long story short power went out the other day, I went to start it and no go. The throttle arm was tight. Got it freed up and still no fuel, so I pulled the pump and found a broken plunger. It broke right at the drive pin. (not sure if that's the proper name). pretty disappointed considering the hours on the machine. There wasn't any corrosion, the plunger was just tight in the bore. I run additives to prevent this so not sure why it happened.

Looks to me like a machined as a pair part, I'm willing to bet it and the head are machined as a pair and not available separately. Does anyone know otherwise? Anyone have any parts units they can sell a pump off of? I'd be willing to try to swap just the plunger.

 

[Ray70]

You have the old American Bosch PSU pump. Unfortunately I've also had 4 or 5 with broken plungers. I searched everywhere and no one had a replacement plunger and Ambac told me they don't support that pump at all.
Occasionally I have seen used pumps on Ebay, but to tell you the truth your best bet is to try to find yourself an Ambac M50 pump and throw that PSU away.
At least Ambac still supports the M50 and they are easy to rebuild.
The plungers and head are supposed to be serviced as a unit, but that doesn't mean swapping individual parts won't work, if you can find anything.

 

[psu927]

You have the old American Bosch PSU pump. Unfortunately I've also had 4 or 5 with broken plungers. I searched everywhere and no one had a replacement plunger and Ambac told me they don't support that pump at all.
Occasionally I have seen used pumps on Ebay, but to tell you the truth your best bet is to try to find yourself an Ambac M50 pump and throw that PSU away.
At least Ambac still supports the M50 and they are easy to rebuild.
The plungers and head are supposed to be serviced as a unit, but that doesn't mean swapping individual parts won't work, if you can find anything.

Thanks! Just curious how do you know which pump I have? They look very similar to me. (comparing online diagrams to mine)

 

[NY Tom]

I have not seen one but I am curious what these plungers look like and how complicated it could be to make one.

 

[Ray70]

There is a very simple way to tell them apart ( if you can't see the tag near the throttle lever.
Your PSU pump has a large flat head screw sticking out of a boss pointing directly down from the bottom of the pump.
The Ambac pump has the boss but no screw in it.

The delivery valve, cap holder , timing button etc is al the same.
The body, drive gear, plunger , timing button holder is different.

The TM shows only the old PSU pump.
The Ambac M50 manual is in the TM section, or I have it but can't see how to attach it her.

 

[Ray70]

The plunger is a simple shape but complicated to fabricate....
It is precision ground made from hardened steel, precision fit to match the pump head and is a shaft about 5/16" dia., tapered on 1 end with a head on the end, looks like a baseball bat.
There is a thru hole for the drive pin and a angled slash cut that controls fuel volume.

I have thought about seeing if a shop that had centerless grinding capability could make them, but would need a Non-broken sample...
All the plungers I have are broken.

When Ambac took over they changed the design to incorporate the plunger guide ( dog bone ) as the drive mechanism ( which will split open if the plunger is stuck ) rather than snap the shaft at the drive pin hole like the PSU does.

Unfortunately no one currently services parts for the PSU... but it may be worthwhile seeing if plungers could be made.
The only problem is they would not be mated to fit the pup body as they are supposed to be.

 

[NY Tom]

I just figure as you did that if a plunger could be made it might be ground to the same diameter as the broken one was. Or polished carefully down to that diameter.

Angled slash cut could be difficult not sure how it looks.

I ask because there are so many resourceful people here someone might be able to do it or have a good example.

Thanks

 

[Ray70]

Definitely a good idea and one that has been on my mind for a while now.
I have a friend bringing me an 003 parts motor with a good PSU pump on it this week.
I might just get the itch to take it apart and look into having a machine shop look at it.