7MC-2A & MC-2A (USAF) Towable Diesel Air Compressors

[Tinstar]

Have been using the 7MC-2A quite a bit.
About 6+ hours since last post.
This thing is a lot more useful than I thought it would be.

Discovered that I have a crankcase breather problem.
It’s pressurizing the crankcase and forcing oil past the oring.

Had the oil fill cap off to check the amount of blow by and noticed that the leak stopped.
Put the cap back on and the leak started right up.
It does of course have some blow by, but doesn’t seem excessive and always has great oil pressure and starts almost immediately, regardless of engine or outside temp.
Exhaust has some smoke at startup, but is clear after that.

I had already taken off the crankcase breather and cleaned it. All looked fine and all parts accounted for.
What I didn’t do was look down inside the long breather tube so see if there’s an obstruction.
Will do that next time I have the cover off.
This machine doesn’t look abused at all so I doubt there’s anything stuck in there. Only one way to find out.

A plastic Lipton tea bottle (with bottom cut out) screws into the oil fill tube perfectly. Nice and tight.
It also makes a great funnel.
While it’s running, I have that screwed in to keep the oil splatter down and the crankcase can breath.
I have a piece of scotch-brite tucked inside to keep anything from falling inside.
Works perfect and it doesn’t get saturated and clog up like a paper towel would.
As soon as I’m done and shut the engine off, oil fill cap goes back on.
Short term solution obviously, but it’s working great.

Ran it almost 3 hours nonstop and No leaks.
Very fuel efficient machine, the 7 gallon tank will last a really long time.
Have two-stroke oil and seafoam mixed in as usual.

Still waiting on parts for both machines.

 

[Tinstar]

7MC-2A

Did some more maintenance yesterday.
Installed new air filter and new pressure gauge on the compressor itself.



Also cleaned out the automatic air tank water drain.
Dirty and bits of rust in the screen but still works fine.


Took a closer look at the voltage regulator.
Made by Ducati.


Then broke out the voltmeter to see exactly what the alternator is putting out.
Readings were not what I expected.
Jumping around and not much above battery voltage.
Moved probes and when I grabbed the center connector, it was VERY hot.
Not smoking but definitely not normal.
Well……I found the issue.




It’s been this way a long time.
Wire insulation was completely gone for a few inches and then what Insulation was left was hard and brittle.
It’s like that all the way and then disappears behind shroud.

EDIT: I separated the wires all the way until it goes behind case shroud. Did a continuity test and sure enough. It’s shorted together past that point.

I’m not even going to try and save the alternator.
The wiring is probably just as bad it’s entire length and not usable.
I ordered a brand new Alternator from Hetz.
Then the fun begins with installing it.
Not sure yet if the voltage regulator is also bad.

The Ammeter never gave me any indication of this issue.
I’ve ran this unit almost 10 hours so far without any problems.
The hydraulic cooling fan runs the whole time and never ran the battery down and battery still cranks the engine just fine.
The Hatz diesel does start within 1-2 seconds of cranking.

I did check the breather tube and all is fine.
I guess the blow by is more than the crankcase breather can handle.
So it’s either do the tea bottle trick from now on, or remove the heads and all that hardware to replace the push rod tube orings.
If I replace the orings, there’s still no guarantee that it won’t do the same thing again.

$1 tea bottle vs $300 kit?

I’m thirsty……….

 

[Guyfang]

Drink GOOD beer. That will cure it all.

 

[Tinstar]

Drink GOOD beer. That will cure it all.

Says the man who lives in the land of amazing beers……..

Absolutely

 

[Guyfang]

Tonight's beer! New to me, and a real keeper!

 

[Tinstar]

I also ordered a new Hatz voltage regulator.
It's also made by Ducati.
Never hurts to have spare parts.
I'll do the testing procedure on the currently installed regulator when parts arrive and I'm ready for the install.
The cover is back on now and parked.

If I keep this one or sell it later, it still needs to operate correctly.

 

[Tinstar]

7MC-2A

New parts arrived yesterday






Found some issues with the old parts.




It had been like that for some time
Glad I also ordered a new voltage regulator as it was fried as well.

The new alternator was slightly different with a protective covering at the wire attachment points.



Will be starting it today to see if everything works as Designed.
Had to wait for some sealant to cure.
It was required at the point where the wires pass to the outside.



New Voltage Regulator installed and ready to go.




Also did some other maintenance.
The fuel tank support straps were installed backwards, so I fixed that.
Installed tie down shackles.
Now I can properly secure it during transport.

Also discovered the breather tube is junk.
It has deteriorated and is doing nothing but letting junk inside engine.
I have one of those on order as well.



Still wanting to replace the main panel Air pressure gauges.
They work fine but look horrible.
Also thinking about painting this the Vietnam Gloss Green color.
Might just leave it as is.

So far So good.

 

[Tinstar]

7MC-2A

Started engine and the newly installed parts worked perfectly.


Had to replace some connections.
The old one was also hot to touch so I cut out the old female connector and replaced it with same style.
New connection is pictured.
With new one in place, the heat issue disappeared.
The other Connection that was warmer to the touch was also replaced and that also solved the heat issue.
Didn’t find any other warm/hot wires or connections.



Also refreshed a few grounding points.
The main ground from engine mount to unit frame was sloppy and wire felt loose where it was crimped.
Fixed that

Also fixed one to the engine block.
It was also poorly done.




Unit is running great.
The new breather tube will be here early next week.

Slowly but surely getting the issues worked out.

 

[Tinstar]

7MC-2A



New part arrived.
Fit like a glove.
Will run it today to see if it helps with the crankcase pressure problem.

I think the blowby is too great for this tube to matter much.
If so, I will keep doing the tea bottle trick.
I'm not going to remove the heads and related hardware, at least for now, and spend the money to replace the orings and risk it still pushing oil past them.
The unit has enough hours on it to expect decent piston ring wear.
Plus the Davey compressor isn't supported anymore and parts are unavailable. So why rebuild the top-end, only to have the rotary compressor go down and turn it all into a big unusable brick.
Hatz still supports the engine and everything is available.

Right now it's doing a fantastic job and is great to have around. Use it much more than I thought I would.



Still waiting on the final part to arrive for the MC-2A

 

[hrbergeron]

Nice info in this thread. Picked up one of these late last year. Definitely not as pretty as yours, worked 1 day then won't build air pressure.

 

[Tinstar]

Nice info in this thread. Picked up one of these late last year. Definitely not as pretty as yours, worked 1 day then won't build air pressure.

Which one did you get?

The 7MC-2A is the newer version and a lot less complicated.
Using mine for all sorts of projects

The Deutz engine parts on the MC-2A are slow in coming but still available.
They should be here the 15th or so.

 

[Tinstar]

7MC-2A

The new breather tube didn’t make a difference at all on the blow by.
Will continue to use the tea bottle trick instead of tearing into it for that oring replacement.

Unit runs fantastic and starts right up
Pressure is great and is plenty for what I need.

Voltage is still doing great after the parts replacement.

I do need to change the compressor oil hoses.
They’re not leaking but look 1994 original.
More for my piece of mind.
Will change the compressor oil again also.

 

[Tinstar]

New parts FINALLY arrived.
A new Governor adjustment screw and crankcase breather gasket.

Still no service manuals …….so does anyone have the procedure on how to adjust it?
Or pictures?

Installing will be easy but I don’t know how far to screw the screw in to the pump rack.
The other unit is also missing the part so I can’t just look at that.
I know it’s RPM dependent but an initial baseline would be great to know.

 

[Tinstar]

MC-2A


Installed parts yesterday and it was interesting.

How this broke is a mystery. Looks like it ran quite a bit afterwards



When installing new screw, discovered that the governor arm was bent. It would not line up properly. Pair of pliers fixed that.

Actually I think I bent it when I started engine up very briefly weeks ago. Tried to push rack into closed position. Engine runs for a bit when switched off. It never came to full rpm’s. Somehow I must have pushed it towards the gear and it caught the broken adjuster screw. Earlier pic shows it unbent.
(Paper towels are there to prevent me losing parts down into engine)



Installed new breather cover gasket and new hose.



Figured out the engine speed control.
Simply adjust this knob for desired engine speed and lock it back down, then safety wire it to prevent it from backing out.
Clockwise increases engine speed and counterclockwise decreases it.

I still need to fine tune the speed and then install safety wire.
The nut safety is missing so I’ll just make something that will work.




Got everything installed, new compressor oil topped off, already did engine oil and filter.




She fired right up and produces air!
Good oil pressure
No leaks.
Voltage regulator works and alternator is charging battery.
It’s very loud with the housing side cover off.

The oil cooler fan is controlled by a thermal switch.



(Edit: According to the manual, this switch is a temperature shut down switch. So now I get to reverse my “bypass” and figure out why the fan wasn’t coming on.
Pays to actually read the manual.)

Oil cooler housing was getting really hot so I shut down engine.
Tested fan itself and discovered the fan is dead.
Swapped fan from my other MC-2A and now I have a working fan when tested directly to battery.

Started unit back up, fan still not coming on.
I already verified power coming to thermal switch.
Removed switch and checked with multimeter.
Checked ok.
Reinstall and still nothing.
Swapped out with one from other unit and still nothing.

I finally bypassed the thermal switch and now the fan comes on when power switch is on.
(The 7MC-2As fan comes on as soon as the engine is running. No thermal switch).



Put cover back on and ran it for another hour.
Unit runs great and holds air pressure just fine.





Still need to fine tune engine speed and then it will be ready for use.
I don’t have a way to read rpms on this particular engine, so I’ll listen to other units running online and simply adjust by ear.
It’s not a generator so rpm isn’t critical.
3000 rpm is the factory set speed so I should be able to get it close.


So now two out of three air compressors are back up and running.
I use the 7MC-2A quite a bit.

It’s smaller and much easier to work on. (Except removing the steel cover. It’s a pain and is designed for two people removing it)
Quieter too since engine speed it much less.
1800 vs 3000 rpm

Now…….which one do I keep?!

 

[Tinstar]

MC-2A


Adjusted engine RPM and torqued down the nut.
Then I safety wired it for security.



Installed two new air filters.
One for engine and other is for compressor.
(Ordered Wix part number and these were in Wix boxes)



Not sure what went in hole by rpm adjustment knob.
The manuals do not show anything there.
Civilian or Military.
I just used a standard machine screw since the hole is an opening to inside the crankcase.



4 quarts of Mobil 427 compressor oil filled case to exactly where it was supposed to be.




Ran unit for almost an hour today and it went well.
Engine runs great and doesn’t smoke except at initial startup.

Checked all the fluids and topped off fuel tank.
It’s parked now with canvas cover to keep water out.

It’s ready and waiting

 

[Tinstar]

MC-2A

Slight oil leak behind Alternator.
Ordered parts and will install next week.

I also ordered the governor cover gaskets.
Tore one when I installed the other new parts so will replace them both.