DN4M pistons

[loosegravel]

I’ve been down a rabbit hole today while looking for some replacement pistons for this MEP-803A that I picked up last week from GP. This thing is the worst case of wet-stacking that I’ve seen yet since buying these machines. Low compression led me to remove the cylinder head. Then I found that 3 out of the 4 cylinders have piston deposits on them. So much carbon in the combustion chambers, exhaust etc. that it’s falling out in big chunks. I haven’t removed the pistons yet, trying to source the parts first before I have pieces of this thing strung out all over my shop for weeks waiting for the parts to arrive. At any rate, the part number on the current pistons is 751-42670/0.250/5. The Onan part number is 186-6068. As near as I can tell this is a .25mm oversized piston. The reman tag on the side of the block and the tier two reset tells me that it’s probably not standard. Yes, I can find them online but not for less than $300 ea. My friend used to drag race and he could buy pistons for his race engine all day long for about 1/4 of that. I’m just wondering if anyone has a lead on a source for these pistons that’s more reasonable. Thanks! Jeff

 

[jamawieb]

Ive never seen wet stacking cause low compression. In the picture of the cylinder wall that looks like aluminum on the wall. Why do you need new pistons? Usually low compression would point to rings. How many hours is on the unit.

 

[loosegravel]

You are correct. It is aluminum on the cylinder walls. 3 of them. Presumably it’s from the pistons. 145 hours on the clock. My suspicion is seized piston rings, due to high carbon deposits and continued operation with no-load. With low oil and high temperature shut-downs on these machines what else would cause pistons to leave themselves on the cylinders? As I mentioned I have stopped short of tearing it down any further until I have the replacement parts in hand. I’m just looking for a lead on some replacement pistons and rings.Thanks for your reply. Jeff

 

[Light in the Dark]

LPW Diesel 750-41610/025 186-6068 Piston Assembly .25MM ⋆ DP Equipment LLC

LPW Piston Assembly in .25mm Oversize includes Piston, Rings, Piston Pin and Clips 750-41610/025 186-6068 these are used in Military Generators MEP803 MEP802 Std. Size This price is for ONE Assembly 4 cylinder engine = 4 Pistons Required. HW L3A4

dpequip.net

 

[loosegravel]

Thank you! This confirms my findings. My current pistons are .25mm oversized. I’m also in contact with MayiDiesel in Texas. He’s checking to see if he has what I’m looking for. I’ve seen a lot of your posts here. I’m sure that you’ve seen just about every scenario on these generators. What’s your thoughts on this unit? What would cause the pistons to rub off on the cylinder walls other than severe carbon deposits, seized rings and continued operation with no load? Of course when I get this unit running I’ll check the low oil pressure and high coolant temperature shut-downs to make sure that they’re working. Air filter was in place and sealed also. Thanks again! Jeff

 

[Light in the Dark]

There are folks on here (Jamawieb being one) that have much more knowledge on engine failure in these sets than I do. With that said, I've had sets in that I have to chisel the manifold clean of deposits, before even attempting to start. Do you see any material missing from the piston itself, or any irregular wear on the edges? Did you dump the oil and look for material?

I would think if your piston touched the cylinder walls, even for a brief time, you would see much worse damage to this and the piston would probably have a catastrophic failure in place.

 

[loosegravel]

I was going to wait until I had the replacement pistons in hand before tearing it down any further. But after thinking about it some more I really owe it to myself to remove the pistons and mic the cylinders just to make sure I’m ok there. The crankcase was empty, so I don’t know what’s in the basement. I’ll go ahead and remove the pistons and share some photos here. I absolutely love the trap-door design on these engines! I don’t have to pull the engine/generator head to remove/replace the pistons.Thanks again! Jeff

 

[loosegravel]

This piston #1. 3and 4 look just like it. The upper ring is seized on 1, 3 and 4. Not yet on #2. Maybe that’s why #2 is still good?

 

[loosegravel]

#4 was the worst one. I’m confident that I can clean up the cylinders. No scrapes or gouges. Only aluminum deposits. The bearings have very little wear. Indicative of a low hour unit.

 

[nextalcupfan]

That looks pretty bad to my eyes. And you're sure this was caused by wet stacking?

I know you said that was a theory in post #3 but it seems like a ridiculous amount of damage just for running the unit with nothing (or very little) on it.

That being said I have no explanation myself.

 

[jamawieb]

Your low compression is coming from frozen rings. That is why I asked the question of the number of hours. This is very common on low hour units and has nothing to do with wet stacking. Your number 4 piston is concerning because there is no reason for the piston to rub, unless the motor was dry and turned over with no oil in it. I've rebuilt alot of these motors. If it was me, I would hone the cylinder, put in 1 new piston and 4 new rings. With such low hours, I would want a new piston to know that they are all newer. Without seeing the other cylinder walls, I agree that everything should clean up nicely. Also look at the pistons around the wrist pins becuase they crack around that area if the pistons were stuck.

 

[Chainbreaker]

He said that it only had 145 hrs on the Hobbs, though it is a remanufactured engine with .25mm oversize pistons. So, there might be other factors involved other than the wet stacking, as to the quality of the reman engine job.

 

[loosegravel]

That looks pretty bad to my eyes. And you're sure this was caused by wet stacking?

I know you said that was a theory in post #3 but it seems like a ridiculous amount of damage just for running the unit with nothing (or very little) on it.

That being said I have no explanation myself.

I really have no other explanation either. I’m just going to focus on getting it repaired.

 

[Ray70]

Check with Mayi Diesel, I have bought oversized pistons from them before and I think they were in the $75 or less range or a piston with rings.
They can be found on line. They have just about everything for LPW4 / DN4M
.25mm is equivalent to a .010" oversize if they don't list them in mm.

 

[loosegravel]

Thank you Ray!

 

[loosegravel]

Check with Mayi Diesel, I have bought oversized pistons from them before and I think they were in the $75 or less range or a piston with rings.
They can be found on line. They have just about everything for LPW4 / DN4M
.25mm is equivalent to a .010" oversize if they don't list them in mm.

Well, Mayi Diesel couldn’t hook me up with the +.25mm pistons that I need. But DP Equipment in SC had them. They’re shipping them out tomorrow. I should have everything thing I need by next week to get this thing back together. It’s the most that I’ve had to do to get one of these “not running “ GP auction units up and running. I’ll post the progress as it happens. Thanks again for all of the guidance and input!

 

[nextalcupfan]

After this saga any other generator should be "easy" by comparison to get up and running lol.

 

[loosegravel]

After this saga any other generator should be "easy" by comparison to get up and running lol.

I absolutely love the simplicity of these machines! I did a rear crankshaft seal on one last year. Even pulling the engine with the power head as an assembly wasn’t too bad using an overhead Crain. And the two components come apart fairy easy.

 

[G744]

If I had that problem with such high $ pistons (like those for the 3 ltr Benz Diesel), I'd bead blast them, then knurlize the annulus, and reuse them with new rings with honing of the cylinders.

Lots of engines run for years with that treatment, but new parts for the right price is better.

DG

 

[loosegravel]

If I had that problem with such high $ pistons (like those for the 3 ltr Benz Diesel), I'd bead blast them, then knurlize the annulus, and reuse them with new rings with honing of the cylinders.

Lots of engines run for years with that treatment, but new parts for the right price is better.

DG

Now that’s being resourceful! DM Equipment had the pistons for $102 ea. I could rationalize that expense. With the gaskets, the fuel pipe from GMG and these pistons (and my auction cost with fees) I’ll have about $3500 into this unit. (Plus my time of course) I’m hopeful that it will be a solid watt-maker when I’m done!