MEP-803A 751-42670/5 Pistons?

[devilphrog]

Hi all, name is Nick and I'm based in central FL. I purchased a couple of MEP-803A generators, 1 with 7k on the clock, and one with 9k (reset 2010 and put directly into storage, don't think MCB Albany changed the Hobbs meter out, but it's pristine and I got it for a smoking price, considering what these units are going for now. I don't think anybody caught the rebuild tags in the photo, and was scared of by the Hobbs meter).

The issue I'm having is the 1999 7k hours unit (which I obviously overpaid for, and was in way rougher shape than I thought), was having serious blowby at delivery. Because it was a GP unit, it was run, so I ran it as well. Without a load, the blowby was high enough enough to blow the dipstick out and smokes like a mother.

I initially thought it was a busted ring, and I tore it down, but despite a couple small rust areas on the cylinder walls from sitting (honed out no problem), I've got 2 busted pistons. They are broken between the ring lands, but luckily, they didn't score the cylinder walls.

I think they initially cracked when GP was doing their inspection videos, and I finished the job with 10 minutes of running to try out my home brew load bank of 4 5400w dryer elements, With 2 run in series for a total of 12.5 kw. The unit did handle the load, but was in total mosquito control mode during that quick test, as it was an "eff it" moment as that grey/white smoke was a telling sign of something being amiss.

Something to consider when they hook up the NATO plug and "let 'er rip" in retrospect, rather than dumping oil down the injector holes, soaking, hand cranking, and working up to a dead crank and then start.

In researching the tech pubs, both the DN4M IPC and the Lister Petter IPC, I'm getting differing piston part numbers. The unit has 751-42670/5 pistons installed, and L-P is calling out a 750-41610 in their catalog for latest production.

Like the other forum with pclausen questioning the /5 and being a standard bore, I dropped a set of standard rings in the bores and have .001" clearance, so I'm guessing I'm not oversized here and I still had decent cross hatching in the cylinders before I blasted the ridge with a hone to pull the pistons.

Long story short, does anybody know of a source for a 751-42670/5 pistons or know what else will work? The big auction site has a couple pistons available but I want to make site to get the right part.

I'm not interested in paying $250 a piston as well for a reboxed L-P part?

 

[Light in the Dark]

Welcome to SS. Have you picked up all the manuals for this set, including the manuals for the engine? You will need to go through the -24P for the engine for this info. I say this... because they list a standard piston, and then 4 more that are oversizd.

Knowing this went through a reset... you are going to have to take some measurements, and determine which is proper for the level of reset it received. Also from this, once you determine the right size, you can take the part number, and look around online for this (186-6067-01 is the standard piston).

Good luck and let us know.

 

[devilphrog]

I sure have downloaded all of the manuals and studied them at length. To clarify; the unit that has the broken pistons doesn't appear to be a reset unit, with all original hours. I need to get some new batteries for my outside micrometer to confirm, but a standard sized set of rings dropped in a few different points within the cylinder cleared between .001-.0015".

Again, thanks for the welcome to the forum, and I am blown away by the amount of knowledge and assistance of everybody on this site.

 

[Daybreak]

Howdy,
Welcome to the forum.
I also would like to say thank you for the information in your post. It makes it easier for everyone to make a informed decision about what you need. I see you have read the forums. By what you have stated, I can see you have all the manuals, and have read them. You have researched your options and are knowledgeable about engines. +++

It has been said, that sometimes during a reset, that they skipped replacing the hour meter. Probably a really good buy when others thought it was a end of life unit.

I see your unit has the part 751-42670/5 . There are so many working variations. Yes Onan side, the Lister-Petter side, and other aftermarket part numbers. There is nothing like someone else's prior experience with stuff like this. Pete Clausen helped so much with his detailed write-up.

I am sure you will get some answers here soon enough. My post doesn't assist here, since I have not needed to do a piston yet.

Per standard, look over these threads.
MEP-802/803 Filters and Parts
Military MEP generator reset information

 

[jamawieb]

I had the same experience from GL. They tried to crank the motor over when it was stuck and busted the piston. I was lucky to have a parts 802a, that I pulled the piston out of to replace the broken one. I really think they sprayed either in mine to try to start it.

 

[devilphrog]

mid-day update. I think I may be cracking a bit of the code on the 751 series part numbers. From some additional research, it appears that the 751 series L-P part numbers cross over into Lister Petter built generators, as well as FG Wilson generators. It looks like I'm going to go at risk and purchase a couple FG Wilson part numbered piston assemblies (found them on the bay for around 70 bucks a piece, rings included). We'll see what shows up, but this may be an additional avenue to source engine parts at a more reasonable price. The part number I'm going to attempt to install is 941-674, which is listed as a LPW 2/3/4 Standard Piston

 

[csheath]

I sure have downloaded all of the manuals and studied them at length. To clarify; the unit that has the broken pistons doesn't appear to be a reset unit, with all original hours. I need to get some new batteries for my outside micrometer to confirm, but a standard sized set of rings dropped in a few different points within the cylinder cleared between .001-.0015".

Again, thanks for the welcome to the forum, and I am blown away by the amount of knowledge and assistance of everybody on this site.

What measurement are you talking about here? If you are referring to end gap then that is too tight. End gaps should be .010" to .020".

 

[devilphrog]

CSHeath, you're absolutely right. I misplaced my decimals absent mindedly. My ring end gaps are within the .010-.015" range.

 

[Guyfang]

welcome to SS.

What is the condition of the rest of the set? Is it well taken care of. It might be a good idea to also look at that. If it takes a while to get your pistons and rings, you can work off other small problems. There are about a million other things to also check, and you have a good start when you have the top and doors off.

 

[devilphrog]

It's in OK, serviceable condition as a whole, dirty and dust, especially up behind S8.

The unit must have had something dropped on top of it, so I had to straighten out the top panel a bit and work to straighten out the hinge for the control panel cover, but all was working before I tore it down.

Ultimately, I'm looking at selling this set to recoup my cost of purchasing it, transporting it, and fixing it up + load testing it as I really only intended on buying one set for hurricane use and actually won two auctions.

I honestly thought I was only going to win one set, especially with how much these things are going for at auction these days and my offer actually ended up being the winner.

 

[devilphrog]

Hey all, just an update. I finally finished up reassembling the unit last night/this morning and it fired on the first attempt after a good fuel system bleed.

I ended up replacing pistons and rings, rod bearings, 2 pushrods, fuel pump return lines, as well as installing the quad fuse and MOV between terminals 7 and 8 of the voltage regulator. Also in process of replacing all of the nuts with stainless rivnuts for future maintainability.

The FG Wilson style (OEM Lister Petter marked) pistons work great, no abnormal vibrations and EGT's are all within 5-10 degrees of each other. Consider these as an option if you need to replace at 1/4 the cost of the military part numbers.

I did a progressive break in run for about 3 hours. It can take up to 13k for short ribs, but it's dropping off line (overload fault) after a long periods. Same issues as my 2nd unit that I was talking about in another thread.

I'll tear into S8 tomorrow and rework all of the connections and clean the contacts out with contact cleaner. I'm strongly confident that a good cleaning will resolve the fault as it's a strong runner.

I need to clean up the inside of the unit, is pressure washing okay? There's quite a bit of Afghanistan looking moon dust in the unit.

I'm going to sell the unit to a buddy of mine, so I'm making sure it's going to be right as he's got a newborn due in the next couple of weeks. I had to say it put a big smile on my face to show him the capabilities of these units by showing an instantaneous 13.5kw load and the unit not bogging down and only putting out a puff of smoke.

I can't thank you guys and girls on this forum enough for your help, expertise, and knowledge!

 

[Guyfang]

Pressure wash the engine and engine compartment. But the control cube and electrical compartment, well, got to be careful there. I flat out would not do it. Compressed air, good dust broom, or small wisk broom, or a real good small paint brush, will do wonders. And I would do it twice.

Some devices tend to soak up water. The K1 relay is the worst. The thing looks like it could operate under water, but often fills up with water. The main AC is good to pressure wash, just let it dry a few days.

A little shock loading is OK, but it's not a good thing to do a lot of. In real life, it's rare that you dump 13 KW on a set all at once.

 

[jamawieb]

Hey all, just an update. I finally finished up reassembling the unit last night/this morning and it fired on the first attempt after a good fuel system bleed.

I ended up replacing pistons and rings, rod bearings, 2 pushrods, fuel pump return lines, as well as installing the quad fuse and MOV between terminals 7 and 8 of the voltage regulator. Also in process of replacing all of the nuts with stainless rivnuts for future maintainability.

The FG Wilson style (OEM Lister Petter marked) pistons work great, no abnormal vibrations and EGT's are all within 5-10 degrees of each other. Consider these as an option if you need to replace at 1/4 the cost of the military part numbers.

I did a progressive break in run for about 3 hours. It can take up to 13k for short ribs, but it's dropping off line (overload fault) after a long periods. Same issues as my 2nd unit that I was talking about in another thread.

I'll tear into S8 tomorrow and rework all of the connections and clean the contacts out with contact cleaner. I'm strongly confident that a good cleaning will resolve the fault as it's a strong runner.

I need to clean up the inside of the unit, is pressure washing okay? There's quite a bit of Afghanistan looking moon dust in the unit.

I'm going to sell the unit to a buddy of mine, so I'm making sure it's going to be right as he's got a newborn due in the next couple of weeks. I had to say it put a big smile on my face to show him the capabilities of these units by showing an instantaneous 13.5kw load and the unit not bogging down and only putting out a puff of smoke.

I can't thank you guys and girls on this forum enough for your help, expertise, and knowledge!

What part number did you use for the pistons?

Like Guy said, power wash everything except the control cubicle and directly into the back of the generator head. If it has a lot of sand buildup, then be ready to wash it twice. I can power wash all the sand away, then start it up and then its back, lol.

 

[devilphrog]

I ended up finding part number "FG Wilson Piston ASSY STD 941-674 Lister 751-40545" on the big auction site. Still up there with more than 10 available.

The seller (a large generator company out of Miami) has a make offer function and definitely is willing to wheel and deal. The only catch was you need to call them after you make a deal to pay them directly and arrange shipping.

I don't plan on shock loading the unit, it was a one time demonstration, if you will.

Thanks on the cleaning tips, just looking to get the crud cleaned up a bit.