Simplicity 7790 Diesel

I acquired my dad’s 1984 Simplicity 7790 diesel about 5 years ago.  My dad used this tractor for everything for 20yrs and originally bought it from my grandfather back in the 1986.   I have a ton of attachments for this thing including a cool front dump bucket.  Sadly, when I received the tractor it was pretty worn out.  The engine smoked really bad and leaked fuel everywhere.  It was very hard to start and was unusable.  The engine is a Lombardini 10LD400-2 two cylinder 18.5hp diesel engine.  It’s air cooled and is a real work horse.    The only bad things are that it is loud and smells.  I however think it is a neat tractor and like driving it. This is identical to the Allis Chalmers 920 Diesel. All attachments from the Simplicity 7000 series frame tractors fit this tractor and many tractor parts (except engine parts) are interchangeable as well.

I spent the next month rebuilding the engine and getting it back into good working condition.  All the parts had to be imported from Italy which cost a lot and takes several weeks.

I bought all the engine parts from here:

They deal with Lombardini parts and were able to get me everything I needed.  The total cost for the rebuild was $582 which included a gasket set, valves, bearings, and piston ring set.  I did not need pistons or jugs since they were within factory specs.

12-22-14 Update – I guess no longer supplies the Lombardi engine parts.

Kholer now owns Lombardini. Parts might be available with them as well. There are also other parts that fit this engine perfectly. Rings are the same as a Mazda 626 Diesel, part # 2M4210.

The engine leaked everywhere and had grease and dirt all over it.  Debris collects under the cowling and prevents air from cooling cylinder heads.  This will cause the cylinders to overheat and destroy the rings.  After every use you need to blow compressed air through the engine cowlings and blow all the grass and crap out of there.

The first thing I did was remove all the cowlings. Next unbolt the intake and exhaust manifolds and carefully unscrew the injector lines. The engine is a really simple one to work on and has nothing special to worry about other than its a diesel. I plugged the injector ports with a clean rag to prevent dirt from falling in there.

Examining my valves revealed a bad sign.  You can see oil everywhere which came from bad rings and leaky valves.  I needed to order a complete gasket kit, valves, and rings for this engine so far.  I also bought new con rod bearings as well.

I bought this service manual by Clymer for this engine and I am not going to go into great detail on how perform every detail to an engine rebuild.
This book completely covers the Lombardini 10LD400-2 engine and I highly recommend it if you want to get your engine running.  I recommend that you have some mechanical experience before you attempt anything like this.   I cleaned everything and installed new valves and had the valve seats ground.  I also honed the cylinder jugs and installed new rings and bearings.  I took apart the injectors and cleaned them very carefully.  I had to replace all my fuel lines and a small copper gasket that was inside the fuel injector right under the nipple.

The engine is all cleaned up and put back together.  The manifolds are not on there yet but they will be shortly.

The injector pump is a very delicate device.  Do not dismantle this unless you know what you are doing.

The compression on this engine was much higher after the rebuild and my starter could not turn it over.  I took my starter to a local starter repair shop and it had it repaired.   $100 later I had a rebuilt starter.   The tractor now starts easily and even in the winter it has little difficulty starting.   The rebuild was completely worth it.   I was amazed at the power that this little garden tractor has.  I mainly snow blow and rototiller my garden with it and have never had any problems so far.   I’ll try to post some video of it running soon and some pics of the attachments.

7790 Front Dump Bucket

Shown above is the cool dump bucket.

01-26-14 Update

Had some trouble with the trans-axle this week.   Winter is blasting us here in Michigan so I have been using the snow blower attachment every week.   The tractor started to have trouble moving and eventually stopped moving unless I held the hydraulic lift lever open.   Honestly I have never touched the trans axle or changed fluid.  Its probably original since my dad never remembers changing it either.   Anyway,  I ended up changing the fluid and filter.  The fluid was dark red and I only drained about 1.5 quarts out of it.  I used Valvoline ATF type FA transmission oil to refill and added 2.5 quarts.   Wow this thing moves like new now.  I must have slowly lost some response over time because its never been this responsive.   I’ll be changing the fluid every 400 hrs like the manual states now.    I used the replacement simplicity filter which was expensive.  Does anyone have a Napa cross reference for a non brand filter?  Has to be 25 micron.  I also checked the bevel gear box and ended up adding some 80-90 weight oil to that.   I’ll take the old filter up town and see if I can get a Napa replacement.Transaxle Oil Fill

Transaxle Vent Opening

Transaxle Drain Plug

Bevel Gear Box oil fill


Update 01-27-15

I upgraded the snow blower skids to a new heavy duty “Armor Skid” set from Home Depot.  Welded from 1/8 inch and 1/4 inch steel.  My old ones were getting worn away (see below).  One of my skids was less than 50% and my snow blower was getting hard to push because it was dragging on the ground.


The new ones work great and I painted them “Chevy Orange” which is incredibly close to simplicity orange.


Update 03-03-16

I broke the drive shaft today while I was snow blowing.   I knew the couplings were getting old and I hit a small seam in the concrete driveway that jolted the tractor enough to break the coupling.   What a disaster.   The shaft banged all around and twisted itself into a pretzel.  I had to cut it out with a torch.


I also busted a screw off in the engine mount.  Gotta get that out.


Busted up couplings.  The 2nd coupling exploded into a million pieces.  These are kind of expensive and I did not want to buy them.  I made a set myself out of G10 (same material circuit boards are made of).


Getting the new shaft into the tractor is not too bad.  I had to lower one end of the hydraulic cylinder and remove one bracket that was in they way.  It took about 2 hrs to make this repair.

Drive shafts are expensive  ($150+).   I made mine easily with 1″ bar stock.   I ground the original ends off and welded them onto the new shaft.  Total cost for the new shaft was about $5 for the 3ft piece of cold rolled steel from the local steel supplier.    The new drive shaft measures 20 1/4 inch exactly if you want to make one.


New drive shaft shown below.


The couplings can be made or purchased.  Here is the drawing I made for the couplings:

Drawing of Drive Shaft Coupling

I made mine on my CNC mill but you can make them with a drill press and a band saw easily.  Just print the drawing to scale and tape it on your sheet of G10.  Drill the holes and trace the circle on with a pencil.  Cut it out with a band saw or jig saw.



Now I need to get that broken screw out of the engine drive shaft hub.   I drilled it out from the back with a end mill.





Got it out.  I ran a 3/8-16 tap in the hole to clean it out.


Here is the new shaft and the new coupling.  No problems now.


Made some spare drive shafts for later….

Drive Shafts


Update 12-04-16

Getting ready for winter and I found one of my PTO pulleys had a failed bearing.   Stupid thing cost $45 from Simplicity dealer and is an odd size.  I will be looking at Grainger or some other online store to find an equivalent for any other pulleys that fail.

Old Pulley


Don’t forget to press out the center bushing on the old pulley.  It is needed for the new pulley.  I just used a socket and small hammer to tap it out.



All done installed.  Good for winter.


Update 7-30-17

Here are some pics of the Rototiller attachment.   I use it all summer on my garden.






Fluids and Misc Info I use:

Engine Oil :  Rotella T 30Wt   (also T1 works)

Transaxle Oil: Valvoline ATF type FA transmission fluid (2.5 qts)

Gear Box Oil: 80-90Wt Gear Oil

Fuel: Off Road Diesel.   I also have used 100’s of gallons of #2 Heating oil.  No Problems.

Battery:  AC-Delco 34PG  (7 year warranty)   Group 34 Size is the biggest that will fit.

Paint: Dupli-Color “Chevrolet Orange”  part # DE1620.    Can be bought at Advanced Auto.


Simplicity 7790 Operators Manual
Simplicity 7790 Parts Manual
Simplicity 7790 Snow Blower Attachment
Engine Service Manual PDF

Drawing of Drive Shaft Coupling


Engine Service Manual I used:

I also have an electrical schematic for the alternator if interested:

Simplicity 7790 Electrical





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