Hi All, i have the same KGP51AT-40UF , i guess my clutch disc is on its way out. Does someone has a manual for this model? I need to change the clutch, the wheels..and grease all the machine :)

I agree with duodeluxe on the tires. I just bought a KGP51AT-40. I never seen solid tires 30" o.d. Like my pneumatics. Im trying to learn about mine as well, I have clutch disengagement issues and it has the fluid coupler but no brakes just yet. Other than a little e brake. If you guys have luck with parts and or diagrams I'd appreciate it. So could the k be for king? They called these Yale yard king gas powered trucks. Mines gas, 6A-215 Chrysler flat 6, fluid (and dry) clutch, 2 fwd 2 rev gears. Cool old truck but I have clutch and brakes to work on. Just rebuilt the b&b carb runs great.

P is definitely for pneumatic. Back when this truck was built almost all forklift trucks, especially pneumatics, were built as gas powered trucks. It would also be extremely difficult to convert a cushion tire truck to pneumatics due to the clearance in the wheel wells.
Trucks this old were equipped with generators, not alternators.
You might want to post a picture or two of your truck.

I got some work done on the old beast yesterday, and, I found out something interesting.

Previously, I'd noticed that there were some long cut marks in the top of the lift cover in the back. They were parallel marks where it looked like someone had cut something off with a torch. I also noticed a round hole in the top deck that didn't appear to be for any good reason.

I started making a list of all the things that need some attention on the forklift and one of them was a carb rebuild. I went to look at the carb to find a number....and there it was! Right above the carb, before the intake manifold was a propane mixer!!

I'm pretty sure that the GP in the Model KGP51AT is for Gas AND Propane. It was, at one time, a dual powered machine. Someone has stripped off the propane system and left the little mixer (or whatever you call it) between the carb and the intake manifold.

This truck originally had hard rubber tires all around. The owner before the fella I bought the truck from had it converted on the front to pneumatic tires. It still has the old (probably original) small hard rubber tires on the back. So, I don't think that in this old model number system that the P was for Pneumatic.

Also, found out yesterday that the alternator is not charging the battery - so there's another issue to troubleshoot.

I took the beast down to the shed where it will be parked and out of service for a while. I'm going to tear all the top and side panels off of it so that I can get at everything to work on it.At this point it needs quite a few repairs.

So, to update my Model number codes:
K -
G - Gas
P - Propane
51 - Base model number
A -
T -
-
40 - 4,000 lbs Lift Capacity
U -
F - Fluid Coupling

Thanks for everyone's insight and assistance.

I think I figured out the K thing. Back then, Yale also had an electric forklift with the model number K51 AT. So they just model numbered their gas powered model KG 51. I'm not so sure the AT means anything about the mast or transmission. The P is for pneumatic tired. It was originally a 6 volt system so someone converted it. I bet the brakes may be the same as a G51P040 and you can get parts. A customer of mine has a G51P040 with the Chrysler flat head 6, 4 speed stick shift, and the fluid coupler. We freshened up the motor with new rings and a valve job. Also converted it to propane. I love that thing. So rugged, so powerful, big tires, and a pleasure to use with the fluid coupler and 2 speeds in each direction.

Thank you for all the info that you have all provided. I really appreciate it.

Well, I picked up the truck yesterday and got it to my shop. Now that I've had time to drive it a little and check it out in the daylight - here's what I know.

It IS old. I'd say that it is 1950's, as Duodeluxe says. But, it will be perfect for what I need it to do. I will probably only use this forklift a dozen times a year to load or unload some piece of equipment that I buy or sell. (Man - I could have really used it to unload an old 15" Sheldon lathe that I bought last month!!! I had to rig that 2600 lbs beast off the trailer with a 2 ton engine hoist and bunch of blocking - ugh!)

I am the 3rd owner after it was owned by an auto mechanic (who rebuilt the original Chrysler straight 6 engine in it). The auto mechanic and the 2 owners previous to me were all good friends. So, I'm pretty sure what they're telling me is legit. In fact, the 2 owners before me were both there to help load it and give me a rundown of the history.

It runs pretty decent. It's a little cold blooded - so a lot of choke and cranking to get it started from cold. But, once it warms up, it runs pretty well. I'm going to look into putting in an electronic ignition system, I think that might help run a lot better.

It has sat outside at some point for a while. The seat is shot and there is grass, spider webs and all manner of crap inside and underneath the truck everywhere. I will be giving it a good cleaning and I might give it a rattle can paint job.

It DOES have the fluid coupling - although, when using it, you have to step on the brake pretty hard. And, then, that bogs down the engine a little and sometimes the idle won't pick up and it quits. I may have a little work to do there.

It only has a two stage mast. The forks go up to the top of the inner stage and then the inner stage goes to the top of the fixed outer stage. But, it will go up plenty high for me.

The pneumatic tires on the front were put on by the 2nd owner prior to me. They had a local tire shop swap the cushion tires off and put on 12 ply industrial pneumatic tires. They are in really good shape with lots of tread left. The rear tires are probably the original cushion tires. They're getting pretty small - I may need to replace those. But, I ran it around on my gravel driveway that was recently covered in fresh, new roadbase - and it worked fine. Didn't even get close to being stuck - but I didn't have a load on it either.

It has a "4 gear" manual transmission - 2 forward gears and 2 reverse gears. The clutch seems to work fine - but I'd like to see, or find out, how much is life is left on the clutch disc. I am assuming, and hoping, that the mechanic that overhauled it put a new disc on in when he had the engine out. If so, then it should not be a problem for me for a long time.

The brakes are OK, but the parking brake doesn't work. It appears that the cable on the left wheel is frozen and the one of the right is disconnected. I will tackle that at some point. Probably easy enough to fix most everything involved there, but will be H3LL (if not impossible) to find replacement flex cables. Maybe I'll get lucky and find out those cables were also on some 3/4 ton 1953 truck! Who knows?

It has a 12 Vdc electrical system on it. Although, the battery was completely shot. We were able jump it to get it running to load it on the trailer. But, when I got it to the shop, I had to rob the battery out of my diesel generator to put in the lift truck for the time being. I'll get a new battery this week.

So, based on what I know now, here's my model number decoding list. I've only filled in what I'm pretty sure is correct:

K - ??
G - Gas (it has IND30-1037 engine)
P - ??
51 - Base Model #
A - ??
T - ??
-
40 - 4K lbs Lift Capacity
U - ??
F - Fluid Coupling

Overall, I'm pretty happy with this machine. It is big enough to do what I need it to do, but small enough to get in my shop. Sure, it needs some TLC, but for the price I got it for, it was a decent deal. I think it will serve me very well.

Thanks for your help, everyone. I'll update again as I find out more about this lift truck.

It is really old, probably made in the 50's. Yale changed their model designations later. In this instance, I don't know what the U stands for but is is not for a Mazda engine.
A fluid coupling allows the truck to come to a complete stop without pushing in the clutch pedal. The only time you need to depress the clutch pedal is to shift from forward to reverse or first to second.

That must be real old. I think they did away with the K in the early-mid 60's. I know in 1968 the P would be after 51, like G51P040. "A" should mean automatic transmission. Mazda engines weren't in 51 models. They showed up in the GC and GP models in 1972ish.

Sorry, I missed the serial number in the original post.

The codes for the first two letters is confusing. The K is common for this family of trucks, the G generally does indicate a gas engine. The U designation does indicate that this truck shipped with a Mazda engine, it may have been changed out in the past. I have also seen trucks get the seat deck switched so the data plate is incorrect. Do you have a serial number?

Hey, thanks Brewski!

Here's my updated list then:

K - Gas (it has IND30-1037 engine)
G - ??
P - Pneumatic Tires
51 - Base Model or Style ??
A - Single Speed
T - Three stage mast (It does)
-
40 - 4K lbs Lift Capacity
U - ??
F - Fluid Coupling

I wonder what the G means then, if K is for Gasoline?
Also, it definitely has the Chrysler IND30 engine - so I wonder about the U.

I can't remember on the clutch pedal - I don't seem to remember one; only the gas pedal on the right and shift levers. I'm picking it up tomorrow, so I will post back on it.

Thanks for your help.
R/Todd

The K indicates it is gasoline. The T indicates a three stage mast. The U indicates a Mazda engine. The F indicates a fluid coupling. A fluid coupling allows a standard transmission to come to a stop without depressing the clutch. Does your truck have a clutch pedal?