John Deere 720 Pony Motor AUTOPSY - Broken Piston Rods
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- Опубликовано: 8 фев 2024
- Disassembly of a Pony Motor from a John Deere 720 Diesel looking for forensic evidence as to why this engine failed. Two thrown rods, broken push rods, and a lot of rodknock. #johndeere #tractor #broken #engine #farming
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That would have to be a tough life, start cold, run 6,000 rpm and shut down. They did take a fair amount of upkeep to stay running. We had a 1955 '70' with a pony motor and a 1959 '730' electric start. 70 would always start, roll it over and hit on 2nd or 3rd compression! Amazing. Thanks for showing this lttle guy!
Not too mention crank that big diesel motor over too! We have a 70 D too. It's a very reliable tractor.
Very educational. I have never 2:40 seen a pony motor stripped down until now. I always wondered what makes them go. You are quite correct, I've never seen this on You Tube before.
Thanks!
Never new pony engine had 4 cylinder interesting. Thanks for showing us the ends and out .
I'm not sure why they decided on 4 little cylinders, but it is a compact little guy. It fits under an 830 (or 80 or 820) hood a lot easier than a 70 series.
I've rebuilt two of the V4s. Yes they're pricey, though there is a supplier that makes it a little better. The little L bracket is part of the governor/carb linkage, it holds a spring to adjust the high PRM speed. I didn't see the spring. The beat up square "shim" is the lockplate for the flywheel. There are a few variations of the pony motors, yours is an earlier model, with the smaller pushrods. The 830 would have the latest model with heavier pushrods and lots of other changes. Some stuff interchanges, but not EVERYTHING. The transmisson is totally different between the 70 and 80 series.
I'm glad to hear they beefed up some things between this and the later series. Because these pushrods are very flimsy. And I'm glad you told me about the difference in transmissions. I'm sure I would have have figured it out... But it's nice knowing before I started disassembling things! 😂
How pricey? I have one that needs a freshening up. Smokes and doesn't seem like it has the power to really spin the 730 motor.
@@nwyoda8792 I haven't shopped in quite a few years. It's easy to dump 2K. Some parts are actually cheaper and more available now, even oversize pistons I believe
@@nwyoda8792 I haven't used them but have heard good things about H & J Machining
Interesting and educational! Keep up the good work!
Thanks for always watching!
Thanks for the dissection of the pony motor. Nice to see what’s in there. I had a pony motor that leaked oil from bottom. I presume the round gasket at bottom of motor or it was the drain gasket into big motor. Nice video
I never really would have thought to look on the bottom side for a removable cover!
A Cat had their own dipstick. @@jonelsonster
Very interesting, lots of good stuff there. Thanks for Sharing. 😊
You bet!
Excellent video :) amazing how much damage that Pony has and hopefully be saved to on rebuilt up to working again like new!
This will be a "parts motor" for me.
First time I've seen the inside of a 4 cylinder pony motor, most interesting.
I bought it was pretty cool to see how it works too!
Very interesting, keep showing the rebuild process and where you are getting the parts
This is my spare motor... So I probably won't be rebuilding this unless my 830-I's pony engine is bad. My main goal here was to familiarize myself (and other viewers) with the pony motor.
I started fooling with Cats in my late teens and have never seen the inside of a pony motor but sure cranked a few. VERY interesting and the crank looks to be in good shape with maybe a little polishing. Looks like maybe they have sleeves but maybe no inserts. Lots to learn. Sure is interesting. Thanks for the video. My ears are up even more!
I'd be interested to know if cat used aluminum rods with no bearings... I'm guessing they may have been a little more heavy duty? I kinda wanna get my hands on one of those now! 😀
I'd say steel but what I've seen a Cat was slower turning than a Deere starter. The opposed piston jobs, D2's and 4's whizzed but the side by side twins like for the 7's, 8's and 9's were slower. Maybe you can find one. It'll be interesting for sure!@@jonelsonster
My 720D pony motor had a Wico magneto, two coils, Harley style. Mine was actually converted to Harley 6v coils as the Wico ones were marginal. Early Briggs were same bore and stroke with same aluminum directly on the crankshaft. I would not be surprised if the rings were also same width tho I never took mine down that far. It should have an ignition disable with low oil pressure. Use a good plug as when pony is installed one plug is not accessible. The best cold starting tractor at the time. Have fun
A lot of good information here! Thanks!
@@jonelsonster He has a good tip about oil pressure shut off on the ignition. I've used one for years, it avoids burning up expensive ignition coils
There was someone advertising in Green Magazine selling rebuilt ponies for about $1900 with your core.
Very good info to keep in my back pocket!
These engines were a bare aluminum rod on the steel crank, it worked, but if anything went wrong with the oil, bad things happened, very quickly. If the waterpump seal went out, the (quart) of engine oil got contaminated with antifreeze and turned into glue instead of slippery stuff and the engine would die quick. Unfortunately no over/undersizes from factory
I guess the lesson to learn here is check the condition of the pony oil before every start. Based off the wear on the rods, this thing either had contaminated oil or was starved of oil. I found the passageways from the valve train in the heads back down to the crankcase were plugged... So any oil that was pumped up to the top of the engine probably didn't drain back down to the bottom in time to replenish the sump... And the rods paid the consequence.
@@jonelsonster the plugged head passages are one reason for their reputation as mosquito foggers. Then the oil just went down the valves and out the exhaust!
Very interesting tear down. I have alway wondered why JD used 4 cylinders instead of 2 cylinders.
Not sure. But I'm guessing the 2 cylinder pony motors like the R had wouldn't fit under the hood of the 70, 720, or 730. So they designed this one, which was more compact? That's just a guess.
The V4s would fit under the hood of the 70 diesel, and had more power
Can you tell me what size and thread the hole is on the side of the block for the oil pressure line? It looks like you have a pipe plug in it. Mine had a brass fitting there with an oil pressure line that ran up to the automatic fuel shutoff. It broke off when trying to get it out and I’m trying to tap the hole without pulling the pony motor out. Thank you!
1/8 pipe fitting. The early ones just had a pipe plug. I have mine plumbed to an oil pressure gauge AND an oil pressure switch to shut off the ignition power when there's no oil pressure
@@weirddeere thank you! That’s extremely helpful! I am in the process of putting in an oil pressure gauge for it.
@@flinthillacres3009 Books says 25PSI just so you know
@@weirddeere yeah mine is at about 20. I guess I will have to pull the cover and see about turning it up a little bit.
@@flinthillacres3009 20 is fine, I wouldn't pull the cover for that. It's a bit more work than it looks like. Unbolting the engine and lifting up the front of it and lots of other things
I'm kinda new to these Antique JDs. Trying to find more in on connecting hydraulics to my 46 A Slant Dash Power Trol. Can't find info any where. Could you assist with a video?
Not sure what you need to see. Are you wanting to move the rocker arms off the 3pt or connect hydraulic lines and run an external cylinder?, We've always been lucky enough to just put 10W-40 motor oil into the power-trol reservoir, turn on the PTO, move the lever and it works. They seem to be pretty reliable. But we do have a 1944 slant dash with the older pedal style power trol and we've never done anything with it... Maybe I could see if it works.
@@jonelsonster I have 46 Slant Dash A, Want to connect my front loader. It has one hose. Pretty sure it's a New Idea Horn 50 Front Loader. Not sure how to connect it.
Wish you had mentioned you were not going to rebuild
No, I'm not rebuilding this one... Unless my other pony is bad. This was strictly for familiarization and evaluation purposes. I think I specifically started this at about the time mark 3:20 of this video. Next week's video I'm reassembling this engine.
probably already pointed out but the 4 steel balls are your governor, push out against the cup at speed, don't loose your balls!
Yes, that's what I figured. It's a pretty simple but seemingly effective design.
looks like it was out of oil,
There was definitely an issue with lubrication at the rods
By the looks of it, don't look like the oil was ever changed
Your probably right. A lot of sludge and dirt
I wonder if this poor engine was run without oil. I have never seen slop like that!
Often people shut them off without letting them run the carburetor out of gas. The gas ended up in the cylinders washing the oil out. They then wore through vibration of the big diesel.
The maintenance the previous owner performed definitely could have been better!