JD 720 Gas Compression test

[BMAN in Detroit]

I am trying to get an unstuck 720 gas back running. I have removed everything from the tank down to the head block and crankcase sitting in the chassis. I was hoping to find that this engine is tight enough to be reassembled, fired up with fresh oil and able to move under its own power. I just tried to do a compression test and I saw what I believe to be really low numbers. I saw about 70 on the flywheel side cylinder and 80 on the clutch side. I had a note written down from years ago that you should see 120 psi on a cold engine.

First question: Is a compression test a viable diagnostic test with a tractor that is 67 years old? (I used a Mac compression test kit)
Second question: If this engine is 'tight' should I be seeing pressures in the 120-140 range cold?

After doing the compression test, I hooked up compressed air to each cylinder. On the flywheel side, I had a lot of air coming back through the crankcase. On the clutch side I could see water scale dust coming up out of the water jacket of the cylinder head, (this thing has been drained of coolant for a while sitting inside a building).

Third question: With air blowing back into the crankcase, is that a telltale the rings are broken and or shot?
Fourth question: With what I have described, am I at a point the head should come back off and the pistons removed to see how bad they are?

Let me know your thoughts and or insights. Thanks in advance for your input.

 

[chris142]

if its been sitting that long the oil on the walls is gone. that oil helps it the rings seal so your test isnt valid.àir in the cooling system is a problem

 

[BMAN in Detroit]

Hey Chris - Thanks for the reply. I got this engine unstuck a couple weeks ago. I've been shooting kroil on the cylinder walls and baring the engine over at the flywheel every few days to keep it loose. I did shoot some oil in the spark plug holes before trying the test.

 

[620 John]

Not surprised to see 70 and 80. You won't know the true condition until you get it running, WORK IT, and then see what the compression is if it's still a concern to you.

However, it sound like you could have head gasket leak if you're seeing air coming up through the water jacket. I'd try to fill the water jacket with water and see if it leaks into the cylinder. Then you'll know you'll have to pull the head.

 

[chris142]

Hey Chris - Thanks for the reply. I got this engine unstuck a couple weeks ago. I've been shooting kroil on the cylinder walls and baring the engine over at the flywheel every few days to keep it loose. I did shoot some oil in the spark plug holes before trying the test.

if it was stuck the rings may still be stuck in the lands,worn out,damaged etc. If it sat with water in the cylnder the bore is likely ruined

 

[NoDakInMN]

As mentioned, it is not uncommon to have stuck rings after freeing a stuck engine. Put in coolant, fire it up and work it hard before tearing into it. You may be pleasantly surprised to find that it may improve.

 

[JIM MN]

Do as 620John suggested. If no leak into the cylinders, lub cylinder, new spark plugs and see if it will run?? Take it out and see if it will pull. . Give it a chance..

 

[BMAN in Detroit]

Guys I appreciate the input! I'm trying to run down a new head gasket. I've been to the Steiners location, and they are out of the engine gasket sets and they do not offer a single head gasket. Once I get a head gasket, I'll see if I can get this thing back together and fired up. Unfortunately, I do not have any ground to drop a plow or disk in or feed to grind. Is running it down the road in say 4th, 5th, or 6th gear at WOT enough to work that engine?

 

[wore out]

Guys I appreciate the input! I'm trying to run down a new head gasket. I've been to the Steiners location, and they are out of the engine gasket sets and they do not offer a single head gasket. Once I get a head gasket, I'll see if I can get this thing back together and fired up. Unfortunately, I do not have any ground to drop a plow or disk in or feed to grind. Is running it down the road in say 4th, 5th, or 6th gear at WOT enough to work that engine?

Have you checked with a John Deere dealer for.a gasket, MANY 2-cylinder parts are still available there.

 

[BMAN in Detroit]

Hey Wore Out - I'll do that. The JD parts list I have, shows a part number of F 2703 R, which is listed as a pack of 5. I am curious if that would be available. I'll see what JD parts has to say.

 

[wore out]

Hey Wore Out - I'll do that. The JD parts list I have, shows a part number of F 2703 R, which is listed as a pack of 5. I am curious if that would be available. I'll see what JD parts has to say.

Those old gaskets had asbestos in them and were "subbed" to asbestos-free ones under a different part number.

After I posted I checked on the jdparts consumer site and it doesn't show a replacement for F2703R, but sometimes such will show up when the dealer parts dept. looks into it.

Let us know if the dealership is able to cross it.

 

[620 John]

I came up with R520489 as a part number which is subbed from F2703R. JD has 6 available. $176.95 for one head gasket

 

[BMAN in Detroit]

Thanks for the info! I may ask for a parts counter job application to see if there are any discounts for employees! I see that's Steiners has a Valve, Ring, & Cylinder Replacement Gasket set for $220, but that is out of stock. I'll do some more looking and see what else I can find for in-stock inventory along with calling Deere about R520489.

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