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620 compression?

D

dflippin08

Member
Good evening everyone, I want to get some opinions on when I should overhaul my 620. I just got finished checking my compression. It is 108psi on both cylinders. How low should I let it get before I rebuild it? I have checked my main bearing clearance. It is .004 on the left main and .005 on the right.
 
Let it be. It will live longer than you will as-is. Btw- did you have the throttle set wide open?? If not it's an invalid test. It probably only had around 120 brand new. Also it should be hot when you check it.
 
My concern is blow by. On the flywheel side it is leaking between the the slinger and slinger housing. I have .004 of space between the slinger and slinger housing and it is still leaking pretty good.
 
You need to put a new seal in the oil slinger and check the thrust washer on the crank. Just did my JD 630 about 6 months ago. It was really leaking and now it doesn't leak a drop. My brother's 620 leaks a lot also so it is a common area to leak oil. Seal is cheap but the thrust washer is about $60 at Mother Deere. There are plenty of archives to read on how to change the seal. Best to read them as there are some things you will want to do when you tear it down and especially when you reassemble,such as marking the crank and flywheel, getting the oil slinger tab inserted in flywheel, etc.
 
It's not leaking from the seal. It's leaking between the slinger and the housing. The thrust washer looks like new.
 
Not sure what the compression was on them but I overhauled a couple of 620's.
One sucked in a carb. screw and hammered between the piston and the head.
Other not sure if it knocked now or owner just noticed loss of power ? I think it had a ring break and work up top.
Anyhow the aluminum pistons in these have thin top rings and break and the pieces can work their way up to the top.
The bore wear size was past where they should of been bored so these engine had ran along time past where they should of though.
Do a leak down test and see if the valves are leaking. If so the head needs to come off to do the valves. And while there it takes next to nothing more to pop out the pistons for a look and measurement.
 
Did you try to put some oil in through the spark plug holes and retest? This could tell you if you have ring leakage or valve leakage. The factory spec is 135 lbs. but that is taken at rated speed, such as you would get by belting up to the tractor. At cranking speed it would be lower. Mike
 
Are you sure the drain back hole is open?? Also mske sure the crankcase vent system is working & not plugged. I have an A with 20psi compression that does not leak a drop. It has gobs of blow by
 
The drain is clean. I can feel air blowing out of the vent tube pretty good. I haven't put any oil in the cylinders because I figured the results would end up being a rebuild either way. I'm scared that if I go in it will turn into a can o warms. I really don't have the money to do it all at the moment. Just trying to stop some leaks. I didn't think that the compression is low enough for it to be blow by causing it to leak. And both the cylinders are at the same compression so they are worn evenly. The oil pressure is under 15lbs at wot, so it's not that. I'm using deere 10w30 oil. Pulling the first red housing off Saturday. It's putting engine oil in the transmission. I read a lot of post on this. There is one that's about having a issue just like mine, but the person never post their findings. He had 120psi compression. ????????
 
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