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Tractor Talk Discussion Forum

John Deere 70 gas Long Post

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super99

01-09-2008 01:33:27




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I will give some background and then ask your opinions. I have a 70 gas that I bought from a dealer 12 or 15 years ago. It had sat in a fence row for years and then they got it running and traded it off and I bought it for $850. It seemed a little weak, but did what I needed it to do. Last few years, it got weaker and weaker till it would hardly start, I tore the head off and pulled pistons to do a valve and ring job, but the top ring grooves are burned out of both pistons. A friend did the head and it is good and ready to go. I took the block out and took to a machine shop to get checked out and it is OK and will bore out to .045 alright. This site has .045 overbore pistons for about $445 so I think I will order a set for it. I plastiguaged the rod bearings before I took them out, and they had .004 play in them. Machinist checked old bearings and said they were standard with .001 wear in them. I should be alright to put standard bearings back into it, shouldn't I? He said the cylinders had .025 wear in them. I think I should take the crankshaft bearings out and check them for wear and possible replacement. Should I have the crank turned down to next size bearing available? Would you take out the camshaft and have it checked? For no more wear than the rods show, I would think the mains should be wore the same, wouldn't they? It is going to need a new flywheel, the old one is cracked all over, but splines on crank are good. I'll be honest, I AM CHEAP!!! This is going to cost more than I planned on, but I want it to run so I can use it, and with the price of corn I guess this is a good time to spend the $$$. I didn't intend to have new everything in this old girl, but I will never be any closer than I am right now, What would you guys do? I am not concerned about being good as or better than new, I just want to run it till I die. I would use it to pull a 4 row planter and spray 65 acres and run auger, maybe take a 3 bottom plow to a plow day if they ever have one close enough that I can drive to it. Should I try to find a higher compression piston, or go with the standard .045 overbore? The rest of the tractor works ok, and with better rear tires, it should run as long as I will be able to use it. Your thoughts and questions please. Chris

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Paul from MI

01-09-2008 19:24:18




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 Re: John Deere 70 gas Long Post in reply to super99, 01-09-2008 01:33:27  
Take Mikes advise about resurfacing your head and th e manifold. I've seen them so bad they drove an old mower sickle into the manifold joint to cut down on the exhaust leak. It'll save you a lot of grief later on.
Paul



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Mike M

01-09-2008 04:18:38




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 Re: John Deere 70 gas Long Post in reply to super99, 01-09-2008 01:33:27  
From what you describe if I had it in my shop for you I would have the block bored and install NEW pistons and rings and likely new rod bushings will be needed for the new piston pins. I would also install NEW rod bearings providing the crank journal were round and not tapered. I would also not pull that crank out. I would put a dial indicator on it and measure how much free play it has in the bearings. I'd have to check the book for specs. These are a round bushing type main and that's how the book says to check them so you don't need to tear it all down. For example- a 720 Diesel main like these has a wear limit of up to .015" if I remember right ? which is way bigger than most any other type of engine has. These type of mains are made to run looser than you might think. So put a dial indicator on it and check against the specs. An I&T manual will give these.

On that cyl. head I would pay close attention to the valve guide to stem wear /clearance and would also resurface the top where the manifold seals against if it is out any since these are so bad about leaking manifolds. And of coarse check the head gasket surface and redo if needed.

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super99

01-09-2008 14:37:32




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 Re: John Deere 70 gas Long Post in reply to Mike M, 01-09-2008 04:18:38  
Thanks for the reply. I have a new set of bushings for the connecting rods and they are at the machine shop already. I don't have any way to measure the crank journals, have to find someone to do it or loan me the tool. Yes, I'm looking at new pistons and the block will have to be bored for them. I think there is a dial indicator in some of my fil's old tools that I got after he passed away, have to dig them out and see if I'm smart enough to use it. A friend teaches auto and ag mech at local college and he took the head and did it for me, said he knurled the guides and did valves and seats. He said that this is the last time for grinding the valves, they will have to be replaced next time. Not sure about manifold surfaces, will have to check into that. I have a I&T manual, thinking about trying to find a shop manual for it. Chris

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