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John Deere Tractors Discussion Forum
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diesel pistons

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joelrand

01-19-2007 07:28:05




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Is it possible to hone cylinders on 720D and just reuse same pistons with new rings? I was told anytime you take pistons out you"ll need to bore to .45 and buy new pistons, rings for $600-1000. The tractor was running before I tore into it and I"m just wanting to clean it up and make it useable for parades and an occasional plow day.




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jdemaris

01-19-2007 09:25:52




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 Re: diesel pistons in reply to joelrand, 01-19-2007 07:28:05  
Top ring grooves in the pistons is the biggest worry. If they're sloppy the rings will evenutally shatter - new or used. It used to be standard procedure to "rebuild" pistons instead of replacing them when the original bore size was used. Years ago we used to knurl the piston skirts and recut and shim the ring-grooves.
Hastings still sells the repair shims cheap - for the piston grooves. You just need to recut the worn grooves, straighten them out, and resize for the repair shims. Local machine shop did my last set for $8 per piston.

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Mike M

01-19-2007 10:25:47




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 Re: diesel pistons in reply to jdemaris, 01-19-2007 09:25:52  
What if he has a late style with the keystone type top rings ? How do you measure side wear in these ? or can't you ?



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jdemaris

01-19-2007 12:17:25




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 Re: diesel pistons in reply to Mike M, 01-19-2007 10:25:47  
I don't know. It's never been an issue that I had to worry about too much. I've never come across Keystones in old machinery - where pistons are more apt to be hard to find - or extremely expensive. By nature, Keystone rings are usually extremely durable - often Chrome-Moly, and the piston grooves are usually armored with a steel liner. With most that I ever pulled apart with Keystones - I just eyeball the triangle shaped groove for any sign of wear. Not very scientific, I guess. But - Keystone rings have always been known NOT to seal as well as standard flat-sided rings. That because of the excessive side-clearance created as the piston moves and tilts a bit while making its strokes. I suspect that is why they tend to be built with better materials than the flat-sided rings. The reason why Keystones are often preferred in diesels or two-stroke-cycle engines is they are self-cleaning - and tend not to get plugged up like flat-sided rings often do.
Guess I really didn't answer your question. Seems to ought to be a "no or no-go" gauge for them somewhere - like you use in a rifle-receiver to check for excessive clearance.

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Mike M

01-19-2007 12:30:23




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 Re: diesel pistons in reply to jdemaris, 01-19-2007 12:17:25  
I guess I was under the impression from JD manuals that these late 2 cyl. diesels went to the keystone type top ring so they WOULD seal better ? is this not the case in the real world ?

I never really had to much question about the ones I've worked on as "being shot" was usually obvious. LOL



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jdemaris

01-19-2007 14:27:30




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 Re: diesel pistons in reply to Mike M, 01-19-2007 12:30:23  
In theory - the Keystone seals worse than a square-cut ring - when both styles are in good working condition. However, since Keystone rings #1 clean themselves, and #2 tend to be much more durable than the average square-cut ring - over the long run they do more sealing. I have pulled apart countless Deere diesels with broken square rings - but I've never seen a broken Keystone except for some rare cases where all was broken - piston, ring, etc. Cold weather ether starting really does a number on square rings.
Also - I don't recall every working on a diesel that had more than one Keystone - i.e. the top ring. So, any blowby that gets past it gets caught by the next square-sided ring. Since the Keystone ring was invented, there have been many patented "improvements" in attempts to make the Keystone seal better. I don't know if any have ever been used. Ever try to break one of those Keystones like Deere or GM diesels use? It's pretty hard to do, that stuff is like kryptonite.

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Mike M

01-19-2007 09:01:07




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 Re: diesel pistons in reply to joelrand, 01-19-2007 07:28:05  
What was the main reason for the tear down ? Burning oil ,blow by ? or did you just tear it apart for something to do ? It depends on what was going on as to what to really look out for. But if everthing checks out good there should be no reason you can't reuse them. Example--my cousins 720 was burning oil really bad ! torn it down expecting to see the worst engine ever,but everything looked and measured up good. Was a head scratcher for a little while. Then we took the old oil rings off the pistons and stuck them in the cylinders. WOW ! huge end gap. oil problem found. his had .090 oversized pistons and it looked like standard sized oil rings. No wonder it burned oil. His cyl. bores were good hardly any wear so with new proper size rings it was like new again.

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Bob

01-19-2007 07:31:59




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 Re: diesel pistons in reply to joelrand, 01-19-2007 07:28:05  
If cylinder wear is within limits, OF COURSE you could break the glaze, and install new rings and the old pistons. Check for ring groove wear, too, before re-using the pistons.



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F-I-T

01-19-2007 07:58:30




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 Re: diesel pistons in reply to Bob, 01-19-2007 07:31:59  
Check the wrist pin bushings, too. You might eliminate some noise if those are tightened up.

Frank



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