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 11-25-2001, 20:33 Post: 33410
critterpainter



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 870 with a knock and blowby

A couple of months ago I posted about a JD 870 fwd that I was thinking about buying in spite of 4K hours on the clock, a miss and white smoke at start-up and pretty bad blow-by. (Former owner had overheated engine) I detrmined that #2 cyl was the problem, and then used the tractor for about 10 hours with the problems. I found as it warmed up the knock almost went away (except under heavy load) and that it took about 1 minute for the exhaust to clear from cold...Blow-by remained constant.
This weekend I tore into the engine and found that 40% of the top two rings on #2 has siezed to the piston and the piston had a lot of wear on the skirts and near the top 90 degrees from the wrist pin. There was evidence of blow-by on the cyl walls. On all 3 cyl's the cross-hatching is still visible including where the top ring comes to the top of its stroke. The rod bearing looks almost new.
I think this is going to be a cheap repair. (when cold the engine started instantly without pre-heat)
Bill






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 11-26-2001, 05:41 Post: 33418
TomG

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 870 with a knock and blowby

Bill: thanks for getting back, I was curious about the problem. Sounds like you know what you're doing with tear-downs so maybe I'll be commenting for other people. I guess that a new piston is required. I'd still plastigauge the rod and main bearing clearances and check upper rod end for diameter and out-of-round. I'd probably consider a new rod just on principal. l If it's sleeve type cylinder and it's not going to be bored or replaced, I'd get the cylinder specs and measure the bore with a cylinder gauge. Actually, I used to use OD Mics and telescoping ID gauges, which is less accurate and sort of a pain. Considering the problem, there's a good chance the cylinder is out of spec.

You might comment if there was a ridge on the bore at the top of the upper-ring travel and if so how big. I've never tore-down a diesel, but gas normally worn gas engines always have a ridge that has to be reamed as part of a rebuild. The ridge size is a rough indication of the miles on a gas engine. Probably the same is true for diesels, but I couldn't say how much would be normal for 4000 hours. If there were no appreciable ridge, then I'd guess the engine had some rebuild work done. I'm not familiar with the cross-hatching you mention, but anytime I hear cross-hatching, I think of knurling and a rebuild on the cheap. Of course, pistons rather than cylinders were knurled--sometimes valve stems too.

What I'm thinking is wondering why the engine over-heated originally. I guess there would be a small chance that something wrong with previous engine work could have caused the original problem. If that's possible, I'd want to think it through before putting the engine back together. For example, since the compression rings seized only on #2, there's the possibility of insufficient oil to the upper cylinder. There’s a chance of obstructed or insufficient valve-guide clearance. The problem would be worse in a hot engine. Such a thing could happen could happen if somebody put an over-sized valve in a worn guide without reaming or replacing the guide.






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 11-26-2001, 19:48 Post: 33447
critterpainter



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 870 with a knock and blowby

Tom:
When I pulled out the offending (offended?) piston there was NO ridge. The cross-hatching on the cyl that I was refering to is the
pattern laid down by the factory when the block was machined. (it is not a sleeve engine unfortunatly). Without measuring I feel that there is no appreciable wear or taper. there was no sign that the engine had been opened up before I came along. I do not plan to do anything more than just replace the piston, as I feel that any slight wear in the cyl will be compensated by the new rings on the one cyl and the slight wear on the two cyl's I am not touching should just about equal the one I am fixing. The engine REALLY looks new and clean on the inside....much better shape than i would have thought for 4000 hours. I hope that after fixing it will have another 6-8 thousand hours in it....It looks that nice.
By the way it overheated because the screen was missing infront of the radiator ($3.50) and the radiator filled with debris and dust (not to mention daytime temps of 105 degrees.
Bill






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 11-27-2001, 05:30 Post: 33454
TomG

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 870 with a knock and blowby

That's a sad story about the over-heating. I've heard that a rad can clog in a very short time during some ag operations, screen or not. I don't have that type of work to do, and I still blow out the screen and rad regularly.

I had a thought about the lack of ridge. Perhaps lower rpm diesels don't make ridges like gas engines, and if you've torn-down diesels you might know what is normal better than I. A pronounced ridge is normal for the 70,000+ mile gas engines I've been into. Possibly the question of why only the #2 cylinder was affected could be expected. Some engine designs produce cylinders that run hotter or cooler than the rest. Guess you'd have to know what is normal for that particular engine. Maybe I've explained away all my questions, but I do try to reason through what causes things to happen. My attitude if that if I understand the problem, then I might save myself a second tear-down.

To me it sounds like your approach is a good one. The engine looks good, and there's an explanation for the seized rings, so just fixing the obvious problem has a good chance of being the least costly fix in term both of time and money. If the reasoning is correct, doing more work wouldn't make the engine last longer either. The only other thing is that I wonder about is the knocking. I guess this isn't the typical diesel knock at startup. I don't think I can reason through what about seized rings would produce a knocking, but maybe so. I'd probably fret that something else was going on, and probably would end up spending a bunch of time 'micing' parts.

Anyway, this sounds like a pretty good result. Problem found, and fix not too bad. Hope you get it running and working soon.






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 11-27-2001, 20:42 Post: 33481
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 870 with a knock and blowby

BILL, I just read your post and I thought I'd add my two cents worth. It seems unlikely to me that overheating caused the damage you found without also scoring the cylinder wall. You should probably have the injectors tested to be sure they are O.K. A bad injector could wash down the cylinder and cause similar damage. Generally , overheating causes damage first on the rear cylinder.
Kenny






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 11-27-2001, 21:16 Post: 33482
critterpainter



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 870 with a knock and blowby

Since I did not own the tractor when the damage occured my information is second hand .
To my understanding the engine had the blow-by and knock (rattle?) problem on start-up after the cooling problem was fixed. I have not yet dropped a hone or mic down #2 cyl (too cold when I get home from work) however my fingernail could not detect anything and an inspection using a strong light showed nothing. I the little I ran the tractor I did notce that the knock subsided GREATLY when the engine was fully warmed up....hence my thought that the piston had "shrunk" from overheating. The engine ran fine on all cyl after a short warm-up so I do not believe an inector problem is a concern. (also the "spray pattern" on the top of each piston and on the head look the same on all pistons).
I hope to reassemble the beast either this weekend or next,and post an update
Bill (wearing my shade-tree mechanic hat)
P.S. The last diesel I overhauled was an old case crawler about 10 years ago...it had a BIG ridge on each cyl. But it had been run with a hole in the air cleaner housing.






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 11-27-2001, 21:24 Post: 33483
Roger L.



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 870 with a knock and blowby

When I was just starting out as a mechanic I used to tear engines down completely and replace or refurbish every part that showed measureable wear. Gradually I got to where I just replace the failed part, rectify the cause, and button it back up again. I realize that it is just one person's limited experience, but it does seems to me that the two different methods yield about the same results.






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 12-02-2001, 16:01 Post: 33611
critterpainter



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 870 with a knock and blowby

IT RUNS AGAIN. I spent a rainy weekend in my doorless shop and put the 870 back together. While cleaning the bore of the cyl that was getting the new piston, I found some aluminum from the old piston stuck to the cyl wall. I guess that means the piston siezed when it was overheated, and when it cooled it broke loose. There was no damage to the cyl. I ran my "stiffback" hone down the bore and did find a little wear in the cyl where the top ring reaches its max height. Very Little wear, I do not own a cyl mike but I would guess the wear to be less than .002 of an inch which is well within spec. (yes I did buy the engine shop manual before working on this engine)
So nice to have a tractor that starts without a cloud of white smoke..
Bill






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 12-03-2001, 06:43 Post: 33626
TomG

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 870 with a knock and blowby

Hey! Congrats! Sounds like a real good result and fortunate there seems to be little engine damage. I never had a cylinder gauge either. It's sort of a pain to use telescoping gauges and OD mics, but I did it anyway. It's good to know that things put back together are within tolerance and likely to last awhile. I guess you took the piston out from the top, in which case thee wouldn't be much of a ridge or it would have been difficult to get the rings past the ridge. The times I've done that sort of work without removing the crank, I've had to rent a ridge reamer so I could get the pistons out. But, I guess I'm just excising my fingers here since the repair is all finished. Hope you enjoy the newly smokeless tractor.






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 12-10-2001, 12:23 Post: 33725
Art White



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 870 with a knock and blowby

Tom I just picked up a new style gage and was thinking about getting rid of my old Central Tools unit which is style like new just a little harder to use than the new one. Would you be interested?






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Discussion Boards > Active Subjects > Messages as Posted > John Deere Review Forum

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