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Ive got a problem with my 400ex and need some professional advise. Sorry about the long post but there is a lot of small details that may give more hints on what the problems may be.

I bought it with a head gasket leak so i tore it down to replace it. When i had it apart i noticed some abnormal wear on the piston and slight scoring on the cylinder(figured the previous owner ran it low in oil). So i honed the cylinder and replace piston, pin, and rings with new oem. While i had it apart i also cleaned out the screens in the oil cooler and in the bottom of the engine. I then replace oil filter and filled with honda oem oil(It did seem to take slightly more oil than normal but i didnt think much of it since i had the oil cooler and side case off to clean out the screens).

Started it up and it ran great. Did about 8 heat cycles and some mild riding on it and then checked the compression which was around 120. Then i rode it fairly hard for a while till i noticed some smoke coming from the pipe and a very slight abnormal knock/ping while under load (the smoke continued seep out of the pipe for a while after i shut it off. It was a used pipe i just bought and put on, maybe it had some oil residue inside and was just burning out).

After it cooled off i removed the valve cover with intentions of getting a look at the cylinder but then decided to do another compression test before tearing down. So put the valve cover back on and tightened a few of the 6m bolts, did the compression test and at around 50 psi the engine seized. Took the cover off the side case and tried to spin the engine backwards, and it broke loose fairly easily. I tried to continue spinning it counter clockwise, and after a bit it got tight again. I pulled the valve cover off and the found the timing chain was tight all the way around (with tentioner in). Then i pulled the tenioner out and whenever the engine would get tight the timing chain work get tense the entire way around. (Tentioner seems to be working correctly)

Ended up pulling the head and cylinder off and found some fresh scores in the cylinder(much worse than the previous scores). The top of the piston also had a thin layer of black residue in the center.

At this point im stuck and need some professional input on whats wrong. If your feeling extra helpful and dont want the confusion of commenting below please send me a pm ...thx
 

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Here is where i think you went wrong. Unless you are a professional machinist or took it to one and didnt say it. When you see scoring in the cylinder that is wear. Not just marks on the surface. At that point you take the cylinder to be measured for out of roundness and wear. The machine shop then tells you what you can do to fix it. Hone and rings 99.99999% of the time dosnt work. There are specific tolerances that have to be met here. This is why when you take it to a machine shop and they say i can bore it without the piston in hand you turn and run! They will taylor make that cylinder to the piston. They will measure for cylinder to wall clearance and account for heat expansion and contraction.
 

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Discussion Starter #3
Im by no means a professional. I took small engine classes in high school, but that only teaches so much. Your right i never took the cylinder to a machine shop, although now im wishing i would have.... I did take measurements of the cylinder and I thought it met all the specs i found in the manual, but again im not a professional. What would have cause the engine to seize during the most recent compression test and not at any other time?
 

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My best guess is the cylinder is in need of a over sized bore and piston. And did you measure after honeing? Cuz the hone does actually remove alittle bit of the metal and could have made your problem worse. I would bet the piston got wedged in a position when doing the compression test and caused those scores with mostlikely the piston skirt. With a piston to wall clearance to big it can do that or worse. Your lucky it didnt bust the skirt off while riding it and send a bomb into your bottom end. The noises you were hearing when you noticed all the smoke i would bet was piston slap. All caused by the same thing.
 

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So any ideas on what to do for the rebuild? I want something fairly budget friendly, but at the same time while im doing it i want to put something that makes good power in it. Ive heard bad things about 440 kits and heard better things about stroker kits but know very little about them.
 

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Best budget friendly build is a 416. Any engine machine shop can bore your stock sleeve from a 85mm to a 87mm. You will want to get gt thunder hd studs which means you will have to send you cylinder to snure interlocking or have your machine shop install them. The other option would be get the 87mm piston of your choice and send it all to snure and have them bore it to match and not have to worry about it being done wrong by a local machine shop. Unless of course you know a reputible one that is. If you look at my signature you will see a very low budget build. My quad sits right next to my brother kfx450 in a drag race. And will beat him on a track. Keep the questions coming and we will get you all fixed up.
 

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Call G&H and ask them what they can do for you. Is your camshaft and your head still good? Pull the cam out and look at the bearing areas if you haven't already. May need a new sprocket and chain too... wrap a new chain around your old sprocket and if the new chain doesn't lie down real nice all the way around the old sprocket its junk.

Did you scrub the cylinder in hot, soapy water after honing it? Really work at it scrubbing and scrubbing...?
 

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Did you scrub the cylinder in hot, soapy water after honing it? Really work at it scrubbing and scrubbing...?[/QUOTE]

I cleaned it up in my parts cleaner....figured that would be just as good....?
 

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Did you scrub the cylinder in hot, soapy water after honing it? Really work at it scrubbing and scrubbing...?

I cleaned it up in my parts cleaner....figured that would be just as good....?
Nope, not much difference than if you didn't clean it out at all. After machining and honing the bore is loaded with grit. The amount of grit in there that the bore holds can amaze you... You have to use HOT, Soapy water and scrub until you can't stand it anymore. Then rinse, dry with cloth and test your work with a clean white paper towel or cotton cloth with a bit of fresh motor oil on it. Rub the oil in really good then look at your wipe. If that wipe is not PERFECTLY clean you gotta mix up some more fresh HOT, Soapy water and scrub it some more! Oil it well once it is clean.

The amount of grit in there that the bore can hold after machining can truly amaze you... Failure to get it all out = early or immediate topend failure. This cannot be underestimated...
 

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Did you scrub the cylinder in hot, soapy water after honing it? Really work at it scrubbing and scrubbing...?

I cleaned it up in my parts cleaner....figured that would be just as good....?
Nope, not much difference than if you didn't clean it out at all. After machining and honing the bore is loaded with grit. The amount of grit in there that the bore holds can amaze you... You have to use HOT, Soapy water and scrub until you can't stand it anymore. Then rinse, dry with cloth and test your work with a clean white paper towel or cotton cloth with a bit of fresh motor oil on it. Rub the oil in really good then look at your wipe. If that wipe is not PERFECTLY clean you gotta mix up some more fresh HOT, Soapy water and scrub it some more! Oil it well once it is clean.

The amount of grit in there that the bore can hold after machining can truly amaze you... Failure to get it all out = early or immediate topend failure. This cannot be underestimated...
Neither me nor my family friend that has been an engine machinist for plane engines and race engines for 40+ years has ever done more then cleaning with a parts washer and then use atf on a no lint cloth through the bore. Probably 1000+ engines between the 2 of us. He has even built 1000+ hp engines and not done any of that.
 

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I have too. Methanol fueled racing motors were my forte... My niche (and love) was design engineering. I thoroughly enjoyed beating up on my competition on the race tracks! I did the design and build of small, lightweight V-8s that would produce massive torque figures on the dyno. I was/am a humble and quiet pioneer. I put motors on the race tracks that made over 1000 ft lbs of torque at the crankshaft from as little as 441 cubes and a single 4-bbl carburetor, per sanctioning rules. As far as I know, today, some of my torque production figures still stand unmatched, per cube.

I explained the importance of cleaning out freshly machined bores and what can happen if you don't. If your wipe came out clean after washing, you got it all, so please disregard my warnings above. Being a forum environment here, it is difficult to assess one's skills and knowledge, so I always end up over-explaining things. I hope you'll forgive me.
 

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soapy hot water I have read in numerous manuals is the proper way to remove machine shop grindings , I use G+H for my boring and on the break-in/ installation instructions that are sent along with the package , phara-phasing it says in a warning box marked WARNING : wash jug in hot soapy water , cleaning fluid may not remove all grindings from machine work done to jug --------- I am going to keep on washing mine with hot soapy water then blowing it off with air
 

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Did you scrub the cylinder in hot, soapy water after honing it? Really work at it scrubbing and scrubbing...?

I cleaned it up in my parts cleaner....figured that would be just as good....?
Nope, not much difference than if you didn't clean it out at all. After machining and honing the bore is loaded with grit. The amount of grit in there that the bore holds can amaze you... You have to use HOT, Soapy water and scrub until you can't stand it anymore. Then rinse, dry with cloth and test your work with a clean white paper towel or cotton cloth with a bit of fresh motor oil on it. Rub the oil in really good then look at your wipe. If that wipe is not PERFECTLY clean you gotta mix up some more fresh HOT, Soapy water and scrub it some more! Oil it well once it is clean.

The amount of grit in there that the bore can hold after machining can truly amaze you... Failure to get it all out = early or immediate topend failure. This cannot be underestimated...
Thats good to know, i thought i cleaned it pretty well, but im usually one to go the second mile.
 

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I have too. Methanol fueled racing motors were my forte... My niche (and love) was design engineering. I thoroughly enjoyed beating up on my competition on the race tracks! I did the design and build of small, lightweight V-8s that would produce massive torque figures on the dyno. I was/am a humble and quiet pioneer. I put motors on the race tracks that made over 1000 ft lbs of torque at the crankshaft from as little as 441 cubes and a single 4-bbl carburetor, per sanctioning rules. As far as I know, today, some of my torque production figures still stand unmatched, per cube.

I explained the importance of cleaning out freshly machined bores and what can happen if you don't. If your wipe came out clean after washing, you got it all, so please disregard my warnings above. Being a forum environment here, it is difficult to assess one's skills and knowledge, so I always end up over-explaining things. I hope you'll forgive me.
Wasnt meant to offend at all. I completely understand and appreciate your knowledge! I talked to my engine guy and he said yep alot of people do wash in soapy water. We here do not because of the minrerals and crap in our water. He said his parts washer filters to microns and he has never had one come out dirty enough for it to be necessary. Again sorry if i came off like an A-hole as that was not what was intended!
 

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@SlammedRanger,

Nah, not at all... I figured I may have come across a bit too blunt cause many times I do! Rarely do I ever intend to unless I say so up front... I appreciate someone who challenges something I say or do or has a better idea!!! My partner was like that, a great friend, a smart guy with a lot of knowledge and a lot of experience... He was a great driver, very skilled and a deep thinker concerning the physics of things. We used each other to bounce ideas off. Without him and our long brainstorming sessions (and constructive argument!) neither of us could have ever achieved our goals. Hit me with all your best shots I always say... Please don't hesitate to empty your clip on me whenever you feel like it!
:)
 

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Slamme'R , LOL , there is a 100 ways to skin a cat , as long as it taste good and there is no grit in your teeth , who cares how you skin't it !

I myself didn't like the idea of using water as water will cause rust , I am 60 years old and never heard of soapy water till maybe 2008 , but they scared me into it , it does a really good job of taking all the oil and grease off of it , so maybe that is a key , no oil or grease to hold grit , who knows !!! I do wash it out a lot , every hole , journal , passage way with hot water and Dawn dish water liquid , then blow it over with compressed air , if I feel one piece of grit I do it again , much like cat ! LOL
 

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Discussion Starter #17
Im still having a hard time trouble shooting all this stuff...I dont want to just go and put a 416 kit in it when there could be other causes or problems. Im getting a measurement of 85.00 and 84.95 at the base of the cylinder. Shouldn't the piston made a fairly noticeable knock if the cylinder was bad enough to cause damage?
 

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Also should the piston be able to rock side to side on the connecting rod? I cant feel any up and down play, but side to side seems kinda odd. Im wondering if maybe the oil pump is weak causing poor lubrication....?
 

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You said earlier that the motor turns over freely for a bit then gets tight. You mentioned the chain getting tight all the way around the sprockets when the motor became tight. Did you put a new chain in the motor recently? On the old sprockets? At this point I would want to know whether the oil pump is working or not too... maybe you can get to it? Any chance you put the oil filter in backwards?
 

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I just re-read this thread and you mentioned having the side cover off to clean screens etc. Was there a reason for that? Crankcase have junk in it? Someone sink the motor? When you put the side cover back on, did anything get left out of there or did anything fall out of place (an o-ring, washer, oil pipe etc) as you were working? For some reason I have a sneaking suspicion that your motor wasn't getting any oil pressure...
 
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