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Discussion Starter · #1 ·
I recently bought a 400ex non running. I got her running, but she smokes a lot hot or cold. I’m about to take it apart, but I’m wondering what I should purchase and replace while I’m inside the engine. I’m also wondering what brand piston I should go with. I want stock bore and have heard good things about wiseco, is that what I should go with. If the cylinder is too big to be used, what cylinder would you recommend buying. I’d rather keep this on the lower budget and have the engine as stock as possible. Thanks!
 

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Use Honda, Namura, Vertex or Pro-X piston - Wiseco is a good piston, but it is forged rather than cast - it has to be fit looser when new, requires a longer warm up and costs more.

Depending on what you discover taking it apart, you should plan on installing a new cam chain, clean the head, clean the valves and ports, install new valve seals and check the big end rod play (side to side is fine - no up and down play) the cylinder can be bored to 1mm oversize without needing a special head gasket - if you need to buy a new cylinder, I would go with Athena over Niche.
 

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Discussion Starter · #3 ·
Use Honda, Namura, Vertex or Pro-X piston - Wiseco is a good piston, but it is forged rather than cast - it has to be fit looser when new, requires a longer warm up and costs more.

Depending on what you discover taking it apart, you should plan on installing a new cam chain, clean the head, clean the valves and ports, install new valve seals and check the big end rod play (side to side is fine - no up and down play) the cylinder can be bored to 1mm oversize without needing a special head gasket - if you need to buy a new cylinder, I would go with Athena over Niche.
Thanks for the reply. As i am waiting for my compression tester to come just so i can know for sure it needs a rebuild, i am looking for parts. anyways, i cant seem to find a stock 85mm Athena cylinder. If they don't sell stock size, would you recommend then to go with niche?
 

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Thanks for the reply. As i am waiting for my compression tester to come just so i can know for sure it needs a rebuild, i am looking for parts. anyways, i cant seem to find a stock 85mm Athena cylinder. If they don't sell stock size, would you recommend then to go with niche?
Niche is not totally junk and will do the job, so yes - to stay at 85mm use a Niche cylinder, just don't expect it to last as long as the Honda cylinder - Chinese steel is a little softer than Japanese steel and the castings are not as perfect as Honda (might be a bit rough, have seen blemish holes in places where it does not hurt anything and sometimes there is extra flash where the mold comes together - can easily be cleaned with a file if you have OCD)
 

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Discussion Starter · #5 ·
Niche is not totally junk and will do the job, so yes - to stay at 85mm use a Niche cylinder, just don't expect it to last as long as the Honda cylinder - Chinese steel is a little softer than Japanese steel and the castings are not as perfect as Honda (might be a bit rough, have seen blemish holes in places where it does not hurt anything and sometimes there is extra flash where the mold comes together - can easily be cleaned with a file if you have OCD)
Thanks. After further diagnosing, I did a compression test and got 145psi. But like I said, it smokes when hold or cold, but when it’s warm it only smokes when accelerating and not just idling. What would you recommend checking next? My owners manual says to check valve seals, Valve guides, head gasket, or excessive crankcase pressure.
 

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Valves, guides, seals and oil ring on piston - the head gasket is almost never the cause and excessive crankcase pressure only if the crankcase vent hose is pinched or plugged.

Worn valve guides and bad valve seals are indicative of long periods of idling and a worn oil ring is indicative of the oil not being changed regularly (if at all).
 

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Discussion Starter · #7 ·
Valves, guides, seals and oil ring on piston - the head gasket is almost never the cause and excessive crankcase pressure only if the crankcase vent hose is pinched or plugged.

Worn valve guides and bad valve seals are indicative of long periods of idling and a worn oil ring is indicative of the oil not being changed regularly (if at all).
I just got done doing valve seals and cleaning and inspecting the whole head. I put the head back on and got the engine back running. It still smokes the same amount. At this point I’m assuming that the oil ring on the piston is bad since I’ve checked everything else. Since I’ve got good compression there’s no point in doing a top end rebuild, so I’ll order a niche ring kit. Is it bad to replace just the oil ring and leave all the other rings stock? Im thinking of doing that so the rings don’t have to reseat and won’t require a break in.
 

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I just got done doing valve seals and cleaning and inspecting the whole head. I put the head back on and got the engine back running. It still smokes the same amount. At this point I’m assuming that the oil ring on the piston is bad since I’ve checked everything else. Since I’ve got good compression there’s no point in doing a top end rebuild, so I’ll order a niche ring kit. Is it bad to replace just the oil ring and leave all the other rings stock? Im thinking of doing that so the rings don’t have to reseat and won’t require a break in.
Since the oil ring has to seat in you might as well install the compression rings also - seating rings only requires about an hour of run time at moderate speed and load. Since rings don't wear evenly, how would you feel if you only replaced the oil ring, put it all back together and then had no compression because the compression rings were not installed in exactly the same position as they were in when taken apart? I've seen it happen to more than one novice mechanic and one experienced mechanic who was experimenting with stopping oil consumption by turning the oil ring over. He took it apart turned just the oil ring over put it back together only to have less than 90 psi of compression and never got it started - in another failed attempt, he took it apart and inverted the compression rings and ended up with 60 psi of compression - finally he installed a new set of rings, brought the compression up to 110, got it started, ran it for about 10 minutes and had over 120 psi of compression - after about 1 hour of run time the compression was back up around 150.

If you are buying a set of rings, just install them all, the compressions rings have an up side, but the oil ring is non-directional.
 

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Discussion Starter · #9 ·
Since the oil ring has to seat in you might as well install the compression rings also - seating rings only requires about an hour of run time at moderate speed and load. Since rings don't wear evenly, how would you feel if you only replaced the oil ring, put it all back together and then had no compression because the compression rings were not installed in exactly the same position as they were in when taken apart? I've seen it happen to more than one novice mechanic and one experienced mechanic who was experimenting with stopping oil consumption by turning the oil ring over. He took it apart turned just the oil ring over put it back together only to have less than 90 psi of compression and never got it started - in another failed attempt, he took it apart and inverted the compression rings and ended up with 60 psi of compression - finally he installed a new set of rings, brought the compression up to 110, got it started, ran it for about 10 minutes and had over 120 psi of compression - after about 1 hour of run time the compression was back up around 150.

If you are buying a set of rings, just install them all, the compressions rings have an up side, but the oil ring is non-directional.
Would you recommend honing the cylinder before installing the new rings?
 

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Would you recommend honing the cylinder before installing the new rings?
You can hone or just glaze break - if the cylinder is smooth and shows no abnormal wear, just break the glaze with a flexible ball hone - if the is a deep scratch or water stains on the liner, then hone the cylinder no more than .002 inches - any honing over .0025" means the cylinder should be bored to the next oversize and honed for .002" clearance - if the cylinder is honed to remove a scratch and less than .002" of material is removed, the engine will be a little noisier, will run normally, but will not last as long before it needs to be rebuilt again.
 
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