It's time . . . top end re-build thread '82 xs400-RS


XS400 Enthusiast
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Oakland, CA
Hello everyone,

I've had my XS400 since February as a learner bike to learn to ride and wrench. It has been a lot of fun riding and working on it and I have gotten a lot of great advice from the folk in this community on the fuel-air and exhaust systems. Due to an oil leak (likely valve seals) causing oil burning recently I have decided its time to start a re-build project. I am going to re-build the top end, re-hone the heads and cylinders, and get the block cleaned and coated. There is first pass on a parts and specialty tools list below. If anyone thinks I'm missing anything, please let me know. I will use this thread to track my progress and compile advice.

On my early research I have had trouble finding a clutch re-build kit for the XS400. Mike's XS has a kit for the 650. Does anyone know if these parts are common to the 400? I also have not been able to find the motor fin dampers, gas tank dampers, and motor mount dampers. Anyone know a source for these? Also the 'standard gasket set' on Mikes XS says it is for the '77-'81 model years. Will these gaskets fit my '82? I have a gasket kit from the PO but it's missing a few that I assume he used.

UNI air filter
Top end gasket kit (includes valve seals)
Muffler gaskets
Exhaust gaskets
Motor fin dampers
Motor mount dampers
Gas tank dampers
Clutch repair kit
Top end copper seal kit
Exhaust mounting studs
Piston rings
Valve shims??
Carburetor boots??

Specialty Tools:
Valve compressor
Valve lap kit
Vacuum gauges
Clutch holder tool
Feeler gauges
Tappet gauges

Thanks in advance to anyone who decides to read this and offer advice. Any lessons learned on what to do or what not to do are appreciated. Additions to the parts and tools list would be great too. With some work and a little luck I should have a nice build to add to this community when its over.
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Thats a big step going from a leaky valve seal to an entire top end rebuild.

Replacing the valve seals and reseating the valves is a job that can be done in a weekend for about $50 in parts..

Honing and rings requires a machine shop that will need to measure the bores to determine if youre good with just a hone, or it should be bored out. The easiest solution is to just do a rebore. This will put you in the hole a few hundred dollars and you will have difficulty sourcing parts along the way. Youll be buying a piston here and a circlip there and maybe a pair of wrist pins comes up or a complete set of the correct size rings...

Dont do a rebore if you dont have to, thats if your compression is OK. Instead, I would just do the valve job and gather the parts you need for a rebore and put those parts aside for when the time comes. I know its a tough call since youre already that deep in the engine and it makes sense to just do it all at once.

Do not remove the connecting rods from the crankshaft. Those bolts are one-time use and are impossible to find.. Pretty sure there are compatible bolts that are for a Mitsubishi but Im not sure, they were ARP bolts too.

I use Partzilla to search for part numbers and cross compatibility. EBay is your friend and worst enemy with availability of parts and insane prices. I see stuff that is still for sale that I saw over 5 years ago on there. One day, someone will be willing to pay $180 for a used xs400 ignition coil!

Clutches, dont bother unless it slips bad. Even when that happens, its very easy to replace and can be done within a couple hours. Get a new clutch cable, youll be much happier lol.
Thats a big step going from a leaky valve seal to an entire top end rebuild.

You're not wrong Mike. My plan was always to do a rebuild anyways and I would rather do a bunch of work in one go than keep chasing problems. I could be totally wrong and it's not even the valve seals. The block, head, and cover are pretty corroded which is bad for heat transfer, and all the bolts are rusty. If may have never been opened up for it's whole life. My exhaust needs work too. I would rather tool up for as much as possible before tearing things apart to increase my chances of success.

My hope is that the cylinders and gasket surfaces are in good shape, along with the rings, and a chemical cleaning is enough. It does have good compression after all. Since this is a small job I'm just getting a quote from a machine shop and a coating shop now so I'm not waiting around for a month to get the parts back. I'm definitely not planning to split the crank case or do anything with the crank shaft or transmission except a good oil flush.

Clutches, dont bother unless it slips bad

That's a good point about the clutch. My lever is really sloppy so I'll just check that out and replace the cable as suggested and see how it feels. I'll check out Partzilla for P/N references as well.

Overall I think I'll be in good shape. My bike runs well enough now. I'm just trying to head off problems that I can see coming to keep the motor healthy. I work at a machine shop, so I have a lot of resources at my disposal. Just too bad I can't get one of our machinists to work on the motor for me . . .
Getting back in to this after a busy month.

I am looking around for piston rings and I see two different part numbers out there. Partzilla lists the rings as 2a2-11610-00 but I also see 12r-11610-00. Anyone know which one is proper for an '82 SOHC XS400SJ. My title says XS400RS but the bike matches picture I have seen of many XS400SJ.

Sorry if this has already has a thread. I did some searching and didn't find definitive answers.
2A2 should be the sohc bikes. 77-81 will also have this. 82 was a two model year so you need to be careful. If you know for sure, use the 81 part numbers and that should help.
No idea on those. Make sure your motor is good enough for standard size rings and your pistons are still in spec. Most often an oversize is needed after a hone job is done ( jugs out of spec).
Typical honing jobs are less common than they used to be, along with knurled pistons.. Most usually opt for the next bore size.

Personally, Id stick with OEM stuff if I could find it. You can also do research on other Yamaha/ Japanese makes. There was talk about using rings from other engines, as long as the bore sizes are the same, it shouldnt matter at all.

I think if you remove the 3 digit prefix off a Yamaha part #, you should still be able to find what you need, it just wont be XS400 specific. It may be for a quad or something. There are other engines that have the same stock bore size and same oversizes.

The last time I purchased a set of rings for one piston, ran me $50 for OEM.. On top of that, I already had a good set of rings and the compression ring broke as I installed it. I was very angry.. USE THE RING TOOL!
Is it safe to assume that my cylinders are in good shape if I have good compression? A guy I work with has honing tools so I am hoping to get away with a chem clean and a light hone of the bores. Going to the next OS bore is going to double my cost and more importantly the time required.
As I understood it, yes: that's the whole point of measuring compression. If your seals and gaskets were good opening it up would be frivolous at best
Sure it may be 'frivolous' but so is owning this bike in the first place really. Mostly I think it will be a fun project and since the motor is in decent health, it should be low risk. These machines are well engineered. It's not like I'm tearing into a Harley shovel head.

That being said, it does have bad valve seals and exhaust seals. The exhaust studs are also stretching cause someone over torqued them. I also want to clean some of the bad aluminum corrosion off the head and block. It has likely never been opened and it has 37k miles on it. I think this job will get ahead of a lot of future problems. Plus, with some luck, it will turn out as a beautiful custom bike when I'm done.
Cant help with painting engines. I keep things very simple. Use a wire brush and lightly scrub the aluminum if the block is raw. This is what Ive do.

It seems youre philosophy on this bike is "might as well do it now since Im in there"... This doesnt really apply here IMO simply because you should be able to remove the engine in about 30mins, no sweat. You don't need to remove the engine to do a valve job either.

Engine internals are VERY EXPENSIVE and HARD TO FIND.. I highly advise against breaking the engine down other than doing a valve job, granted the compression is OK. It is a complete waste of time with an engine that runs. Do not let curiosity get the best of you! Im just saying...

As long as you don't beat on the bike everyday, an XS400 (all motorcycles) will go for a very long time. Id suggest picking up an additional XS400 and having a spare engine/parts that you can work on in the meantime. Its all about the maintenance.

The valve cover doesnt have a gasket. You will have to clean and prep the gasket surface and use Yamabond. Get the engine to TDC before removal to make sure theres no pressure from the valves pushing against the cover.

Oh, and pricewise, you ARE tearing into a shovelhead... HD parts are probably easier to find due to actual aftermarket support.
I guess I should think more carefully about this then given y'all think it may be a bad idea. Maybe I'll just do the valves and exhaust while I work out the electronics. I guess I'll see what it looks like when I get into it. I've already got a lot of the of the internal parts for the motor.

Is there a way to clean carbon build up out of the cylinders without cracking it open?

Thanks for the tip on the Yamabond.
Got the motor out and cam cover off yesterday. I was a lot easier than I thought it would be. I was concerned that the bolts would be seized. Everything looks in good health, but I haven't taken any measurements yet. The LHS cylinder had low compression at 120 psi when I got it in to the shop so I feel more confident that doing the whole top-end is a good idea.

cam and valves.jpg
I got the head and block off today. nothing too difficult came up. I made a scraper out of a chunk of copper tube and went to town on the heads, valves and pistons with that and a brass brush and some WD40. There was a good amount of carbon built up on the aluminum and on the exhaust valves. I'm guessing that is the cause of my low LHS compression since the bores look good. I'll get them measured before I re-build. The valve collars were all stuck in the retainer plates, so much so that it bent the spring tool. Tapping the spring with a rubber mallet when compression was applied freed them up. It looks like some grinding and cleaning will be enough to get the valves back in good shape. I didn't take any picture cause everything went so smoothly but I will get some next time.

The only thing that wasn't to spec was that one of the hollow dowels in the block wouldn't come out, but I'm not even sure that it needs to. Any advice on that?

Next step is to start measuring everything, get the head and block cleaned, and prep the frame for paint.
I used Easy Off to remove the carbon off the head chambers and valves. worked very well when I put the valves in ziplock bags for a while.

I dont think Id be overly concerned about 120lbs, as long as the other cylinder was close and at least 115lbs. Thats like the very bare minimum though. If the bike idles fine, then its probably OK. Lapping the valves will be good. I had a real pain getting the valve train assembled because the valves are so strong and the clearance is weird for a normal valve tool. I think I used a large C-clamp and a O2 socket or something.

The dowel is just that and also the oil galley. The gasket kit will come with new rubber.

Lapping the valves is a very easy and gentle process. A lot of people turn the valves too much or apply too much pressure and end up ruining the valve lands. Youre looking for 2 clean thin lines on the face.
Is there any risk in just doing the pistons rings anyways? My cylinders look pretty good and measure to within 0.01mm of spec. I'm going to give them a light hone to give the rings a nice surface to seat to. I've done valve lapping on globe valves for water before so I have a feel for that task.

Thanks for the easy off tip. I have a ton of carbon on the exhaust valves and ports and I was not looking forward to cleaning that.

I'm in the process of finding a shop to do glass bead blasting on my motor exterior. If it's too expensive I'll just give it a go with a brass brush.
Over the past week I have gotten in to the cleaning and re-work portion of the job. I used a wire wheel to clean up the exhaust and intake valves. Theres a before and after of one of the exhaust valves. the seat seems to be in good shape so I'm planning to lap the valves rather than cutting new seats. It's pretty clear now that my compression problem was the exhaust valve seat. I also got the old piston rings off, though I haven't got around to cleaning the pistons yet. I have also honed the cylinders, which turned out pretty good. The ring gap measurements for the new rings checks out. I'm off to clean, port and polish the heads today, and maybe lap the valves if I have time. If all goes well I could have the motor back together this month. Sorry about the picture quality of the pre-honed cylinder.clean valve.jpg IMG_20191030_193412.jpg before hone.jpg hone2.jpg