Piston Ring Replacement

[animated]

Hey all,

So I finally got the top end pulled apart.

good:
The rings look to be in decent condition. There's no cracks or busted up bits. I haven't measured the gaps yet, but I'm not too worried about them at this point. Cylinders look fine too. There's visible vertical lines, but you can't feel them at all with your fingertips...

probably not good:
There's some carbon buildup on the top of the piston, around the top of the cylinder, and all over the head around the valves. I'd say 1mm at it's thickest.

definitely not good:
There's about 2mm vertical play in the right side con rod. I've got the pistons off, and when I tug on the hole where the wrist pin goes, the thing has vertical play as well as side-to-side.

Is that a bottom end re-build? Can I do it myself?
What should I do?

 

[lhaolpa]

I don't see why not,I've done it many times on cars,trucks and a few old tractors.However,on the bikes you have to split the cases,mic the crank,and go from there.If the crank is good,then just replace the bearings.If not good,then you'll have to regrind the crankshaft,have the rods checked for cracks,etc. Assembly is the reverse of removal! HaHa. lha

 

[animated]

Hahah.
I'm not really worried about assembly... But if the parts cost more than a used XS400 is worth...

I guess I have nothing to lose by trying.
Except a whole whack of time in the shop.

Just so I know... "mic the crank" = specialty measuring tools, right? A local bike shop would have the equipment?

 

[lhaolpa]

Micrometer or good set of calipers. lha

 

[bcware]

Clean out the grooves real well and make sure the rings are staggered. I had no problems before my first top end rebuild. but once everything went back on I lost compression. Once I took it off a second time my rings were stuck and some were not staggered. Mine are within spec or close enough. The cost of parts is fairly high, so consider the costs of doing a proper rebuild.

 

[lhaolpa]

I wouldn't worry so much about the carbon on the piston,that's from a rich mixture.The rod being loose is the biggest problem and when that goes,the motor goes.There is a motor on ebay for $300.00 or so. That may be a better deal if you want to get going right away.The bearings problem is one that you won't know about until you crack open the cases and measure. On the tech pages,you'll find a tutorial made up by a member,showing how to split the cases. Do this and use your manual,and you shouldn't have much trouble having a good,safe motor to give many years of service.Buying another bike is a coin toss,as you might get one like the one you have. After fixing the bearings,then assess the cylinders by putting a new ring into the cylinder with a piston to square it,then measure with a feeler gauge.Do the same in the middle and bottom of piston travel. compare the readings with the clearances in the manual,then you'll know what condition you're in. Good luck. lha

 

[lhaolpa]

By saying HaHa,I wasn't making fun of you,it's just that I find it ridiculous that manuals always say that after describing how to remove a part or a bunch of parts.We all know that it's more important to have directions on how to assemble the bike than to disassemble it! for that reason I put the HaHa! in there. By the way,if you didn't know, there is a good manual in the tech section.Good luck. lha

 

[animated]

Cool thanks - I'll check the tech section for that step-by-step and a manual.
I've got a Haynes, but it's a little hit-and-miss.

You can make fun of me all you want. I'm a worthy target - in over my head again and again with this bike.
If I wasn't laughing at myself I'd be crying for sure.

I should have the cases split by next weekend, so I'll know more then.

It just sucks 'cause this is my first bike and I have WAY more hours wrenching on the thing than riding it.
I guess I'll just keep daydreaming of a new Bonneville.

 

[volcomskater77]

join the club! ive had mine since last november and i still have yet to ride it. i did the same thing your doing. but all my rebuild costed was $50 for new cases, $40 for the gasket kit and rings, but my dad ate the cost on that one cuse we did some math wrong and the deal was if it dont work, then he buys the new rings. thank god, those suckers are expensive. other than time and labor, it was relatively easy. think theres a write up on here about splitting the crankcase... dont remember who did it tho...

 

[lhaolpa]

If you need a good manual,I've got one for the early models[before '82]that I bought before I snapped that I have an '82 DOHC model.It's a good one with lots of pics and detailed instructions and I put 4 pages of electrical diagrams in back.It's in a folder with plastic page protectors.PM me if you could use it. lha

 

[jthiessen]

An old thread here, but I find myself in a similar position.
I was wondering what else should a guy do while the engine is apart replacing the rings?
I heard someone say something about lapping the valves?
And if the engine needs to be rebored, how much oversized can one safely go with these bikes?

 

[rshutchinson]

If you need an overbore I would suggest you try to find pistons and rings first and then bore to that size as the oversize pistons seems to be the hard piece to find.

I got away with just a factory size new ring set and honing the walls since they were smooth (no scoring) but had no hatch left. My low compression was from seized rings from sitting though.

Aside from that, new gasket set, valve stem seals and a valve lapping should be all you really need to do if everything else is in good working order.

You could replace leaky oil seals while you're at it if some are starting to sweat like the clutch pushrod seal and neutral switch o-ring.

 

[Arrowrl206]

Hey where can I find rings for an '80xs400?

 

[xschris]

http://www.ebay.com/itm/Yamaha-XS40...m5447136414:m:mznqZJlEWugrpOgQGTMV16w&vxp=mtr This guy sells standard size only. I have a set but haven't used them yet. I think a few people on here have used them in the past. If your pistons are still in spec and the bores also you should be ok with these.