boring out cylinder(s) on an XS850?

Question

Here's a little more info:
I'm a college student, hence not a huge budget
I picked up an '81 Yamaha XS850 for $50, it's been sitting for 18yrs
One cylinder was stuck from light surface rust (cylinder closest to cam chain)
I got the cylinders off, and am undecided what to do about boring them
I've called around, and it's about $110 for new oversize pistons, and $35 for rings per piston
It's about $40-50 per cylinder to bore and hone.
It's $95 for a complete engine gasket set

So, if I were to oversize all 3 cylinders, it'd cost quite a bit of money (to do it the right way)

BUT, a buddy of mine suggested maybe just getting that one cylinder bored to the 1st oversize...

I asked, wouldn't that make the engine out of balance?
he said, well, not significantly enough to worry about it

I can get a complete used engine with 150psi per cylinder for $300-400 which would be cheaper than boring out all 3 cylinders...

I have one more year of college, then it won't be hard to find a good paying job, which in turn means, I won't have a problem with costs of boring out all 3 cylinders, and whatever else comes up...

For now, I'd like to just be able to ride it, then when I graduate, I can tear it down again and REALLY go thru it... strip it all down, repaint everything, etc... new bearings everywhere, etc

What do you think?  Would boring out one cylinder to .25mm (.010") oversize throw the engine out of balance significantly enough to cause damage?  ten thou isn't much of a size difference.. but then again, turning at upwards of 8kRPM might be more noticeable???

let me know what you think...
On a side note, the bike has 46k miles... she's an oldie, but a goodie... they all are... Or, I could just sit on it til next year, and redo it then... but I like to work on stuff...

Bob A · Accepted Answer

First off boring one cylinder is a big no no.It's not so noticeable on a a 4 cylinder but on Triple it's a disaster.The next thing is that the XS 750 and 850 Yamaha's were pledged with problems starting with electrical and mechanical.They had notoriously bad transmissions and blew crankshafts on a regular basis.
  If you are going to go ahead with the project get the used engine and do a complete swap it will be the cheapest and simplest fix.

Anonymous · Answer

First choice:  I'd go for the used engine.
Other info:
-1st oversize would not present a significant imbalance in a non-racing motor.
-I would just hone the cylinders to clean them up, and replace all the piston rings.  You don't want to have to disassemble again, and have to buy another set of gaskets.

Stephen · Answer

I think you should put another engine in it,and work on building that old motor while you ride on the new one,take youre time  do it in stages,so the cost wont be all at one time,this way you will always have a work horse bike running,even if you up grade you still got a spare set of wheels,its best to go one or two sizes over all the same.

Anonymous · Answer

I think first you need to ask yourself why are you boring them at all? Is the rust or damage sufficient enough to warrant boring or would a simple hone clean them up enough to leave them within stock limits obviating the need for all this boring jibberish? Clean them up first and measure with a snap gauge and caliper and see if they are out of spec. Then you can make a decision. Oh, your buddy is a nice sort but keep him well away from engines, Just sayin'.

Andy Pandy · Answer

It's a heck of a long time since I was boring (girls) cylinders out for racing bikes but from memory if you don't have a ''match'' between the pots you'll nacka the big ends up, disproportionate load department, and should this happen then you have trashed the thing completely.

Honda XR650RRRRR · Answer

Go for the used engine, cheaper in the long run and a LOT less work.

ridingorracing · Answer

molitar is correct, don't bore if you don't have to.  very little is gained in HP.
first check and see if it needs it.

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boring out cylinder(s) on an XS850?

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