Engine rebuild -- got the oversized pistons. Now what?

[robsGS850]

Hey everybody.
So I've somehow gotten ahold of four .05mm oversized pistons + rings, and with the rainy season's coming, figured it's time to give my 110k+ bike a rebuild. (Compression numbers are all hovering about 5psi or so over what Clymer lists as service limit.. runs like a top though)

I'm guessing it's a matter of taking the engine apart and bringing the cylinders to a rebuilder for a rebore ( leaving the pisons stickin' out of the lower end and the head on my workbench, right? All they need is the barrels?), then reassemble. Right? Close? Sorry if this is basic mechanic stuff.

Anything I'm clearly missing? I'm sorta aware of something called bushings that sometimes get replaced... I think they're the wrist pins in the crankshaft? Am I way off?

Any advice or links to point me in the right direction before i screw this up would be appreciated.
Thanks!

 

[txironhead]

Get a factory service manual! That's the most important thing. You might even look at www.repairmanualclub.com to see if you can find one for your bike you can download and print out.

It's generally a bad idea to order pistons before taking the cylinders to a machine shop, because say you need 10 over instead of 5, then you get to try to return the 5 over. Most places won't take 'em back. With your compression numbers, though, you should be okay.

You're going to need, minimum, a top end rebuild gasket kit. It's also a good idea while you're there to lap the valves (I'd spring for a professional valve job) and replace the valve seals. Check the tension on the valve springs. MAKE SURE you understand how to properly set the cam chain specs or you'll end up punching your valves through your nice, new pistons. This is where the manual comes in. New exhaust gaskets, if not included in the kit, are also a good idea. Also, not all kits include the four rubber half-moon valve cover seals.

You're going to need a piston ring compressor made for motorcycles. Also, be very careful removing the wrist pins as they can be damaged if you're careless.

I cannot stress enough how important a manual is. Get one and read the chapters on top end rebuilding several times before you pick up a wrench. Trust me, it'll save a ton of headaches later. You may not need all of the "special tools" listed, but some of the basics are the piston ring compressor, valve adjustment tool, feeler gauges, torque wrench (lb-ft and in-ft(?)), and a rubber mallet. Not to mention a good-quality set of metric sockets and wrenches.

Not trying to scare you off of the project, just don't want to see you make a mistake that could cost a few hundred to repair.

 

[Nessism]

Take the pistons along with the cylinder to the machine shop. They will bore the cylinders to match the pistons. Get the clearance specification from the manual and make sure you tell the shop what you want the clearance to be.

I strongly recommend you take the head with you and get a valve job done. With the mileage you note on the engine you may have worn valve guides, and maybe the valves as well. The only way to know is to measure the various parts.

You are going to need a gasket kit for sure and I recommend new wrist pins and cam chain. These parts are not expensive but you will need a tool for installing the chain. Check Z1, or give them a call. They can hook you up with most of everything you need.

Good luck.

 

[focus frenzy]

it would have been best to have measured the cylinders first before ordering pistons as how do you know you need the oversized pistons?
might just need a good honing and fresh rings, or you might need the 1mm over pistons. my 1000G was just short of 100,000 miles when I did the top end and the cylinders were worn 1mm over. .5 over pistons would have been to small.

a good machine shop will need the new pistons so they can bore/ hone to the individual pistons. that way you compensate for slight variations in the pistons and the tolerances match across all four.