When to replace pistons and valves?

Shamrox · New Member Joined: Sep 09, 2006 Posts: 6

Hey guys. New guy here. I picked up a 73 TX650. I'm doing a top end rebuild b/c it's been sitting so long and had a lot of rust build up and other stuff all over the motor, I figured it would be wise.

So, I have torn apart the motor (first rebuild for me). The valves and tops of the pistons look pretty bad to me, but how do you really know when they need replacing? Attached are a few pics of them after opening the motor. I've cleaned them up a bit since. Any thoughts?

royfisk

I cannot find your pictures but I would have a machinist or better yet a local bike shop measure your cylinder bores and inspect them for wear and out of roundness. Then have him inspect your pistons. pistons you would be looking at the sides for deep scores and wear. It is more important that the piston rides true to the cylinder then the tops of the piston. You see the seal between the cylinder and the piston is what builds compression. Yes I know higher dome pistons build more compression, but ALL PISTONS ARE USELESS IF THERE NOT SEALED CORRECTLY. Unless your racing I wouldnt be to concerned with the tops, ( UNLESS THEY WERE DAMAGED DURIN DISSASSEMBLY). As for valves and seats you probably dont have the correct tools to properly do the heads anyhow. I would bring them and have them done by a notable mechanic. Now ALL of the items you have talked about are covered in the service manual. WHICH YOU NEED. Most of the bikes that I have encountered only need to have the valves re seated. real easy with some valve grinding compound and a $5.00 hand lapper at your local auto parts store. Are there ANY scores running up and down your cylinders? If so when you run your thumb nail across them does it catch the score? If so you will probably need to bore and buy new pistons, If you cannot catch a nail you may be able to get by with honeing your cylinders and buying new rings. You need to decide what your plans for the bike are, simply because it is cheaper to do the job once, and if performance mods are in the future now is the time to do them. Good luck and get a manual. I personally like the clymer better then the chilton I dont have a haynes to compare to I need to get one but I am sure that any of them and this web site will get you through your new endeavor. again good luck

Shamrox · New Member Joined: Sep 09, 2006 Posts: 6

Thanks Roy.

There is def not any scoring on the sides. I checked that area first, even with my limited knowledge of engine internals, I knew that would be a problem area. That looks ok. The tops where pretty black but have cleaned up decently. It wasn't a runner when I got it so I couldn't check compression. Also not sure why the pics aren't loading.

Good info in your post, much appreciated. Keep you all posted.

royfisk

I wouldnt worry about the pistons but would check out the cylinder to make sure it isnt worn out . they have a tendancy to wear a arch into them opposite of the wrist pins. This wear will cause the rings to tighten up and loosen up while the piston travels up and down the cylinder. You would check this with a cylinder bore gauge. If this is all good, your styleing and need to order new rings and hone the cylinder, then reassemble.

Shamrox · New Member Joined: Sep 09, 2006 Posts: 6

Do you happen to know a source for rings for a 1st over piston? Apparently there isn't much demand for 256 1st overs....

royfisk

Do you need them? What size pistons do you have now? They should be marked on the tops of the pistons. if no # then your stock bore. if it says .010 then your that over bore etc.

Shamrox · New Member Joined: Sep 09, 2006 Posts: 6

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