TOPIC:

Got the bad piston off...do I need to replace the good one too if I don't need a rebore? 



Been working the scuffs with muriatic acid, lovely stuff .  Its either getting cleaner or getting closer to the scoring, but I remain optimistic that I won't need a rebore especially after pricing it. [$100 each bore, and definitely need 2 pistons for another $150, not to mention any valve work] . Maybe just a little sanding/honing??

The only way to know for sure is to have a machinist carefully measure it. Typically, it takes six measurements using a high quality bore gauge. I am pretty sure the fsm  describes the procedure and lists the acceptable tolerance. 

I'm not sure you need a new piston, but I'm wondering why it was rubbing on the cylinder wall?  Were the rings missing at that point on the piston?  Was the wrist pin worn, causing play?  Or was the connecting rod side play outside of spec?  

If you have a micrometer you should carefully measure the wrist pins and the inside of the wrist pin hole on the piston.  Also check the side play on your connecting rods.  If you don't have a micrometer....take the parts to a machinist to be measured.  Your manual will have specs for those measurements, along with the pistons themselves and the cylinders.

 I want to know too.  I went back 3 years and found the last post of mine when it was still running:

My KZ305 has been getting harder and harder to start, and was burning lean on one cylinder. Then I noticed the tiny wisp of smoke coming from the lean side of the engine. Leaned over and saw the oil bubbling out of the cylinder head gasket.

Does this description along with the pictures of cruddy intake valve on the right side help me understand what caused my problems in the first place, so I make sure I fix the cause? I wish I wrote down which side was lean back in 2018, but I didn't.  However the right side has the scuffed piston and the right side has blued exhaust so it may have been the right side. The exhaust was blued a lot more than 3 years ago however.  I do seeeeeem to remember noticing the HG leak on that side too, which all points to that being the lean side.

I will look into getting things measured. My hard copy service manual came today which is way easier to use than the pdf. But I didn't notice anything amiss when I pulled the piston. It was snug on the pin and the pin was clipped. I don't notice anything wrong with the rings but a)I'm a newbee and b) I might have to take em off to notice uneven wear.

You are clearly burning oil in at least one cylinder possibly fromm the  piston having excessive clearance and rocking in the bore resulting in blow by.

ideal top end rebuild minimum:
buy NOS oversized pistons, wrist pins, rings, circlips  $400-500? ebay has a couple items hard to find many options
bore and hone the block $120 - 180
buy new valve seals - $10
3 angle valve job, deck head, and rebuild - $300-400
gaskets $40-60

Its possible to rebuild the head yourself with only a few special tools ($40-60), but if you need the head decked and valve job (likely) its better to have the machinist assemble for a small amount more.  i would rate cylinder head rebuild as medium to advanced (if it only needs lapping  - no machine work)

Since you are on a budget it may be possible to find a decent cylinder block with matching pistons. If not excessively worn you can get away with a hone and new rings, i would have machine shop measure and evaluate.
 

I want to mention something that I read a couple weeks ago - under the clutch cover and behind the oil pump there's an oil strainer. 

 

If that's clogged it will have affected oil flow - a new clutch cover gasket is less than $20, I think, so it may be worth checking. 

As far as your cylinders: if the non-scored piston and cylinder are in usable condition, it may be cheaper to have the damaged side resleeved instead of buying oversized pistons - worth looking into?

Ah yes, thanks for reminding me about that oil uptake. That "gauze strainer" as he called it was the ridicule of a guy in britain/europe who tore down a GPZ305, the only semi-useful record I have found of a teardown of this engine. I may have linked to that thread earlier but my short term memory only goes back a couple days. If it is easily accessible I will have a look at it, I already have the clutch gasket.

I appreciate everyone's input. I found a machine shop that specializes in bikes and dropped the head and jugs off. While I waited he did something in the back with the cylinders, honed them I presume, and came back to show the skid marks were gone...almost. He said it would absolutely run and that it would just run fine like that, it just wouldn't be perfect. He said if I was racing it he would suggest a rebore.  He wanted to leave it up to my judgment and I told him I have no judgment on such things. I'm going to wait for him to give me a quote on the head/valve work before deciding. Of course he said that piston would have to go, at the least.

 

There you go!  Things are moving along quite nicely!  Is he going to measure the cylinder bores?  

Considering the low mileage on the engine the bores will be fine.  

The head shouldn't take much to clean up either.

A new piston and two new sets of rings and you will be well on your way!

Oh, and tend to the oiling system.  There was a thread here recently where the weakness in the system were discussed so you might want to find that thread and do all you can to fix that.

wrote:

There you go!  Things are moving along quite nicely!  Is he going to measure the cylinder bores?  
 

I presume so, but I will make sure he does. I found the maximum spec.

wrote:

Considering the low mileage on the engine the bores will be fine.  

The head shouldn't take much to clean up either.

A new piston and two new sets of rings and you will be well on your way!

Oh, and tend to the oiling system.  There was a thread here recently where the weakness in the system were discussed so you might want to find that thread and do all you can to fix that.

 

Unless his quote on the head changes my mind I already decided to go as is and just replace the one piston. It saves me $160 for the two bores, $75 on a piston, and saves me from having wasted the $60 on standard rings that I foolishly (or presciently!) already ordered. This $$$ can all go into the kitty for snagging the next cheap KZ440 that comes along needing TLC and I'll have this to ride while I TLC it. And it really wasn't hard tearing it down so if I have to do it again in a few years, no biggy. maybe I'll be richer by then. But the machinist thought it was probably just from running lean, so if I fix that I shouldn't have a repeat any time too soon.

I found this thread that talks about a long bolt blocking a gallery and also mentions the lame strainer mentioned a few posts above. I presume this is the one you referred to. I guess it won't hurt to pull the clutch cover and see if I'm frightened by what I see or not...a little more info on where to find that bolt might be helpful but maybe it will be obvious.

www.kzrider.com/forum/2-engine/614557-kz...blowing-their-cranks

 

It's up to you but it's an easy measurement and your machinist will have the tools.  I would never pull the cylinders and not check the bores.  At least you'll know they are OK.