Cam chain idler gear, wear and tear. Service limit pistons?

Hi
I`m rebuilding my engine on my ZN1300 86-model witk 127.200 miles. I have a tread in another forum (American Voyager Association), but it seems to me that there are not so many active users there so I`m seeking advice in this forum.

I have to split the crank case since there are some problem with the transmission. At the same time I also will replace pistons rings, hone the cylinders, replace cam chain, gaskets, valve steams on so on.
I have already recived a new cam chain idler gear from Liska, I attach a picture of new and old cam chain idler gear. My cam chain have 128,7mm and the service limit is 128,9mm. So is about time to replace it.

I have measured the pistons and for ZN1300 the service limit is 61,85mm. My pistons have 61,82-61,85mm diameter. The question is, how critical is this? I`m thinking not to replace the pistons only the piston rings. The bore in cylinder block is below servic limit and okay.

The cylinder head will need some work, thinking to delivered to a machin shop. I have measured the cam shafts and they have exceeded their service limit, is that critical as well? They have no marks

Thanks for all help

New Liska and old oem gear


Marks from cam chain idler gear at the crank case

Page 183 in the manual quotes the "as new" spec for piston cylinder clearance @ .031 - .058 mm (.0012"- .0023") which is tight in my mind. I don't like to see less than .076 mm (.003"). Normally I would shoot for .076 mm (.003") on a rebuild. Less chance of scuffing a piston on break in.

If you pushed the service limits of the piston (61.80 mm) and cylinder limit (62.1 mm) that would be a piston/cylinder clearance of .3 mm (.012") and that's getting sloppy. (pretty loose)

In my mind, any more than .2 mm (.008") piston /cylinder clearance should be dealt with, especially since you've torn the engine down. There's no way you can rebuild this engine now with new rings and a cylinder hone and end up with a reasonable seal between the piston rings/pistons/cylinder walls. Loose pistons in the cylinder bore will cock around and that can lead to piston ring flutter which will allow ring blow by and blow down past the piston. Blow down leads to dry cylinder walls and eventually can cause a piston seizure from lack of oil between the piston and cylinder wall. Even more so on high revving engines.

I also have to ask- Do I see a crack in the upper engine case ?

Thanks for your reply, Kawboy. No, I don’t think there are cracks (don’t give me a heart attack) I will take a better look when I get home

Hi. I’m trying to split the crankcase, but it’s stuck at the alternator rotor/stator side.
I have followed every points in page 98 and every bolts are out. It seems to me that I also have to remove the pickup coil assembly? Is this correct? Is the pickup coil assembly attached to the upper and lower crankcase?

I believe the pick up coils bracket is screwed to both upper an lower case. You'd think the holes in the rotor are made to undue them... but they don't align in your pics. I think you'd need to remove the rotor.

Special tool for removing the rotor arrived today Splitting the crankcase was easy. But what do you guys think about the crankshaft bearings? They don’t look good...right?