1973 Z1 900 Cylinder Condition

I posted on my project page, but wanting to post here for more visibility.

I have the head off doing valves on my 1973 Z1, and have questions on the condition of the cylinders. Pics below. Should I bolt it back up as is? New rings with a hone? go .5mm oversize? Something else? Currently bore measures the stock 66mm. Funds are limited due to the rest of the $$ I've already put into this project.

Compression was 125+ on cylinders 1, 3, & 4. Was down to about 105 on cylinder #2. I didn't do a leak down test before I pulled the head.

Valve seats were clean, just a decent amount of carbon in the domes. Seats clean makes me think they're seating OK. Intake guides were pretty loose, and a couple seals leaky on 1 & 2.

Cylinder #2 looks OK at a glance (once again more carbon), No major scoring or gouges from what I can tell. If I shine a flashlight on it, I can see some vertical marks, but doesn't seem too bad, though I'm no expert. Nothing that I could even catch a finger nail on, felt completely smooth. (Last Picture) I can feel a slight lip at the very top of all of the cylinders.





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Just to note, the bike seemed to run pretty well beforehand.

Do you have access to a bore gauge? If the out of round and taper is .001" or less than a bottle brush hone could be used to prep the cylinder for new rings.

Nessism wrote: Do you have access to a bore gauge? If the out of round and taper is .001" or less than a bottle brush hone could be used to prep the cylinder for new rings.

I don't, but looks like harbor freight has a cheap set that might work good enough.

Pics of the pistons?
Steve

SWest wrote: Pics of the pistons?
Steve

Anything specific you want to see? Right now the cylinder is still on the bike.

Wear etc. Be sure to get all the carbon. It will scratch the walls.
Steve

gvsu141 wrote:

Nessism wrote: Do you have access to a bore gauge? If the out of round and taper is .001" or less than a bottle brush hone could be used to prep the cylinder for new rings.

I don't, but looks like harbor freight has a cheap set that might work good enough.

If you use an actual bore gauge with dial you need a micrometer to set it up. You can get by with a set of calipers but micrometer is the way to go. They also have T bore gauges but I prefer the gauge.

Set the micrometer to the bore diameter, install specified adapter to bore range on gauge, set the dial bore gauge within the micrometer and set the dial to zero, insert the gauge into cylinder and read the runout. Bottom, middle and top 12-6:00 and 3-9:00. If the runout falls within spec, you're good.

I rechecked my notes from a while ago when I did the compression test. Turns out my actual readings were 130/105/118/119. However, this was with several tight valves, so hard to tell much from that alone.

I'm may just end up going a size or two over anyways since it's apart. Would be pretty much a brand new engine.

It's tempting to go with a set of Weisco 1015 10.5, but I should just probably keep it stock. I have my ZRX to hot rod in.

See how the top of the pistons are clean around the edges, carbon mostly in the middle? An engineer once told me they call that the "halo effect" from excessive oil getting by the rings.

if the bore is good a hone and new rings with the proper gap should be all that's needed.

save the bores!!!!they are only that size once!

leon