Top end rebuild

[tommer2]

Hi All. Need some advice. Just took down my 75 r90s top end. Looks in great shape. A couple things. First the Cylinders base gasket was installed dry. Nothing at all. I thought it needed something like Yama bond or something similar. I had no leaks or anything so I'm not sure. I've always used Yamabond on other builds. Secondly the cylinders look really good. I can still see a little bit of cross hatch. Not much but some. The piston and rings look clean with a little carbon on top. Question, do I need to hone and use new rings or put it back together. It ran well and never smoked at all. I'm on a budget like most of us!! Thanks in advance for your input.

[Rob]

No leaks = VUNDERBAR!!!

You got a good one! Who knows if it will leak if put back together dry, or not. Your call. Sounds like you already know to be careful of oil passages.

[Major Softie]

I would measure for taper since they're open, but I wouldn't mess with perfection. I'd just like to know what kind of future I'm looking at.

Some might do a light hone. I'd just bolt it back up. I'd also Yamabond it this time, even if it was nice and dry without it. If you're re-using the base gasket, it's already been compressed once. If you're not, who knows if you'd be as lucky as the last guy.

[enigmaT120]

Why did you take it apart?

[tommer2]

I want to make it PURDY!! All kidding aside I'm doing a total job. Powder coat fame and swing arm , polished the rim,relaced the wheels with SS spokes and nipples, new paint ect. ect. The jugs and cylinders looked like crap!!!

[Duane Ausherman]

We put many hundreds together from all sorts of condition. You didn't tell us the compression and how it was measured, but now it is too late. This is an important item.

In this case we would have run a ball hone (often called rabbit turd hone) in the cylinder lightly with some oil. It should just be enough so that the carbon on the cylinder just above the wear marks for the top ring is cleaned off. Or, you can scrape it off too. What you want, besides a very light hone, is to examine the cylinder wear. It is always the most in the top 3-4 mm. If you can feel a tiny ridge with your fingernail, then I might measure it for wear. In your case, since it was working just fine, I probably wouldn't bother. You can learn how to measure it without any special tools on my website. W6REC.com If you wish to measure ring wear, remove the top ring and install it into the bottom of the cylinder where there is no wear. Measure the gap with feeler gauges. The gap should be .003" for each inch of piston diameter, or about .010" total. I probably would reuse it even if the showed .020" as it is so easy to remove later for more work.

We assembled engines with all used parts that didn't meet specs and they never saw each other before. Generally they worked just fine. Also one can assemble an engine with everything perfect and in spec, still it can burn oil and smoke a bit. The variation in oil consumption varies widely from new bike to new bike. Go figure.

It is common for the base to not leak oil and also common for them to leak when installed dry. We would use the thinnest layer possible and reuse the metal gasket. They can be reused many times.

More questions, just ask.

[tommer2]

Thanks Duane. I was hoping you would respond. Great info!!!

[Duane Ausherman]

I fixed a few typos.

Let us know what you find and how it works out.