This thread details the reassembly of the top end of my 1980SG. Everything's already cleaned up and measured, but I'll backtrack a little and show how to do some measurements.
All manual references used here are from the 1978-80 Yamaha Service Manual. All points of view (left and right, fore and aft) are referenced to the riders point of view
I'm writing this for anyone who's never done a top end before. If you have, you probably won't learn anything new.... or maybe you will. Keep in mind, this is how "I" do it. If you do something different that will add to the usefulness of this thread, by all means jump into the discussion.
Before we get started, you need to check the big and little ends of your connecting rods. I did this while the bottom end was disassembled and unfortunately, didn't take any pics of it. Pg. 27 of the manual tells you what to check for and the dimensions. You need to do those checks before starting the top end reassembly. If you can't figure out how to do it, bring it up in the comments and I'm sure someone will do some 'splainin'.
Let's get started.
First off: Sealant. I use Permatex Aviation sealant....
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The brush inside the cap is a little unwieldy. I use a Q-tip for better control and less mess. I've overhauled or repaired hundreds of engines in my career and this is all I've ever used. It's never let me down. If you prefer Yamabond or some other sealant... then by all means, use what works for you.
Since you already have the top end apart, now would be a good time to replace your stretched out cam chain. Use a piece of safety wire to join the new chain to the old one and work it into place.
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Once the chain's through, use some safety wire to tie both ends together and tied to one of the studs. Ya don't wanna lose it back down in the engine...
The next thing I did was install the cam chain tensioner. It's a new one from Mikes because I couldn't find an OEM one. I hope I don't come to regret that. There's 2 long and 2 short screws. The long screws go on the right.
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The 4 screws are JIS... not phillips. A phillips screwdriver will strip the heads. You can modify a #2 phillips by grinding the point like this.....
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I find that this works just as good as a JIS screwdriver. Oh... and I put a little blue Loctite on the screws, then tighten to 85 in.lbs. If you're worried about dropping screws and washers, do it this way....
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Next, make sure the two alignment bushings are installed (red arrows) and install the base gasket.
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I know some people don't use gasket sealer here, just grease or oil.... I do. Just make sure you get it on the right side/bottom of the gasket so you're not makin' a mess as you go (test fit it if you're not sure. It only fits one way). Make sure you re-secure the cam chain after you slide the gasket down. Rotate the crank so that the connecting rods are up at TDC and secure with rubber bands as shown. This keeps the rods centered as you install the pistons.
OK, on to the cylinders. I like to hone them right away and do all the measuring afterward. Honing knocks the glaze (shine) off the cylinder walls. This gives a scratched surface that aids in ring seating and in oil retention (lubrication). There's tons of videos out there if you've never done it before. Seems everybody has their own techniques and opinions on the best way to hone a cylinder. I've found this short one that does a fair explanation on how to do it.
You can look at some of my pics below to get an idea of what the end result should look like. If you've never done this before, it might be worth getting an old cylinder and practicing first. It's up to you.... If, after honing you still have score marks in the cylinder, you might want to put a good pic of it up in the comments and let the "gurus" here give you their opinion on whether they're acceptable or you need to do an oversize bore job.
Providing the honing process went good, now's a good time to install the big o-rings in the bottom of the sleeves (top arrow) before you forget em.
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The bushings for the cam chain guide, technically called guide bar nuts (bottom arrow) have copper crush washers for oil sealing. There's several ways you can go here.... your best bet is to install new ones. Or you can use a torch to anneal them....
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All it takes is a propane torch to heat them to a bright orange and then quench them in water (thanks Dick). Don't hold the heat on em' for too long or they'll start to deform/melt on you. Now they're soft again and can be re-crushed into an oil tight seal. Your third choice is to spread a little sealant on the threads and torque em down. any one of these will work just fine for oil sealing. A word of caution here: There were reports in the past of the MikesXS bushings being too long, causing the cam chain guide to stick too far into the cylinder. If you're using aftermarket bushings, measure 'em against the originals and make sure they're the same.
OK, assuming you've got your cylinders all cleaned up, it's time to check them for serviceability. The next thing we're gonna' do is check the bores for excessive wear, roundness and taper. You'll need a telescoping gauge (usually just called a "T" gauge) and either a micrometer or caliper that can measure down to 4 decimal points (this is all written for inches, not metric). I've found that even many seasoned mechanics don't know the correct way to use a T gauge, so I've made a short video showing the correct way to use it.
Your manual (Pg. 26) has the dimensions and tolerances you're looking for and a picture showing where to check. There's 6 places to measure... towards the top, middle, and towards the bottom, then again at 90deg. to the first checks. From these measurements you can determine taper, roundness, and diameter. Write these down.
Now measure measure the piston skirt as shown on Pg 26. I use a wooden block dropped down inside the cylinder to rest the piston on for this check.
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This dimension, subtracted from the bore dimension (I use the mid point measurement for this) gives you your skirt clearance dimension. The specs. on Pg. 64 give a skirt clearance of .002" to .0022".
Lets talk about cylinder, piston and ring matching briefly. No two pistons are the same.... no two cylinders are the same... no two rings.... well, you get the picture. Measure both skirts and compare them to both cylinders.... then decide which piston is left and which is right. Think balance..... If one skirt is at the minimum and one at the maximum..... and you swap the pistons and now both are towards the max. but are the same..... Which configuration do you think is better? Exactly... you want balance. Do the same for the rings (next section). Mix and match until you can get the best balance of ring gaps.
An alternate way to check the skirt clearance is to use a feeler gauge.
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With the piston pushed to about midway into the cylinder (correctly oriented), a .002 feeler should be a snug fit and a .003 should immediately start to bind up as you insert it. I prefer to use tapered feeler gauges for this, but mine seem to have grown legs...
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OK, now let's see how flat the face is. You need something absolutely flat and rigid. I use a piece of 7075T6 aluminum angle.
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Move it around to various places on the face and see if you can slide a .001 to .002 feeler under it at any point. If you can, you might want to consider refacing it. I'm not gonna cover doing that here, but if you need to do it, bring it up in a comment and I'll (or someone else will) explain how to do a "backyard" refacing.
If all these checks are good, you are now the proud owner of a serviceable set of jugs.
I'm guessin' I'm about to hit my character limit on this comment. So, we'll continue below on the next one.
