What constitutes a top end build?

Paddy399

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I'm at a loss. I picked up a bike ('81 Special) in the spring, have been riding it when I can, troubleshooting the little problems when I can. I think I have all the problems worked out except a pop at low RPM from the right cylinder that I'm 99% sure is a carb issue.

Here is my problem. I had planned when I bought the bike to ride the kinks out of it during the summer then, come winter, build it into a bobber or street tracker or something. I was planning on doing a top end rebuild as part of the build but now I'm wondering if I really need to.

That started another train of thought. What really goes into a top end build? Is it just gaskets and seals that get changed? Piston rings and pistons or just the rings? I understand it all depends on what it looks like when I open it up.

I suppose I should also state that I'm not sure that I've ridden all the kinks out of it. I didn't get as many chances to take it out this summer as I would have liked.

Tell me what to do! Rebuild the motor or not?!?!? I should just stop my blabbing and start the build and a build thread...
 
When I do one, I check everything from the top down.

Do the rockers look pitted or damaged? Do the tappet ends have pitting or damage? Then do the rocker shaft pins look ok? Is the valve/valve guide clearance within spec? Are the springs the proper length? Are the valves pitted on the end of the stem, near the valve seals, or on the sealing surface in the combustion chamber?

If any is NO, then I replace/repair it to be within spec. All the specs are in the book. I always go a valve grind and fully clean the head/combustion chamber and ports to like new.

is the cam ok? is it varnished with old gummy oil or pitted with rust from being dry too long? If needed, I'll polish or replace the camshaft.

For the pistons, I look at the skirts for damage or wear. I measure the bore to be within spec. I check for damage to the bore (pitting, rust, deep grooves, etc..) I almost always replace rings, and fully clean the cylinders and pistons to as new.

While i have the pistons off, I check the rods for wear. I like to see 80% or more of the copper colored coating in the small ends of the rods still intact, and I check the big end for too much play.

All these specs are in the book, if you get some measuring tools and a sit down with the book, you can easily find your trouble spots and fix em good.

I always replace the head washers with Brass, and I chase every thread with a good tap to ensure good clean threads and proper torque specs...


I always replace every seal and gasket, along with a new camchain and front guide (even if they don't "look" bad, I like the peace of mind)

Replace copper crush washers on the oil pipe banjo bolts.

I hope that helps. I'm sure I'm missing a few things...
 
If the engine is not using tons of oil, has reasonable power, starts up when you want it to, does not make strange loud mechanical noises, no need to do a re-build. Sometimes a compression test is used to gauge an engine's wear, but its not a perfect indicator.

You are correct, in that you don't know what parts need replacing and/or machining until you open the engine. The internal parts are measured (special tools) against the specs that Yamaha put in the Service Manual. Out of spec parts are replaced or may need to be machined.
 
Since you don't seem to be putting lots of miles on it, I'd say ride it a couple seasons and work all the little kinks out. I rode mine 2 seasons before going into the top end. One of the main reasons I did and probably why many, if not most, others do is that failing front cam chain guide. They're going bad on most of these motors now mostly due to age.

In the 1st two seasons, I got the bike running well, fixed the suspension and handling, and diagnosed that I had a failing front cam chain guide. I gave it a new chain and front guide, mild porting, a valve lapping and new guide seals, and rings. I could have bored it but decided not to. It will be bored next time I'm in there, probably within the next few years.
 
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