First time engine rebuild

DB90

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So, I acquired a large amount of XS parts a while back and in that bunch of parts is what I believe to be a 78 basket case motor. The heads, jugs, pistons, cam chain, and cam chain guides have already been taken off. I know absolutely zero history about this motor. Judging by the condition of the pistons, they appear to be scorched like they got really really hot, I think this was taken apart for a top end rebuild but I don't know for certain.

I figured that this would be a good motor to learn how to work on these motors more in depth. I have zero experience with rebuilding motors. The extent of my knowledge ends at adjusting timing, points gaps and dialing in carburetors.

I know, before anyone says it that I need a manual which should be ordered within the next couple days. Suggestions on which manual is superior are welcomed!

So with all that said, what do y'all think my starting point should be? Should I dig into the bottom end and see if there are problems there before just doing a top end? I kind of figured since it's already almost all apart anyways that I might as well.

What do you gurus think?

I will post some pics this evening.
 
The bottom end can be checked out without disassembly pretty well. I wouldn't disassemble it unless you do run across a problem or an unknown. First thing I'd check is the copper cladding inside the con rod little ends. Give us some good pics of 'em.

I like the Yamaha book better than the rest of 'em... cept it don't go into great detail for engine rebuilds. It covers it, just not as thoroughly as it could.
For that, look on Amazon for an XS650 engine book by Hans Pahl.
Edit: found it. https://www.amazon.com/Yamaha-XS650-Engine-Including-Electric-ebook/dp/B071W5SSPY

Also, for just the top end, some yahoo here wrote a thread on putting 'em together. It's in Tech. I've never used it, so I can't vouch for how good it is... :wink2:
https://www.xs650.com/threads/xs650-top-end-buildup.52041/
 
The bottom end can be checked out without disassembly pretty well. I wouldn't disassemble it unless you do run across a problem or an unknown. First thing I'd check is the copper cladding inside the con rod little ends. Give us some good pics of 'em.

I like the Yamaha book better than the rest of 'em... cept it don't go into great detail for engine rebuilds. It covers it, just not as thoroughly as it could.
For that, look on Amazon for an XS650 engine book by Hans Pahl.
Edit: found it. https://www.amazon.com/Yamaha-XS650-Engine-Including-Electric-ebook/dp/B071W5SSPY

Also, for just the top end, some yahoo here wrote a thread on putting 'em together. It's in Tech. I've never used it, so I can't vouch for how good it is... :wink2:
https://www.xs650.com/threads/xs650-top-end-buildup.52041/
Thanks Jim! I'll get some pictures of the con rod small ends!

I downloaded the Hans book on Kindle just now. Great info in there! I'll definitely be buying the hard copy!

I'll download the PDF but I really enjoy having the hard copies. I love sitting down in the shop and just reading through them.
 
First thing I'd check is the copper cladding inside the con rod little ends. Give us some good pics of 'em.


To my eye they appear to not be damaged but they are rusty.

My second image won't upload but it looks almost identical to the one in the image
 

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That little end don't look happy. Shade tree test, just for fun, if you push a gudgeon pin/wrist pin in it should be a very close sliding fit. But if you can wiggle the ends of the pin up and down it's boogered.

Come to that, if you can feel roughness with a finger tip on the bearing surface, it's boogered.
 
So I was digging into the head on this motor. Taking everything out so I can get the whole motor vapor blasted. I came across what looks like a poor attempt at porting.

Pretty sure the valve seats need recut and the valves replaced. The angles on the valve seem kind of concave which I'm pretty sure in no bueno.
 

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