White Trash Bobber Resurection and Top end rebuild...IT RUNS!!!!!

I also forgot to mention that since everything is all apart, I didnt want to take any chances on the carbs either, so I sent them off to Rick at Oldskoolcarbs for a complete cleaning and rebuild. From what I've seen and heard here, his work is top notch, so I'm excited to get them back good as new.
 
No updates today. Still waiting on parts. I did get a new addtion to my tool chest, a 3 drawer middle chest. So I spent a little time cleaning and reorganizing my tool chest. Hopefully the parts come in soon, and I can really get moving on putting this all back together.
 
GLJ said:
You're at the upper end of specs. Will it run Yes. For how long? Who knows.
Click to expand...

I’m just getting caught up reading this thread. It really is amazing how these motors can run when they are really out of shape. When I bought my XS2 , the guy started it up, and I shot a video that’s posted at the very beginning of my build thread. The motor was running with These Jugs !
A767B50D-17AA-4D21-8D5B-4693776F9618.jpeg
D5941D71-B7B8-45C1-B51F-F613490FFD35.jpeg
 
Got me a little ol' box o' parts from MikesXS today. Now I just have to see when I have some days off again, and I can start gettin this sucker back together.
 
All right, I had a few minutes to measure up the new rings. I measured both sets in both cylinders. I had trouble measuring the oil rings because they are not "springy" enough to hold themselves snugly against the cylinder wall, but was able to get an approximatation. I know the 2 oil rails are the same size, and the oil control is obvious, but which which compression ring goes in the top grove? One of them appears slightly thicker and wider edge to edge. I think the smaller of the 2 goes in the top grove on the piston?

Heres the numbers I got when I measured:

Set #1 in Left Cylinder

Top ring: .008"/.203mm
Second Ring: .010"/.254mm
Oil Rail 1: .021"/.533mm
Oil Rail 2: .018"/.457mm
Oil Ring: * .025"

Set#1 in Right Cylinder

Top Ring: .008"/.203mm
Second Ring: .010"/.254mm
Oil Rail 1: .021"/.533mm
Oil Rail 2: .017".432mm
Oil Ring: *.025

Set #2 Left Cylinder

Top Ring: .008"/.203mm
Second Ring: .011"/.279mm
Oil Rail 1: .019"/.483mm
Oil Rail 2: .021"/.553mm
Oil Rail: * unable to measure - ring nearly overlapping

Set #2 Right Cylinder

Top Ring: .008"/.203mm
Second Ring: .011"/.279mm
Oil Rail 1: .020"/.508mm
Oil Rail 2: .020"/.508mm
Oil Ring: * unable to measure - ring nearly overlapping

So based on all of that, I should be able to file/grind/set the gaps as needed. I saved a thread somwhere that had all off the correct gap specs, but now I cant seem to find it. Anyone got an idea of all the proper gaps for the the upper and lower compression rings, and the rails and control ring?

I also noticed that on the base of the cylinder there is no grove for the cylinder O-rings. It is a MikeXS 750 big bore cylinder. Should I put them on anyway? Or will that interfere with properly sealing the cylinder to the base. Or should I just leave them off?
 
Reading back through some other posts about rings gaps on a Mikes 750 kit, it seems like my oil rails and control rings should be ok, and maybe just a need a couple thousandths off the compression rings. Maybe I can hone the cylinders a little more to clean up some of the vertical scratches and may open up the gaps to right where they need to be?

Thoughts or recommendations?
 
lidirtrider said:
Thoughts or recommendations?
Click to expand...
Be mindful of your skirt clearance.
A better way would be to hone until either your scoring disappears or you reach the skirt clearance limit... whichever comes first. After that's done, adjust your gaps accordingly.
 
Thanks Jim,

Right...I guess I didnt clarify that to well. I was meaning to hone a little more to clean up the cylinders, and THEN, check and adjust the gaps as needed. But yes, I will also need to consider the skirt clearance. There is a lot to account for, and I love learning, so thank you for the advice. Looking at the above numbers for the rings, what do you think of how it all stands as is? Just open the 2 compressioin rings?

I want to make sure I have pretty good grasp of all this before I either try more honing or adjust the gaps.

And unless I just keep missing it in both my manuals, and on here, I cant seem to find the oil conrol ring gap listed.
 
Here's the gaps from the Yamaha book.... including the oil ring. The idea is to make the gaps as small as possible while staying inside the range listed.

IMG_20190730_170003.jpg
 
Wow... .030" max ring gap is.... rather large. Granted, I'm not an XS expert, but I'd question how Mikes (or whoever made these pistons) came up with that. Lycoming (my world) has a factory max the same.... .030".... but that's for a piston that's over 5" in diameter. Air cooled, steel jugs and rings... basically the same design. If it were me, I'd be sorely tempted to stick with Yamaha's tolerances for the itty bitty pistons we're talking about.

Untitled.png
 
Agreed. That was an old spec sheet that (I think) 650skull saved long ago, which I revised to correct translation errors for another member, a few years ago. The gaps may have been made generous because it may have been assumed that those 750 kits were going into racing applications, or somebody just did a wild guess.

OTOH, that 0.030" max may be the service limit. It's in line with the Lyc's 0.047".

Going from a 75mm bore to an 80mm bore is just a 7% increase, which would call for a minuscule 0.001" increase in the ring gap specs...
 
Last edited:
Hmm.. interesting stuff to consider.

Here's what the cylinder O-rings look like. I've read that some guys with the 750 kits omitted them with no problems. Think I should leave em on or run without? I'm concerned that the cylinder wont be able to compress down enough to seal properly.




IMG_6040.JPG


IMG_6041.JPG
 
There's no groove machined into the aluminum casting for an o-ring... I'd just leave 'em off.
I've never understood why Yamaha used a gasket and o-ring in the first place. The gasket by itself will work just fine.
 
Cool, thanks fellas. Been a busy couple weeks going back to work. Gonna try to make some progress this weekend. I've been studying Jim's rebuild thread like crazy, and as long as I can carve out the time, I think I can get it all back together this weekend.
 
Didnt have as much time as I thought I would this weekend so didnt get to do any work. I’ll have time this afternoon, amd I’m going to cut the rings/set the gaps, amd install the rings onto the pistons.

I was thinking that if I have time, I’d install the pistons and the cylinder. My questions is should I wait to do that until I have time to fully assemble the engine? Would it be bad to just set the cylinder and wait a few days to instal the head? Or will that not really affect anything as far as torquing everything down. I just want to do it as correctly as possible.
 
As long as you're using a non-hardening gasket sealer, You'll be fine.
 

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