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I think the issue is the bore being out of round though, it's 7 thou out at the bottom (too narrow). 4.6 thou where the rings get to.
I can not fathom that the piston even fits into the cylinder if the bore is .007 inches to small from side to side.

The front to back piston to cylinder clearance has a normal maximum of .0028 inches of clearance.
Can you turn your piston sideways and insert it into the cylinder bore (without the rings on it)?
Then one can use simple feeler gauges to measure the piston Skirt to cylinder clearance at many different points inside the cylinder bore.
Not quite as accurate as a Good Dial Bore Gauge, but pretty darn close!
 

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Discussion Starter · #63 ·
OK so with the piston skirts going front to rear (as per a normal install), I get a bit of clearance. Annoyingly my feeler gauges are missing the one closest to .0028, but I can get .0015 in ok, and .00315 will push in but it pushes the piston over a bit.

With the piston skirts being side to side, my smallest gauge (.0015) will not fit in. Can just squeeze it to the side of the skirt, and then gets locked. So basically the skirts are touching, or very close to touching the sides of the cylinder wall.
 

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Discussion Starter · #65 · (Edited)
Same result, side to side the piston just locks up in the bore, front to back it's fine. It catches about halfway down the piston skirt when I'm pushing the piston in from the bottom. The piston ring area fits fine in to the bore in any orientation.
 

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Take the individual rings (top) & (second) insert them into the bore(one at a time), square them up by using the crown of the piston and use the feeler gauges to measure 'ring end gap' at different elevations in the cylinder. Let's see where you're at.
 

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Rectangle Font Parallel Pattern Number

From JE pistons website, use the top application listed HP/street strip.
 
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Discussion Starter · #68 · (Edited)
RIng gap top is good, .019" (schnitz says .018", is that right?) .Ring gap at the bottom is next to zero, can just get 0.0015" in there. Frankly I think I'm lucky this thing didn't bust any rings!

According to the above, I need 18 thou top ring, 20 thou 2nd ring? When we initially did the 685, we did both rings for 18 thou gap.
 

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I reckon they screwed the pooch on that bore job!
Precisely.

Have a 'quality' machine shop square it up and use a 'plateau' hone to finish it. If they give you grief about it let them install a new sleeve and get it right.

The second ring needs to have a slightly wider gap than the top ring.

I trust EM machining more than a lot of others. Most other 'retailers'/'shops' are farming out the machine work. Over time they become complacent and quality suffers.

Have patience, you'll get her dialed in and it'll all be worth it in the end.

Best wishes
 
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I still think it's a good idea to toss some fresh stem seals because you do have symptoms pointing in that direction as well.

Is there much 'oil reversion' in any of the ports or are they looking proper?
 
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Forgive me, when referring to reversion I'm talking about a phenomenon that causes the air/fuel mixture to re-enter the ports(intake usually) as the piston comes up to TDC just before the valves close. The airflow in a running engine basically oscillates with every intake cycle. It deposits residue/carbon in the port and onto the back side of the valves especially if oil is present in the mixture (ie: from leaking stem seals ect.). Hope this helps.

Side note: Most OEM heads/ports have this problem. Most of the expenses in modifying a head for racing applications is spent eliminating/minimizing this issue. Even going as far as to create sonic wave pressure during valve overlap to force feed the chamber. Likewise, a lot of work goes into exhaust gas scavaging but that's a whole other topic.
 

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Discussion Starter · #75 · (Edited)
Alrighty then, I got the cylinder to a very reputable machine shop. These guys are known around here to be very good. So, they did confirm the out of round as I had pointed out here. i.e. too narrow side to side at the bottom. But, they don't think it's a problem. They gave it a hone, said put new rings in it. They mentioned the valve seals too as a possible culprit, and that potentially I boofed something up putting it together. e.g. Putting oil on the barrel. Now I would have put it together as per instructions, but it was a long time ago, so who knows.

They said they can't fix up the bore being too small at the bottom, would need a new sleeve to work with. I don't understand why, but they are the experts.

So I guess I'll put it together, change valve seals, and hope for the best. They also said fire it up, and ride it pretty hard, don't let the thing idle.

Thoughts?
 

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Ok, couple cents for ya as follows:
e.g. Putting oil on the barrel. Now I would have put it together as per instructions, but it was a long time ago, so who knows.
It's perfectly OK to wipe a 'light' coat of oil onto the bore/rings so it has some lubrication on initial start-up; doesn't need to be heavily coated. DO NOT use synthetic oil.

Next:
. They also said fire it up, and ride it pretty hard, don't let the thing idle
What they mean is take it out riding and 'vary' the RPM and 'load' on the engine to help facilitate good ring sealing. Don't just idle it or ride at a steady rate.

Lastly:
Seeing how the dimensional issue was at the bottom of the cylinder where the rings/piston are moving slowly (relatively speaking) and not under compression load, they're right about it.

Upon re-assembly make certain that the 'second ring' is oriented in the proper direction and that the ring end gaps are sufficient. Based on the look of the chamber and piston crown I believe your biggest issue was the valve stem seals but, also potentially that second ring being upsidedown.

Edit: do a check and make sure your crankcase vent line is unobstructed and free flowing.

Keep confidence, you'll get it done.

Best wishes and remember, ride hard, ride often, for life is short.
 
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