Kawi2strokes.com Forum

I finished it up for now. It did take a long time that's for sure. I need a better way to measure the bore. I am using a snap gauge and the same mic I am using for the piston. Accuracy aside, repeatability is what I was after. So even if the mics are not the best, Id expect them to be repeatable anyway.
I took 8 measurements of each, the resulting clearance, tossed out the high and the low and took an average.
I was shooting for .0059 and I am at .0058 as best as I can tell. It "feels" about right.
The crank I have was messed up as well...it really was sort of a rough motor but the price was right and I have a lot of extra parts already. The crank that was in it has a broken drive pin in it from the rotary valve and it is a hardened pin and below the surface.
The short of it is, I was able to make a plate to press the crank apart and I managed to do so last night at work. I have a spare crank, but the bearing surface is a little small...it is a bit of a slip fit. I know some people do that on purpose on smaller engines but Im not sure its a good idea and the big end bearing feels a little sketchy anyway.

So next I want to try and do my own crank. That would be the last thing I haven't done myself. This entire project is mostly something to mess around and learn with. At work, if my work orders are light and calls are quiet (nothing is breaking down) it can be tedious getting through the night so I like to have something to think about, plan etc. And I am able to carry small items in and work on them. It beats nodding off in a chair like some of my cohorts do.

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1972 S2 350, 1972 F9 350 Bighorn, 1975 KX400, 1981 YZ465, 1980 XT500 (Built for MX) , 1987 KD80, 1995 MZ Skorpion, TTR 125, 1994 KDX200 , 1978 Mobylette, 1985 KX350R-F91m (Frankenstein) and a Doodle Bug

What made you go with six thou clearance? I used to redo my top end when it got to six.

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If it surges, that's normal, upshift.

Piston clearance is well past "worn out, rebore" level. Ring seal will be compromised as well.

I bore 'em when they get to .002 and looser.

Lets hope you can get the crank back together, and trued correctly. Usually, Hurley and I had contests, which one of us could get them .000, or darned close to that.

I was a college kid commuting to school on the H2. I was getting about 10,000 miles to 0.006", starting at 0.002 with stock Kaw pistons. I was just going by the recommended clearance in the manual. It worked out OK, since I never had a piston failure. Wossner specifies 0.0027 clearance for their pistons.

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If it surges, that's normal, upshift.

I can't remember where the clearance ended up on my 80,000 mile original engine, probably about 6 thou or so - They do get a bit noisy tho!

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"One day, your life will flash before your eyes. Make sure it's worth watching." : anon.

I bet that one was more than 6, Rod.

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If it surges, that's normal, upshift.

A good cast piston likes close tolerances.

A forged piston needs looser clearances, because they are not expansion controlled.

The forged H2-R factory pistons collapsed their skirts in less than two laps. Pistons were gone then, ring seal compromised to 15 percent, lost power.

The factory developed a cast racer piston for us, .0015 clearance, replace at .002. Pistons kept their clearances far, far longer, ring seal loss was kept to a very reasonable 4 to 5 percent in a nominal 200 mile race, engines ran crisper, kept their power, ran cooler, quite, better throttle response, overall, noticeably significantly better.

Now, go ahead, tell me worn out pistons and giant clearances are better for the street, I'm waiting to see it all.

We found the same occurred to the Yamaha two stroke twin air cooled road race engines as well.

Not at all. I always get mine bored to the close side of what the manufacturer specifies. As I said above, Wossners (forged) at 0.0027".

I'm still wondering why 2stroke bored his to 6 thousandths.

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If it surges, that's normal, upshift.

The Wossner pistons are good. Former forged pistons, both factory H-R, and especially the worst forgings on the planet... Wiseco, were pure garbage.

One issue with the forged pistons is they usually don't have strengthening ribs on the inside of the skirts, so, collapse city for the skirts, which significantly increases rock over angle at both TDC and BDC, killing ring seal, and power.

A good austenitic, autothermic cast piston is far, far better. When we went to the factory cast race pistons, the piston and ring issues magically disappeared,

Wow...my brain is fried. I was thinking about the clearance issue....I was shooting for halfway between F5 and F81M specs from the factory...but then there is the wiseco piston issue...I read the posts here, and then some about wiseco clearances, etc. I went back to my spreadsheet to see what I had because it did seem I was shooting for a high clearance, based on an old service manual...

I had slowed down when the piston started to just about go into the cylinder...I didnt go too far beyond that point when I took several measurements and did an average. Well, as luck would have it, I was paying more attention to the average than the actual measurements....made a boo boo in my spreadsheet calculation for the average....forgetting to put the additions in parantheses....
the average said .58 something or other..... but looking at my measurements, they were not in that range. fixed my average and it came out to .00175 for my clearance as it stands. As I said, it felt right So, my mistake with the average calc saved my ass I reckon. I can give it a little more breathing room perhaps...after I take 38 more measurements that is.

Oh, and the hone I used was the 15000 with the micrometer knob and replaceable stones, they are held fixed and move in and out with the knob, changeable stones...it runs 2 stones and 2 felt wipers at once. The pivot it is on makes it rather self centering.

losing my mind over truing the crank at the moment.
I got both needles to swing the same, .005 right now, but my maximum and minimum are happening at 90 and 270 degrees in respect to the pin vs. the indicator needles. For a pinch or spread, Id have expected to have the range peak and/or diminish at 0 and 180.

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1972 S2 350, 1972 F9 350 Bighorn, 1975 KX400, 1981 YZ465, 1980 XT500 (Built for MX) , 1987 KD80, 1995 MZ Skorpion, TTR 125, 1994 KDX200 , 1978 Mobylette, 1985 KX350R-F91m (Frankenstein) and a Doodle Bug