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xr80 piston clearance, taper piston argh!

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Im converting the xr100 to a xr80 and inserting the 80cc sleeve into the 100cc sleeve, the new piston has a taper of .1mm or 4thou on the skirt, i was going to set the clearance at 4thou but that would create heaps at the top, ie 8 thou, what is the norm, i was thinking +1thou at the bottom = +6 at the top. Its a genuine honda piston.

Cheers, Matt.

Hi, Thanks for the posts, i have completed and run this project. Here are some tips for anyone wanting to take it on.

Turn up a bush reducing sleeve for the little end,I used phosphor bronze, the 100cc gudgeon pin is 14mm and the 80cc is 13mm. Dont forget to drill oil holes once pressed in.

The gudgeon position is higher on the 80cc piston, Face of 50% of the domed crown thickness on the 80cc piston.Note this will reduce 4 grams off the piston weight which is already 20 grams lighter than the 100cc piston. Balance flywheels to accommodate.

Raise the cylender a couple of mm to suit piston height with consideration to cam chain length.

I turned down the 80cc cylinder by .8mm to be a .05mm interference fit to the original 100cc cylinder.

Heating the 100cc cylender to 125 deg Celsius gave me .06 expansion internally,

but freezing the 80cc liner gave no apparent contraction difference.

Very quickly push the 2 together, Don't get this wrong you get no second chance.

Bore the new 80cc cylinder to 4 thou + of the bottom of piston skirt (the largest point)

I get the pro's to do my head flow and triple cut the valve seating with the springs set to tension rather than height.

Put it together re tune carby and presto, a fat xr 80.

I dont do a lot of mad mods to the boys bike as he has to learn to ride the thing hard first.

But as i grow older i like my own engines to have me to the first corner first, im getting that old guy what if syndrome.LOL.

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no piston would have a taper. and you simply cannot drop an 80 jug on 100 cases, tje oiling hole is in the wrong spoy

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The ring land area is always smaller than the skirt to allow for expansion. And the skirt isnt round; it is egg-shaped with it narrower at the pin bosses; again for expansion. A piston only assumes proper shape at running temp; therefore is is essential to not only follow the piston makers clearence but also WHERE they want you to measure it on the piston. 'nutcase

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