Welcomed discovery in the pile, an XR500 Wiseco piston!

As I sort out the parts in the pile, I discover a piston which doesn’t seem to be an NOS Honda part.  Its diameter is 90mm and its dome has markings which seem to be a part number, “4117 P4”, as well as “1.00mm” which probably indicates that it is 1mm over original in diameter.  A quick search shows that this is an oversized Wiseco piston for a Honda XR500 motor.  Glad it was in the pile!

XR500-26a
Markings on the Wiseco piston: “1.00mm” oversize and “4147 P4” part number.

The Wiseco piston provides a compression ratio of 10.5 compared to the original 8.6.  With this oversized piston (1mm over the original 89mm), the displacement moves from 497cc to 509cc, a 2.5% increase.  According to Bowling’s Rough Horsepower Calculator, installing this piston should increase the power from the original 32hp to about 39-40hp at 6500rpm.  Nice!  And the extra bonus is that one of the cylinders found in the Basket Case pile is already bored to 90mm, honed and ready to be used with the Wiseco piston.  Ahh, life is good.

XR500-44a
At the left, the original Honda XR500 piston with its flat head (XL pistons are identical), and on the right the higher compression Wiseco piston .  Note the Wiseco’s shorter skirt and carved higher dome.

The Wiseco piston is missing its pin and rings.  Fortunately Wiseco’s web site has these items available as replacement parts (see “Related Products” section).

XR500-27a
Wiseco piston rings.

The Wiseco copper looking ring is the top ring, followed by the silver and finally the oil scraping spring ring set.  The oil rings can be mounted with either side up, however the two compression rings need to be mounted with the “N100” marker facing up towards the top of the piston (see picture below).

XR500-28a
Markings on the compression rings indicate the face of the ring which should be facing up towards the top of the piston.

Wiseco provides a handy chart which indicates the correct position for each ring gap when inserting the piston back into the cylinder:

XR500-29a
Ring end gap placement.

With the piston and cylinder all set, I’m moving on now to rebuilding the carburetor…

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3 Responses to Welcomed discovery in the pile, an XR500 Wiseco piston!

  1. mike disney says:

    This is good! Does Wiseco require different valve clearence or gasket stack at the head?

    • Mike says:

      The 10.5:1 Wiseco piston is a bolt-on replacement. Wiseco offers though an even more aggressive piston which provides a ratio of 12:1 and it’s unclear if this one requires some adjustment on the valve side or gasket stack. The 10.5:1 will be borderline with premium fuel — fortunately ok here at higher altitude in Colorado — the 12:1 will most certainly need racing fuel to prevent pinging.

  2. Pingback: Re-assembling the XR’s engine top end | Mike's Basket Case

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