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!
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.
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).
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).
Wiseco provides a handy chart which indicates the correct position for each ring gap when inserting the piston back into the cylinder:
With the piston and cylinder all set, I’m moving on now to rebuilding the carburetor…