S&S 106 Big Bore Kit For Harley Big Twin Features At 3.927 bore, these cylinders have the largest bore that S&S feels can safely be used in a cylinder with stock spigot diameter This kit is only offered with S&S cylinders because the cylinder liner material in S&S cylinders is much stronger, and has proven its ability to stand up to the added power in numerous dyno pulls, street miles, and a lot of quarter mile drag strip runs There is a trade-off for the ease of installation, which is that only one piston oversize is available for this kit No need to bore or even split the cases, just do the cylinder swap In addition to the ease of installation they look great because the cylinders match the stock finish of your engine 10:1 Compression ratio with stock heads, 9.7:1 with S&S 89cc heads