I plugged mine. I also plugged the big return hole from the timing case into the engine to raise the oil level in the timing case. I am using 8:1 low expansion pistons. I have ridden about 10,000 miles now with these mods and have had no trouble at all with pistons. The pistons still look pretty much like new. They are running .0025" clearance. The reason I plugged the oil off to the cylinders, was to try to fix the excess wear I was getting on my cams. I wanted more oil to the cams. The Vincent oil pump has a very low circulation rate. The main wear problem was with the rear inlet cam and to a lesser extent the front inlet cam, these are the ones where the cylinders are really going to steal the oil from (the cylinder oil holes are at the inside ends of the cam spindles right after the outlet holes for the cams. I altered the cam bushes so the oil would be emitted constantly from the cam oil hole, instead of just the brief instant when the holes lined up. This wear was most likely caused by valve springs that were to strong, as it is working well now. Also I have Garry Robinson 105 stelite cams I guess they never wear out. I know tons of people will say that Tony Rose went 100,000 miles with a stock engine etc, but the factory did have some problems with cam wear. My engine isn't stock, so I can't expect it to work just like the original one. The newer pistons probably don't need as much oil as the old .006" clearance ones.