34.1 grams!  Compared to the stock solid shim over bucket lifters (at 47.0 grams), 28% lighter is significant, especially since that is weight off of the valve train.  The weight reduction was only a bonus though.

The reason for the shim under bucket setup has more to do with the amount of lift on the cams and the threat of the OEM shim over bucket lifters spitting out the shims at higher RPMS.  There is also a concern that the camshaft would come in contact with the top perimeter edge of the OEM bucket.   Bucket lifters are shaped just as the sound, like a bucket turned upside down.  In a shim under bucket (SUB) configuration, the lifters are shimmed (slightly raised or lowered) from the bottom of the lifter, where it touches the valve.  In a shim over bucket configuration, the shim is placed on top of the lifter where the cam makes contact.  To keep the lifter in place, there is a ridge around the outside of the bucket.  In situations where only a thin shim is needed, that ridge my protrude above the shim and could potentially come in contact with the cam, which would damage the camshaft over time.

The Maruha website’s racing cam page and “early NB8C” F-Cam page  does a great job of explaining the benefits of SUB lifters and includes several helpful diagrams too.

PICTURES: Maruha SUB lifters with info/captions.