BE-FAQ - Mixed sets of STD/+025 shells
Let me explain why the current sets of BE Bearings are mixed STD (lower) and +025 (upper) shells. It's all about quality control.
When BE receives bearing shells from Clevite, we measure the thickness of every single shell. As with any manufacturing process, there is variance in thickness. The variance follows a near-perfect normal distribution curve. The thicknesses ranges between 0.XXX00 - 0.XXX50 inch thick. The midpoint is 0.XXX25 inch thick.
When mating two nominal, 0.XXX25 inch shells together, the clearance is typically above their nominal 0.0024 inch recommendation, usually about 0.0028 inch. Furthermore, when BE ships all STD shells, somebody's going to get too much clearance, and somebody's going to get less clearance than recommended. Due to normal distribution of bearing thicknesses, there's really no way around it. That's where the +025 shells come in.
The +025 shells are exactly 0.00050 inch thicker than the STD shells. They also follow a normal distribution curve of manufacturing thickness tolerance. These shells run between 0.XXX50 - 0.XXX95 inch. By mixing the STD and +025 shells together, BE can basically guarantee everybody receives the same spec clearance -- as measured on a nominal thickness rod journal and brand new connecting rods of nominal big-end bore size.
Here's how this works:
BOTTOM, TOP SHL, CLEARANCE
0.XX000, 0.XX095, 0.00235
0.XX005, 0.XX090, 0.00235
0.XX010, 0.XX085, 0.00235
0.XX045, 0.XX050, 0.00235
0.XX050, 0.XX050, 0.00230
See how taking advantage of normal distribution curves and mixing STD and +025 works to control quality?
Now here's a word about plastigage. It's not accurate. It's just a sanity check. All of the BE Bearing wiki specifications were developed using bore gauges and micrometers as a race engine shop would do to measure clearance. So you can't really directly compare plastigage to bore gauges and micrometers. Here's a real case where this came to light. BE had a customer measure 0.0010 inch with plastigage. He did the right thing and immediately stopped the installation. BE paid to have the bearings sent back to them next-day air. Bert measured them with plastigage too, then I measured them with bore gauges and micrometers. I think Bert told me when he used plastigage, he measured about 0.0015 inch. However, when he measured with bore gauge and micrometer, it was 0.0026 inch clearance.
I hope this answers some questions that people may be asking.