Just doing some big end bearing clearance checks on my 74 T120V crank, before putting it all together and have had a bit of a surprise! The crank is still on standard journal size and measures closer to the upper limit than to the lower limit, so all well and good and has since been tuftrided and dynamically balanced. I bought some new std size bearing shells fitted them to a pair of new Thunder Engineering con rods. Having measured and set the micrometer to the journal diameter, I then zeroed my 0.0001" bore gauge to check the bearing clearance after having torqued the bearing cap.
The big end bearing clearance from the workshop manual is supposed to be between 0.0005"/0.0020" and the new shells are reading 0.0022", so just out of top limit. I thought I'd try the old shells as this engine had always had good oil pressure and to my amazement the clearance was 0.0005" bang on bottom limit as you can see below.
It goes against my better judgement but it looks like I'm going to have to build this engine using the old shell bearings and not the new ones! I'd be interested in any thoughts on this as it goes against what is supposed to be good practice. Cheers, John
The overlay plating on the Clevite and the VP trimetallic bearings (VP bought a manufacturing license from Clevite for their VP2 bearings so both are Clevite 77) is microns thick. It's very soft to improve embedibility ie to absorb grit and stop it wearing the journals but once the overlay is full it has the opposite effect as the grit is hard. So fit an oil filter so the oil is clean and the grit build up in the overlay is minimised. Also as the overlay protects the next layer of leaded bronze from the acids in the oil attacking it as soon as any bronze shows from behind the overlay then the bearings need changing.