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If you get new bearings, keep in mind that inexpensive bearings do wear over time. Datsun used triple alloy bearings which do not wear out, but only if the engine loses oil pressure they will be damaged, or if debris gets into the oil.
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Measuring Bearing Clearance
Measure Bearing Crush when installing new bearings.
It is difficult to tell by visual inspection if bearings are worn. It is normal to have some pattern on them.
Direct measurement: The best way to check is to use plastic thread. Buy the Plastigauge (just a couple of dollars) and put it back together. The amount the thread is squished tells how much actual clearance it has.
Plastigauge see wikipedia article
Indirect measurement: Measure the journals with a precision micrometer, then calculate the clearance.
It is suggested to measure clearance after fitting/refitting bearing shells to confirm actual clearance.
Datsun Factory Service Manual:
Measuring Main Bearing Clearance
Regarding rod bearings, it says "Measure connecting rod bearing clearance in the same manner".
Connecting Rod Bearings
A12/A13/A14/A15 12111-20910 BEARING-CONNECTING ROD STD <> 12111-20900 12111-20900 BEARING-CONNECTING ROD STD 12111-20901 BUSHING-CONNECTING ROD UNDERSIZE 8 12111-20902 BUSHING-CONNECTING ROD UNDERSIZE 12 12111-20903 BUSHING-CONNECTING ROD UNDERSIZE 25 (0.25 mm undersize) 12111-20904 BUSHING-CONNECTING ROD UNDERSIZE 50 (0.50 mm undersize) 12111-20905 BUSHING-CONNECTING ROD UNDERSIZE 75 (0.75 mm undersize) 12111-20906 BUSHING-CONNECTING ROD UNDERSIZE 100
1984 Nissan Competition Catalog said: The -25720 ... bearing shell produces the most desirable bearing clearance for racing purposes when applied to a standard size crankshaft. The other shells exist in order to obtain proper clearance when compensating for the variations in the crankshaft or connecting rod journals.
8 each required:
* 12111-25720 .0590"
* 12111-25721 .0588"
* 12111-25722 .0585"
* 12111-25723 .0592"
* 12111-25724 .0583"
Main Bearings
Crankshaft bearings
BEARING SET A12/A13/A14/A15 * 12207-H1001 SET-BEARING STD <> 12207-M7201 <> 12207-H1000 $71.58 * 12208-H1000 SET-BEARING 0.20MM * 12209-H1000 SET-BEARING 0.25MM * 12210-H1000 SET-BEARING 0.50MM * 12211-H1000 SET-BEARING 0.75MM * 12212-H1000 SET-BEARING 1.00MM
1984 Nissan Competition Catalog said: The ... -H2310 bearing shell produces the most desirable bearing clearance for racing purposes when applied to a standard size crankshaft. The other shells exist in order to obtain proper clearance when compensating for the variations in the crankshaft or connecting rod journals.
MAIN BEARING (#1/#5, SHELL THICKNESS) 4 each required
* 12215-H2310 .0720"
* 12215-H2311 .0718"
* 12215-H2312 .0715"
MAIN BEARING (#2/#4, SHELL THICKNESS) 4 each required
* 12231-H2310 .0720"
* 12231-H2311 .0718"
* 12231-12312 .0715"
MAIN BEARING (CENTER, SHELL THICKNESS) 2 required
* 12247-H2310 .0720"
* 12247-H2311 .0718"
* 12247-H2312 .0715"
Thrust Bearing
See: Thrust Clearance
Main Bearing Cradle
Although probably unnecessary, you can fabricate a main bearing cap girdle (ladder frame support) such as many modern engines have. The theory is that it stiffens the block for high HP applications. Also called a main cap girdle.
Garage Nakamura halo cradle
Specs
Bearing Clearance * Main: 0.020 to 0.062 mm * Rod: 0.020 to 0.050 mm
Main Bearing Journal diameter * 49.951 to 49.964 mm
Crankpin Journal diameter * 44.974 to 44.961 mm
Journal Taper and out of round * measure multiple spots * less than 0.03 mm
End-play * crank thrust: 0.05 to 0.15 mm
Bearing Crush * main: 0 to 0.03 mm * rod: 0.015 to 0.040 mm
Part Numbers
Tri-metal was used as original equipment. Very very durable bearings.
ROD BEARING Nissan 12111-20900 BEARING-CONNECTING ROD STD (8 required) Mahle/Clevite CB980P (pair) P-series (Tri-Metal) Mahle/Clevite CB980A4 (set) A-series (Aluminum) Sealed Power 467320CP
MAIN BEARING Nissan 12207-M7201 set STD Sealed Power 7135M
Wheel Bearings
See main article: Wheel bearings