Bearings

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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.

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 cradle (ladder frame) such as many modern engines have. The theory is that it stiffens the block for high HP applications.

Garage Nakamura cradle