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Datsun 521 L20B

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Im not  diff expert on the trucks but assume it would fit but don't know if that is SAE threads or Metric going from 521 to the 620.  example the leaf spring U bolt/ nuts might be different


as for the head gasket you put a new one on there and will assume its good.Not really hard to fuck that one up unless you actually see water coming out from there.


One really should have the head pressure tested but I have put heads on and they worked.


I used to ck my plugs and if the gasket is bad on the intake side the spark plug will be really clean. If bad on exhaust the radiator will get hot.  Or start truck when cold and actuall see the leak happen.

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...good to know, is this left when you are at the front of the car?


For a vehicle, left and right is always as looked at from the driver's seat.


I will definitely stick with the L series motor for now, lots of variables and I'm pretty invested at this point.


I did a compression test on cylinder 4 and I got a reading around 126, and I would have liked to believe it came from the heater hose but I double checked. (leak was coming from spark plug side)


Will take some photos tonight


Are 620 diff's interchangeable? I have access to one that came with a 4-speed... did a quick search and it's seems it will work but I'm not 100% certain


What would you guys do? Should I pull the head and inspect? Pull the engine and throw in a different one? (will have to source one)


No lectures on doing it right the first time, I really thought I did it right..


I am running a 5-speed out of a 280z




It's very possible that the gasket could leak fluid but the cylinder is still sealed.


Your 521 will have a 4.375 ratio differential. The 620 differentials will fit and were....


'73-'78 manual.................. 4.375

'79 manual........................ 4.11

'73 automatic.................... 4.625

'74-'79 automatic.............. 4.375

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When u did compression check, did you use plug in type or screw in type? Plug in type is not that accurate and you get lower reading.

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I like the 3.899 rear end pumpkins out of the 80 720 regular cab 5 speeds.  All you need is the third member (pumpkin).

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The 720 is even easier. The differential ratio is stamped on the credit card sized engine tag located on the passenger side inner fender below the hood hinge. Bottom line will say ...


TRANS/AXLE............. FS5W71B.......... HL38  (for a 3.889, HL41 for a 4.11, HL37 for a 3.70 etc...)

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