72 750 Crank bearing question

A

a1984mm

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• Apr 22, 2026

• #1

I was looking at the Nocnsw.org.au bearing reference. It lists NJ306A as the needed bearings for the crankshaft. My engine has one MAJA30 cylindrical bearing. NJ306A looks like it could be a replacement for it. But my other bearing is an MJ30 ball bearing unit. Can this bearing be “upgraded” to the NJ306A?

Thanks in advance for any advice

Mike

 

L.A.B.

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• Apr 22, 2026

• #2

If it is truly a '1972' (200000) series then it should already have two roller bearings. The correct replacements are NJ306E C3 which is what it says at nocnsw.org.au.

Andover Norton - CRANKSHAFT MAIN BEARING NJ306-E-XL-M1-C3#N1 (NM17822/4) FAG Schaeffler - 06.4118

CRANKSHAFT MAIN BEARING NJ306-E-XL-M1-C3#N1 (NM17822/4) FAG Schaeffler

andover-norton.co.uk

Both original roller bearings would be MRJA30.

 

Last edited: Apr 22, 2026

Torontonian

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• Apr 22, 2026

• #3

Not A , but an E .

 

A

a1984mm

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• Apr 22, 2026

• #4

Thanks for the info guys. I did misread the original roller bearing. It is a MRJA30 not what I originally typed. I will have to check on the series number as I do have 1 roller and 1 ball bearing.

 

L.A.B.

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• Apr 23, 2026

• #5

A pre-200000 model would have had a roller and a ball bearing so likely not '1972' production. If so it would also have a timed camshaft breather. The ball bearing can be replaced with the roller.

 

K

kerinorton

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• Apr 27, 2026

• #6

Replace both bearings with the correct ones. My 71 with the original bearings, the original ball bearing crapped out at around 10,000 miles. The ball b earing cage scored the r/h crankcase 1/2 but no real damage. I heard it start to go and knew it was a main bearing noise so pulled it apart before it did real damage.