7 replies to this topic

[maieth]

Howdi.
Does anyone know what the tolerances are for diameter of main bearing journals? I've had my 1275 crank lapped/polished but not reground Centre and RH journal are still fine at 50.84mm, but LH journal is a little shy at 50.79, 0.04mm below what Haynes says.
Is that one going to need a thicker pair of bearing shells, and can they be bought individually?

[mini13]

How did you measure?

I make that clearance about 1.5 thou which is fine

[ACDodd]

This is undersized for the after market shells that are available. The minimum size for a road engine is 50.827mm, using the heavy duty bearings. Competion engine can run easily smaller than this at 50.81mm. If for road work regrind.

AC

[maieth]

Mini13 - metric micrometer. Unfortunately it only goes up to 50mm officially, but the micron dial only went up a further 79 points for that journal while all hd others went up to 83 or 84.

AC - I'll triple check my measurements, but sounds like a reground is the wisest option. Thanks.

[Guess-Works.com]

Std size for the main bearing journal is 2.0005 to 2.0010 inch, outside of that, regrind....

[mini13]

Ok,

often people try to measure with a vernier hence the question.

I misread the post last night ( one too meany beers!) theres about 1.5 thou wear on the LH journel over the other two, I'd deffo go for a regrind.

have you checked the bigends yet?

[maieth]

Yeah, big ends and camshaft all seem fine.
I'll get the crank to a machine shop this week.
I've picked up some +10 main bearings for now.
Anyone know what measurements I am going to need to ask for crank to be reground to for those? Haynes only lists metric measurements

[ACDodd]

Just to clarify journal tolerances are related to bearing type. The heavy duty lead copper shells have a nominal journal size of 2.0011" to 2.0017". The cheaper alloy type have the 2.005" to 2.0011" journal size requirements. The journal size must relate to the shells being used.

AC