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Big end bearing tolerances


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Hi all,

I'm just after some clarification, i'm inspecting the big end bearings and wish to know what is the acceptable range of 'thickness' I measured one bearing with a digital caliper and got a thickness of 0.064 inches or 1.63mm. Is this within range?



Craig Cass

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I believe the way this is supposed to be measured is by using a strip of special measuring plastic that you crush in the bearing by assembling it. You then open up the bearing and measure the thickness of the plastic with a micrometer to know if the clearance is within the tolerance given in the manual. My Vitesse manual gives this value and not the bearing thickness however it does allow me to calculate it for a main bearing. The result is between 1.8225 and 1.8325mm which I would imagine is quite standard for all Triumph engines. This fits in quite well with your measurement especially as it's difficult to measure to this accuracy with a caliper.

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It is even easier than Glang says. Once the special plastic (Plastigauge) is crushed in the bearing, you simply check the width it has spread out to, against a scale provided. You can read off the clearance in thousandths of an inch or hundredths a mm. No micrometer required.

A check for ovality can be made by doing the same check with the crankshaft at TDC and half way between top and bottom center.



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