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Understanding V-belt lengths Li vs La (inner length and outer length)

March 8, 2023

Some vbelt brands (like Bando) like to indicate inner and outer lengths on their belts but not both. The important thing to know is that Li = inner length and La = outer length.

BMW always indicates their belts in La in the ETK (without explicitly writing La or Li)

For example, the 3315 belt below is indicated as 13 x 775, which you may think is too short for an E30 AC compressor, but Li indicates that 775 is the inner length not outer.

So how do you convert from Li to La to know if you have the right length?

La = Li + (*depth of belt from top to tip of a tooth* x 2 x 3.14159)

So for the 3315 belt it would be La = 775 + (9.5x2x3.14159)

La = 834.6902604

*Given that the factory AC belt (64551722990) is 823 outer length, the 3315 would fit just fine.

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