It looks like that belt is just too small for your particular motor. Maybe something was changed in the past, like the pulley on the air con compressor.I dont think you have done any thing wrong. Looks like you need a 13 x 935 belt on your motor. Is that tensioning bolt adjusted up as far as it can go?

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Grumpy Old Diesel Owners Club group

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1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

Thanks. I'll try that.

Regarding the tensioning bolt, it's completely separated from the restraining bolt. You can see the washer end of the tensioning bolt in one of the photos.

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1982 300TD

Everything I know about cars I learned from PeachParts.