I recently fitted a Provent to my TD42, not to stop oil leaks, but partly to catch oil before it got back to the inlet manifold and partly to monitor the amount of blow-by that was occurring.
Very impressed with the Provent. There's no more oil in the inlet manifold and hardly any oil dampness in the Provent.
When fitting the Provent I replaced the standard hose with 16mm hose, maybe that's helped improve flow and reduce crankcase pressure.
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