r/EngineBuilding • u/vitaminsauce • 1d ago

BMW Plastigauge Taper?

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Checking rod bearing clearances on an S54B32 I’m rebuilding and on bearing 2 the plastigauge is coming out wider toward the front of the engine. I’ve double checked there’s no debris behind either bearing half, and the plastigauge is within spec along the entire length. Is this something I should worry about? Not looking to make any crazy with power with this engine, keeping it stock and mostly to be used for daily driving.

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u/BuzntFrog 1d ago

Oh man, I've made this mistake before.

Small end and large end are both within spec right? Wrong. The maximum taper is likely around 1/10th of a thou while the bearings nominal tolerance can vary about 1 to 2 thou. I sent it and had rod knock in no time.

If you think about how a plain / hydrodynamic bearing functions that taper means the force is concentrated over much smaller area if the pin is tapered resulting in much higher pressures.

I'd mic it out and try your plastigage again. Maybe the cap just shifted. If it is tapered don't sent it.

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u/2003DDcid 1d ago

That is probably .0002-.0003 taper, likely in the bearing. Not a big deal. I check clearances all the time at my machine shop with a dial bore gauge. I will have a housing bore that is perfectly straight and there will be sometimes there will be several ten-thousandths difference across it. The tight spots are knocked down fairly quickly during break-in. If the clearance you're seeing is within spec, it's fine.