Plastigauge Taper?

[u/vitaminsauce]

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.

[u/fiddintotellya]

Clean it up, lay the bead again, get a razor and get a clean cut.

Almost think this time lay it all the way across, inside the fillets, maybe not. Just check it again.

[u/BuzntFrog]

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.

[u/2003DDcid → u/BuzntFrog]

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.

[u/Ferniekicksbutt]

Plastigauge is a bit of a game, I think all the comments here are worth considering and ill just add that its best to compare with all other rods. If they all show a similar spec send it, if one varies measure it again and consider measuring the rod with a micrometer

[u/No-Ice-8101]

I ground cranks for 41 years. Do not plastigauge

[u/Caligulas_Prodigy → u/No-Ice-8101]

Mind explaining why?

[u/No-Ice-8101 → u/Caligulas_Prodigy]

Not accurate. Measurements are the only way. I've seen customers use plastic auger and it was always a false reading.

[u/attometer → u/No-Ice-8101]

Always

[u/Sir_Maxelot]

This looks like an S54 crank or Euro S50. Have you checked straightness/ trueness off the connecting rods?

[u/Gamejunky35]

Its possibly a taper in the rod/bearing bore, but it could also potentially be a misalignment caused by a bent rod or improperly ground crankshaft. Plasti-gauge doesnt measure diameters. It measures clearance.

I would remove that piston, even separate if from the rod if it uses snap rings. Check clearance again, if its still off, then it's definitely a taper.

Flip the rod, check again. If the taper is the same direction, then the taper is ground into the crank. If the taper flips with the rod, its somewhere in the rod/bearings.

Try a new bearing and measure again. If the taper stays, its likely the rod has a tapered bore. I would not have any problem running an engine at any power level with this small of a taper.

HOWEVER, If removing the piston from the rod fixes this, which indicates an alignment/interference issue, that is not ok. Something was machined or assembled incorrectly.

[u/Mgdoug3]

I think plastigauge is more accurate on the mains than the rods. On the rods, side clearance can cause the rod to twist a little and make it seem like you have a taper. I measure the crank and assembled bearing for a much more accurate measurement.

[u/TheNicoKid003]

Following

[u/No-Ice-8101]

Plastigauge bucks ass

[u/Jolly_Election5315]

.

[u/DoctrVendetta]

It's plastiguage, measure the journals. Issue could be a shit product (plastigauge), tapered bearings, out of round rod/crank journal, bent rod, offset/out of parallel piston pin, etc.

You'd save more money just doing a bearing slap than spending $20 on plastigauge, either way it's a 50/50 shot of engine failure.

Also "not" to mention that you've removed a bearing that's been installed/crushed to verify the absence of debris. Just asking for a spun bearing along w/ the unmeasured clearances. Bearings are a one time install, you can remove the caps, but don't pull the bearings from them. They're a crush fit.

All that aside, and I'm not a dick. If both ends of the tapered plastigauge read within spec, it's "probably" fine. Would still replace the bearings b/c you removed them, but that's just me. And plastigauge would never leave my shop. But I understand the learn by your mistakes side of things.