Why use oversize rod bearings

While line-boring a block isn't usually necessary unless the engine's spun a bearing or the builder wishes to assemble a perfect creation, crankshaft machining is much more common.

Crankshaft journals bear the brunt of your engine's horsepower assault and are subject to all sorts of damage and wear. Journal scoring or scratching as a result of oil contamination is common. To restore the journals to stock smoothness, machinists remove just enough material from the journal surface to eliminate the deepest scratches.

Undersized bearings compensate for the amount of material removed during this machining process. Bearings don't have to be just oversized or just undersized; many antique engine builders will both line-bore the block and machine the crankshaft to make use of more modern bearings designed for a completely different engine. For instance, say you have an old Bentley engine and you want to get it running, but no one makes parts for it.

If the engine's main bearing bores measure 2. Richard Rowe has been writing professionally since , specializing in automotive topics. The bearing inner diameter will be smaller than standard. Now easy to find that undersized bearing is right choice. Is this result surprised to you?

Is different with what we often say oversized bearing when sell or rebuild an engine. For a friend who have a repair shops for over 20 years, when I asked him oversized bearing use more or undersized bearing.

He told me they are same for 0. He turned to tell me only Caterpillar genuine bearing has overszied bearing, only rebuild one engine to use oversized bearing in so many years. Other engines do not have this kind of bearing.

Little to use real oversized bearing. Once you get your crankshaft back from the machinist and get the bearings of the size they state, then you use plastigage to see if everything is done right. If the machinist is worth their mettle pun intended , everything will match up and your plastigage measurements will be spot on and within tolerance. You never really know though, and is why you always double check things when putting them back together.

You never know if the machinist misunderstood you or you misunderstood them. Things happen. It is far better to figure out you have an issue before you put stuff together than to get the death knock on first startup.

As a side note, there are several things your machinist will tell you which plastigage won't. Things like whether the rod journal is out of round or is tapered.

In most cases the machinist will be able to fix either of these issues, but unless you have a good set of micrometers, you might never know the difference. A good machinist is worth their weight in gold.

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