Turning a Crankshaft

[Tom in Iowa]

We hear the term, "turning the crankshaft," or "I had to have my crank turned."
Please explain. My assumption if that the two end and maybe the middle shaft portions are trued up by a machine shop to get them back to round. My guess is that they wore into an elliptical shape. If this is all so, we end up with a crank that is smaller in diameter than original. Therefore, how do new bearings mate up properly to an undersized crank?

 

[souNdguy]

Different bearing sizes. That's why you plastigauge or mic the journals to determine what bearings you need.

When they turn the crank, they will bring it true, to the next acceptable bearing size.

Soundguy

 

[teddy52food]

The new bearings are thicker. Cranks are turned .010 at a time depending on how much they are wore. You may end up with a crank turned .010 on the mains and .020 on the rods. So you get rod bearings marked .020 and a set of mains at .010. Most cranks can be turned to .030 and some go to .040 if bearings are available.

 

[old]

Turning a crank is simple term for haveing the main and rod bearing jurnals turn down or cut down the the next smaller size that they can be and made true again. Those the term turned
Hobby farm

 

[Bob]

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<img src = "http://www.technicalchange.com/images/crankshaft.jpg">

 

[ricko700]

Just had mine ground for 50 smackers and it took .020 rods and .020 mains...the machinist looks in his charts and determines what the crank was when it was made and tells the client what size (oversize) bearings he needs to compensate for what he ground off or what has been taken off so far..in my case I gave him the specs I got out of my FO manual to save him time...he checked his way anyway.lol.

 

[TheOldHokie]

More precisely crankshafts are REGROUND to the next standard UNDERSIZE (.010,.020. .030...). The main and rod journals may be reground to different undersizes as dictated by their condition. Once reground the appropriate UNDERSIZE bearings are installed. UNDERSIZE bearings hava a SMALLER intenal diameter which compensates for the material removed from the crankshaft. An OVERSIZE bearing is one that has a larger OUTSIDE DIAMETER and is used when it is necessary to rebore the connecting rod ends or main bearing caps. This compensates for material removed from the rod or main caps.

TOH

 

[teddy52food]

I used to work in a machine shop. The connecting rods are resized back to standard by grinding off the face of the cap and rod, then bolting it together and honing it out round to standard specs. Over sized bearings are thicker on the inside not on the outside, to replace what is ground off the crank.

 

[TheOldHokie]

The only machine shop I've ever worked in is my own hobby shop and yes that's typically how you address a hogged out rod or main problem.

Q. How to you reduce the size of a hogged out in block camshaft bore?

A. You don't - you bore over size and install oversize bearings. Larger OD - same or smaller ID as required.

Crankshafts are ground UNDERSIZE and UNDERSIZE bearings are installed to REDUCE the ID. Look in any automotive bearing catalog - that's the word they will use for that application.

From my Clevite catalog:

<font color=red>OPTIONAL .002 OVERSIZE CONNECTING ROD BEARING SETS

The following interchange is a list of our standard Clevite 77 bearings and the corresponding .002 [b:654c4848f0]oversize[/b:654c4848f0] bearings. These bearings are intended for use in connecting rods that have had the housing bores resized .002 larger than stock</font>

What follows is a table of standard UNDERSIZE bearings and the corresponding OVERSIZE equivalent i.e same ID larger OD.

JMHO,

TOH

 

[teddy52food]

What happens if on the next overhaul someone puts in std ( not .002 oversize bearings)?? You will end up with spun bearings. I would never machine a rod oversize. Clevite may have those bearings but I use Federal Mogel and others and have never seen what you have in Clevite. Cam bearings are full round so would have to be oversized if the block is bored. Main bearings caps can be cut down and the block is then line bored back to standard.

 

[DavidO]

As TheOldHokie stated below, when crankshaft journals are re-ground the diameter of the journal is smaller than original, i.e. undersize.
Of the hundreds that I have installed, my observation is that "undersize" bearings are actually thicker and have a smaller ID (as stated by The Old Hokie). Therefore "undersize" is the term used to describe the bearings used on a reground, "undersize", crankshaft, but the bearings themselves are actually thicker than the original and have a smaller ID.

 

[TheOldHokie]

(quoted from post at 06:35:29 10/01/07) What happens if on the next overhaul someone puts in std ( not .002 [color=red:f1351e25c9][u:f1351e25c9][b:f1351e25c9]oversize[/b:f1351e25c9][/u:f1351e25c9][/color:f1351e25c9] bearings)?? You will end up with spun bearings. I would never machine a rod oversize. Clevite may have those bearings but I use Federal Mogel and others and have never seen what you have in Clevite. Cam bearings are full round so would have to be [color=red:f1351e25c9][u:f1351e25c9][b:f1351e25c9]oversized[/b:f1351e25c9][/u:f1351e25c9][/color:f1351e25c9] if the block is bored. Main bearings caps can be cut down and the block is then line bored back to standard.

Indeed - it seems we agree on what [u:f1351e25c9]oversize [/u:f1351e25c9]means when applied to bearings. Here's a link to a discussion on Ford's historical use of oversize (and undersize) rod bearings:

A bit of history from Red's Headers

And it's not just a Clevite thing - I can point you to numerous other bearing suppliers, including FOMOCO, that offer them. As for which repair technique you prefer that's a professional judgment call. But before installing replacement bearings any competent machinist is going to inspect the old bearings for size markings and measure the hole first - whether its the big end of a rod or the camway - to determine what if any corrective measures may be needed. Personally I'd have no more qualms using an oversized rod set than I'd have using oversize cam bearings.

YMMV,

TOH