Crankshaft Bearings Specification?

[Auggie56]

440 Crankshaft bearings. The specifications I'm finding don't match what my rod throw measures. The rods mic out at a consistent 2.352 the mains 2.75 The specs I find say the mains are 2.75 and rods 2.375. I'm going to buy them local am I going to find an oversized rod bearing on the market for the rods? This is a 78' motor home engine with 25K on it. Please advise.

 

[bm02tj]

20 under should be easy to order

 

[33 IMP]

Yeah, but .020 should be 2.355, or 2.354 for an extra thou of clearance. It sounds like a .020 rod bearing would give about .005 clearance. More than i would like.

 

[Fran Blacker]

Check the pad on front of engine for a maltese cross, if you have undersize crank. Then if you could face of counter weights, there could a stamp R and maybe 1,2,3,4,5,6,7,8 or something to tell undersized. Picture is of crank is M5 is main joural R5678 rod journals all .001 under. Some editing at 3:37 pm.

 

[Auggie56]

Thanks for the information people.

 

[gkent]

You want .010 under bearings which will give you the desired .003 clearance.

2.375 - 2.352 = .023 .010 under bearings will add .010 of material around the whole bearing which changes the inner diameter by .020

 

[dvw]

You want .010 under bearings which will give you the desired .003 clearance.

2.375 - 2.352 = .023 .010 under bearings will add .010 of material around the whole bearing which changes the inner diameter by .020

A .010" under bearing fits a .010" under journal. Give or take .001" if you want to adjust the clearance. The rod journals are quoted in post #1 at .023" under. Something is wrong. Mains are quoted as being standard with no wear.
Doug

 

[69L48Z27]

2.375 with .010” under is 2.365 not 2.355. .020” under is what you want but clearance is built into the number. You’ll be .003 + .0015 most likely. Sounds like a another crank is in order. Last machinist that took it .020” under went a little too far IMO.

 

[33 IMP]

If this isn't going in a street strip motor, just a warm street engine, i wouldn't be afraid of grinding the rod journals to .030, even being a cast crank. But i would be sure of a supply of .030 bearings first, and i would make sure that the crank grinder doesn't grind the mains, just because he can.

 

[gkent]

If a crank grinder takes off .010, that's .010 off the whole journal which makes it .020 smaller. The bearings will also be .010 under because they have .010 ADDED to the whole surface.

 

[Cranky]

If a crank grinder takes off .010, that's .010 off the whole journal which makes it .020 smaller. The bearings will also be .010 under because they have .010 ADDED to the whole surface.

If you come into my shop and ask to take off .010 to the side, yup, that's what will happen but in most shops people who want .010 taken off means a total of .010 off and a .010 'under' bearing means that it'll fit a .010 under crank with the proper clearance. The grinder takes .005 off to the side to get what is commonly known as a .010 under crank.....

 

[dvw]

If a crank grinder takes off .010, that's .010 off the whole journal which makes it .020 smaller. The bearings will also be .010 under because they have .010 ADDED to the whole surface.

No. That would be like saying boring a block .030" would be .030" on each side and require a .060" over piston. The numbers given when machining a round part are the difference between standard dimension and the undersize/oversize quoted; .010 under, .030" over etc.
Doug

 

[gkent]

The "boring" example makes a good pont.