Crankshaft Bearing Clearance

[Matt Wilson]

I'd like to get a better understanding of crankshaft main and rod bearing clearances.  I have five rebuild manuals, which indicate the clearances listed below:

Army Technical Manual 9-808, Truck, 3/4-Ton 4x4 (Dodge), dated 11/14/1942:  230 engine should have 0.001 - 0.002, rods and mains (other engines not covered).

 

48 - 49 B-1 Series Dodge Truck Manual:  218, 230 (23" block), 236, 251 (25" block) are all supposed to have 0.0005 - 0.0015 clearance, rods and mains (other engines are covered, but they are much larger flathead sixes in a class of their own with bigger clearances, so I opted not to list them here).

 

Army Technical Manual TM 9-1840A (a.k.a., Air Force Technical Order TO 19-75B-15), dated June, 1952 (M37/M43 manual):  230 engine should have 0.0001 - 0.0021 clearance for rods, 0.0002 - 0.0022 clearance for mains (other engines not covered).  Yes, you read that correctly (yes, I typed it correctly); 0.0001 (one ten-thousandth) and 0.0002 (two ten-thousandths) are the min specified clearances for the rods and mains, respectively (other engines not covered).

 

Chrysler Six Cylinder Industrial Engines Maintenance and Parts Manual, D-12154, Second Edition (only two dates I could find were 1946 and 1953, in the parts list section):  218, 230 (23" block), 237, 251 (25" block) engines should have 0.0005 - 0.0015 clearance, rods and mains (other engines not covered).

 

Motors Truck Repair Manual, 19th Edition (don't know publication year, but last vehicles covered are 1966):  1955 - 1960 230, 251 & 265 engines should have 0.0005 - 0.0015 clearance, rods and mains (other flathead sixes not covered).

 

As you can see, three manuals call for 0.0005 - 0.0015.  The other two allow as much as approximately 0.002 or 0.0022 for the max end of tolerance. 

 

Anything less than 0.0015 seems small compared to what I hear most people like to aim for on these engines.  Half-thou (0.0005) seems very much on the small side and questionable, and 0.0001 or 0.0002 seems ridiculously small and just begging for trouble.  Have any of you guys actually built your engines with anything approaching 0.0005" (I'm not even going to ask about 0.0001 or 0.0002).  In a discussion with someone at Vintage Power Wagons a few years ago, I was told to try to get down as close to 0.0005 as possible, as it is good for maintaining a healthy oil pressure, but it just seems kinda small to me....and yet there are three manuals that allow it, and another that allows even less.  It seems like 0.0015 - 0.002 is what a lot of people aim for, and some are even happy with up to 0.003 in these engines during engine rebuilds.

 

I did find the thread below, in which DJK tried bearings with 0.001 clearance and he couldn't turn his crankshaft by hand.  When he bumped the clearance up to 0.002, everything was free-spinning and the engine ran great.

 

It just kind of bumfuzzles me that Chrysler would specify such tight clearances.

 

 

Edited June 16, 2023 by Matt Wilson

[Sniper]

1 hour ago, Matt Wilson said:

It just kind of bumfuzzles me that Chrysler would specify such tight clearances.

 

Why?  Tight clearances are much better than loose. 

[Matt Wilson]

I've read so much about people who are more experienced than I, and their machinists, tending toward larger clearances, like 0.0015 - 0.002, or larger. 

 

I've also talked with a couple of long-time engine builders who said they think it's a bad idea to aim for such small clearances, as all it takes is some tiny little something not being exactly right, and it'll wipe out a bearing, when a bit more clearance would have prevented it.  And these are guys who are extremely picky about the work they do.

 

Certainly the 0.0001 and 0.0002 seems way beyond the limit for good practice, and even 0.0005 seems like it's taking undue risk.

[Sniper]

It is more work to machine an engine to those tolerances, so basically I hear "I'm lazy".  Bet they run 20w50 oil too.

 

 

 

[soth122003]

Motors manual 1935-1953. Dodge section

Camshaft bearing clearance.001-.003. all years

Connecting rod bearings: .0005-.0015. all years

Main bearings: 1935-1940 .001-.003. 1941-1953: .0005-.0015

 

Now this is the auto edition, but I doubt the trucks are all that different, unless the engine size is bigger than a 230. Dodge put the 217 in cars up to 1941, then went to the 230 in 1942.

 

Hope that helps.

Joe Lee

[Matt Wilson]

20 hours ago, Sniper said:

It is more work to machine an engine to those tolerances, so basically I hear "I'm lazy".  Bet they run 20w50 oil too.

 

 

 

Right, I can understand what you're saying.  These couple of guys I'm talking about, I would not characterize as lazy, but they deal a lot with higher performance engines, and have seen the result of cranks or blocks flexing under high torque and small clearances, which resulted in bearings getting eaten.  They are staying on the side of caution.  That concern may not apply so much to our engines, with their lower power output levels, but they were just advising to be more on the safe side.  Other folks who are pretty experienced with engines, even flatheads, usually prefer to stay on the larger clearance side of things.

 

I know that it's possible to achieve near-perfect crankshaft out-of-roundness and taper within each journal, and near-perfect runout across the whole crankshaft, but I think they were being a little more realistic with regard to what is much more commonly the result, and knowing that blocks and cranks deflect under load.

[Sniper]

1 hour ago, Matt Wilson said:

higher performance engines,

 

Once again, someone is applying race thinking to a street engine.  I see it all the time.

 

You're probably going to be making 100hp and the RPM's will be half that "high performance engine"" limits.  That thinking does not apply here. imo.

 

I think Chrysler, which made millions of these engines for decades had substantially more institutional experience and knowledge in this regard, I would follow those recommendations myself.  But ultimately, your engine, your money so you get the final say.

[Matt Wilson]

2 hours ago, Sniper said:

 

Once again, someone is applying race thinking to a street engine.  I see it all the time.

 

You're probably going to be making 100hp and the RPM's will be half that "high performance engine"" limits.  That thinking does not apply here. imo.

 

I think Chrysler, which made millions of these engines for decades had substantially more institutional experience and knowledge in this regard, I would follow those recommendations myself.  But ultimately, your engine, your money so you get the final say.

Fair enough.  But it was not just race guys saying this, but also guys who have rebuilt plenty more engines than I have, so hopefully you can see the origins of my question.

 

Another guy quoted an old-time machinist who said it's very difficult to achieve the same tolerances as the factory, when it comes to crankshafts, with aftermarket crank work (at that time, at least - talking decades ago) being as much an art as anything else.  Or who knows, maybe that particular machinist was actually lazy.

Edited June 18, 2023 by Matt Wilson

[Matt Wilson]

Then on the other hand, some of my manuals allow as much as 0.003" runout before it's deemed necessary to regrind the crank.  That seems really large.  But I'm sure there's plenty I don't understand that goes into that.

[Sniper]

8 hours ago, Matt Wilson said:

Another guy quoted an old-time machinist who said it's very difficult to achieve the same tolerances as the factory, when it comes to crankshafts

 

Yes, worn out tooling, the hurry to get it out the door, the "it's close enough for government work" thinking all add to that.  One would think today's machinists could match the 70+ year old factory work without an issue.

 

8 hours ago, Matt Wilson said:

hen on the other hand, some of my manuals allow as much as 0.003" runout before it's deemed necessary to regrind the crank.

 

That's the wear limits, not the finish specs.  If you start at 0.0025 well the wear limit isn't far away.

[Dartgame]

If I were to guess I'd say the military spec is a mis-print. I know of no engines that use less than .001 for bearing clearances. Usually .001 to .0025 is good, .003 is getting kind of loose, for a street engine, not so much for a race motor.

[D35 Torpedo]

12 hours ago, Dartgame said:

If I were to guess I'd say the military spec is a mis-print. I know of no engines that use less than .001 for bearing clearances. Usually .001 to .0025 is good, .003 is getting kind of loose, for a street engine, not so much for a race motor.

Loose is fast. I agree 100%. And lazyness has nothing to do with it...an experienced machinist would huck that manual. .0001 is basically metal on metal. No room for heat expansion or a film of oil. Modern engines run tight clearances with thin oil. Tight being .0015.

Edited June 20, 2023 by D35 Torpedo

[kencombs]

Close clearances require oil thinner than most use in old cars, IMO.   But, for years the rule of  thumb was one thou per inch of journal diameter.  Realistically I shoot for .0015 to .0025 for rods, maybe up to .0035 for mains.

 

I would only go tighter on mains if the block had been line honed by a good shop.   An extra thou clearance is sometimes needed to prevent binding due to a little misalignment.   I've seen cranks that wouldn't turn with all the caps tightened, but when all but one was loose Plasti-guage normally.  

 

There used to be a source for oil pump 'overhaul' kits, spacer and longer gears to provide higher volume.   Used them in old Y-block Fords that had oiling issues.  Covered up the oil pressure loss by just pumping enough to build pressure.  

 

I know you've all seen engines with really low idle pressure but OK at speed.   Result of bearings, especially mains, at the upper end (or over) or the spec.

 

edit to add:   If you want to get deep on the subject look up 'hydrodynamic lubrication' .   The theory there helps with understanding viscosity and bearing clearances.     Once started there should never be any metal to metal contact because of that phenomenon.

Edited June 21, 2023 by kencombs

[Matt Wilson]

On 6/18/2023 at 5:54 AM, Sniper said:

That's the wear limits, not the finish specs.  If you start at 0.0025 well the wear limit isn't far away.

The 1952 Army/Air Force manual says 0.003" crankshaft runout is for new parts.  Yes, the same manual that allows down to 0.0001" and 0.0002" clearance for rod and main bearings, respectively.  Crankshaft runout wear limit is stated as 0.005".  The later 50's industrial engine manual says 0.003" is the maximum limit, presumably meaning the wear limit, although it's not 100% clear.  In any case, the manuals are not consistent.

[Matt Wilson]

This evening I pulled out my Haynes manual for 1980 - 1994 Ford pickups.  It has various bearing clearances depending on which engine we're talking about, but there are some that show as little as 0.0001 - 0.0015 for mains and 0.0008 - 0.0015 for rods, and other that show 0.001 - 0.0014 for rods and mains.  These are all new part clearances.  I don't know if Haynes reflects the factory manual accurately or not.

 

I still say the smallest of these seem too small, especially 0.0001, compared to everything else I've read, including clearance information that I've read from companies that make bearings.

Edited June 21, 2023 by Matt Wilson

[Dodgeb4ya]

I rebuilt my 265 in my 52 pu back in the early 70's.Not the first flat head I've ever built.

I remember and some where have the assembled specs..rods where between .0015" and .002".

60,000 miles since and still runs well and no issues.

I have never had rod bearings under .001'. Mains have always been .0018 or so up to .0025".

Have done many MoPar flatheads over the years. I really wouldn't want those super tight bearing clearances on any of my flatheads.

Newer cars ...maybe just the way it should be. JMO.

 

Edited June 21, 2023 by Dodgeb4ya
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