Pony bearing clearance question?

I posted a thread before about crank bearing clearance on a D2 pony motor. Thanks guys for the spec sheets.I have my crank at the machine shop, they told me that .005 clearance sounds excessive.If anyone can verify thats correct. I want the minimum clearance between crank and bearing.I know the there is a wide range that is in spec, but i am looking for the minimum clearance. Thanks

That's what it says......
Also the same for the D6 except it states 0.002 to 0.0055 for babbitt bearings and the same 0.005 to 0.0085 for aluminum.

Is .002 a acceptable clearance? It is babbit.

Are you sure it's not aluminum, that is what Cat most commonly used.

0.002 is snug. A check of my old Federal Mogul Engine Bearing catalog shows 0.0025 to 0.004. Doesn't say if it is aluminum or not but I'd think that's for babbitt.

Only my early D3400 engine manual for "J" series D2's lists babbitt pony bearings, they were used early on and were replaced by the aluminum bearings after a short time. Aluminum bearings became standard long before the "J" series production run ended. Any replacement bearings I've ever found are all the aluminum ones, you need to make sure which type you have because the clearance specs vary between the two.

These are the clearance specs for the pony main bearings from my D2 books covering both the J and U series:

Main bearing clearance (Babbitt) - .0015-.0025"
Main bearing clearance (Aluminum)- .005-.0085"
Maximum permissible clearance - .015"


I've rebuilt a couple D2 pony motors using the aluminum bearings, had the cranks ground to the .005" starting clearance, no problems whatsoever. At first I had the same reaction from my machine shop guys, you just have to remember they're used to rebuilding modern engines where clearances are universally a fraction of what these old Cat engines are. The construction is a lot different between the two time periods and I've learned over the years that it's best to trust the Cat manual on these things.

The fact that they're questioning the clearance spec is a good thing actually, it shows that you took your parts to a place that cares about their quality of work and wants to keep themselves as well as you from having a bad experience with this rebuild. I work on the modern automotive gas and diesel engines at a Ford dealership and can assure you I know where the machine shop guys are coming from on this one, a .005" clearance on a modern main bearing of similar size would be past worn out, but in this pony engine it's just like new. I've surprised my machine shop numerous times with the old Cat specs, but now they know me and expect to get some schooling from the past whenever I bring something in.