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So, after installing new main bearings, I am in a bit of rough spot. I measured main clearance using plastigage, and got:

#1: .006
#2: .005
#3: .006
#4: .0055

The book says I should have .0005 - 0.0015. The crank was polished, not ground, but I don't think the machinist took much, if anything, off, because it doesn't look like the finish even changed.

I have remeasured the crank several times, and my results indicate I should be about .002 to .004 under nominal of 2.7465, so I should have .0025 to 0.0055 clearance. It seems odd, because I checked clearance with my old bearings, and got similar results, but I had 20-25 psi with the engine warm at idle, which seems high for that much bearing clearance. (At least that is what the dash gage read)

Should I spec out 0.010" over bearings and grind the crank, or just run what I have?

The genesis of all of this work was a "simple" ring and valve job. Valves are great, now, but haven't gotten to the piston rings yet. Engine ran ok before I took it all apart, but smoked a little, which is why I started in on the ring and valve job.

I'm interested in what the experts think.

The best would be to go to the .010 size. Excess bearing clearance contributes to oil burning as there is much more oil being thrown around in the the crankcase and the oil rings have trouble scraping it down.

You could however get a pretty good result by finding a set of .oo2 bearings. That was a standard service item for tired engines back in the day and there are many sets of originals floating around. I might even have a set.

And how are your rod bearings?

I like Ross idea of the .002 under bearings. What about the rods???

Even the service bearings in 0.002 undersize would still leave quite a bit more clearance than one would want, though it may be the best "quickie" fix I wouldn't count on it for a long service life and surely won't do the crankshaft any good. Best solution as Ross notes is to have the crankshaft turned to a standard undersize. But another thought if the journals are evenly worn (without flatspots or uneven wear) is to find a set of main bearings in "semi", meaning unfinished to final size, and have them sized to fit your current crankpin dimensions. I did crankpins on a 356 Packard years ago and all I could find at that time were "semi" mains which I had finished to the dimensions I wanted. I don't believe "semi-finished" bearing shells were a listed Packard service part, I got mine from a Federal Mogul facility.

PS - if the main bearing journals show that much wear I'd expect the rod journals to show much more.

When u plastigauged the bearings did plastigause only once or also a second time with the crank turned 90* from the first position f plastigauge test???

My point is (regardless of the polishing) is to determine how out of round the crank main journals are.

Certainly grinding the crank and new undersize inserts is the optimal way to go for very long term ownership and max engine service life.

On the other hand, if it was a dealer warraanty claim "back in the day" then the mechanic would drop a set of .002 under bearings in the engine and the owner would no doubt still get 40K miles out it which is about all any of the prewar and immediate post war engines were good for from the factory when brand new anyway.

THis is one of those 'judgement calls' that can vary from individual situation to situation.

PackardV8's suggestion about checking the clearances at several radial locations on the crank is best practice and he's right of course, you should do that check.

But I can't agree about a 40,000 mile service life for those engines, assuming good care they were good for far more than that. And a "warranty" claim would not have used 0.002 undersize bearings, the warranty period was only 30 days, was it not? Hard to think anyone engine could get amount of wear in 30 days and if the engine did show that much wear within the warranty period, I suspect Packard would replace the engine or do a proper repair or disclaim it entirely if there were signs of customer abuse (like running with no oil).

Your machinist should have checked the crankshaft before and after the work. If there was taper problem or out of round problem then that is a reason for a grind. Unless there is some visible problem with the journals polishing them will not be much benefit.

The service manual specifies .0005 to .0025 clearance for those main bearings. Oil leakage from worn bearings increases with the square of the amount of wear. For example, wear of .002 will result in up to 4 times as much oil leakage (translation low oil pressure) from the bearings.

Of course covering all those bases and doing the necessary paperwork is more expensive. Here in Detroit we are dealing with big money problems. People don't bitch about doing that sort of thing around here.

So unfortunately you do have a problem that needs to be straightened out. I would first put your old bearings back in and check the clearances. If you look in the Packard manual they talk about selective fitting of bearings (the same way they build motors today). Of course we don't have shelves of bearings lying around waiting to be sold so some of these rebuild problems represent a compromise. So you need to do the best you can and don't expect to run the car 85 mph from Denver to Chicago the way they could when it was new.

Hope this helps.

See post #2 above by Ross and i quote:
" .... a set of .oo2 bearings.
.... and there are
many sets of originals floating around. I
might even have a set."

That statement might very reasonably be interpretted as a subtle way of saying that BRAND NAME replacements are STILL avaiable in that size.

THE REASON is that not many engines OF ANY MFG'er remain after 5 or 6 decades that have not already been ground .020 to .040 under. Therefore many of the std. and up to .010 under are still hanging around this world as OLD STOCK TRUSTED BRAND NAME parts.


I've gotten into this problem on non-Packard applicationss from the late 40's thru the late 60's.

I'm not sure that there has been enuf testing and operation of any REproduction modern day parts to determine the quality level and service longevity.

NONtheless, since this particular engine only needs a .010 under size then those are probably still somewaht plentiful too. Anything over that becomes questionable.

I have not checked the crank at several locations with plastigage, but I did check the mains at several different locations radially. I believe my machinist did run outs and recorded either the average or the minimum diameter. For example, my measurements on #1 main:

2.7441
2.7455
2.7445
2.7436

He recorded 2.7405.

I have not plastigaged the rods, but the machinist report between .0078 and .0067 under spec.

To ensure I have the right numbers, I am using 2.7465" (+0 -0.001") for main journal diameter and 2.0944" (+0 -0.001") for specifications.

Assuming I can readily source 0.010 over main and rod bearings, I will likely go the grinding route. I am also going to a swap meet in Rhinebeck, NY this weekend- if I fall upon a virgin crank, I will pick that up instead. I have two cranks, but both are about equally worn, but my spare had the #1 rod bearing spin and or gouge the journal at some point in its life.

Thanks for all the info and advice- please keep it coming!

Also, if I spec 0.010 over bearings, couldn't I have the machinist fit them to clearance of 0.0005 or 0.001 while he was grinding the crank?