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Connecting Rod Bearings


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Hi,

I have a 1926 Packard Eight.

What should the connecting rod bearing clearances be?

These are the original babbit bearings. I have measured 0.004 - 0.006" on two rods so far. Is that excessive? What can I do? There are no shims in the caps.

I do have a knock on start-up which disappears when oil pressure builds, and that odd noise I sometimes hear around 25 mph I am now thinking may be the rods.

Thanks --- Scott

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If I remember correctly .004 is acceptable, before caps are torqued. Are you sure the knock is at crankshaft? Did you check for worn valve-lifter guides, etc. Maybe you haven't measured the rod that's really loose, yet. I doubt that it's the main bearings but it's possible.

Shims work, as a quick fix, if you can locate the ones you need. They last a surprising long time. Of course that is not as reliable as a complete rework of the bearings.

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You arent going to like my answer - the "fix" is going to be expensive.

.003 is pushing the outer limt. ANYTHING over that and you are going to be punishing your crank-shaft. The shock loads of a long stroke engine like yours, combined with the absurdly low final drive ratio, imposes "loads" on your crank-pins, at 45 mph, MUCH greater than a modern car cruising at 100 mph.

First thing you need to do is get a "mike" on your crank-pins and make sure they have not already been "pounded" out of round.

I cant recall when Packard went to off-set balance weights - if your engine has these, they must be removed before the crank-pins can be machine/ground. Very few shops have seen this kind of balance weight - they HAVE to be removed the right way and re-installed the right way.

As for rod bearings themselves, at the risk of repeating earlier discussions on the problem, let's see how brief I can be. There are still shops that can re-pour "babbit" into your con. rod "big ends", but most restorers do not consider this an acceptable shop practice on ANYTHING but the brass era cars where engine rpm is never going above 1,200!

Except for the MOST gentle VERY slow speed OCCASIONAL use, I dont think you will get good service out of a poured babbit bearing. Poured babbit worked out when no-one expected an engine to go much behond 25,000 mi. But by the late 1920's, ever higher road speeds and engine rpm made con rod bearing failure an epidemic - finally solved by the introduction of the "modern" or "insert" type steel backed rod bearing that was pretty much standard by the mid 1930's.

True, GM did "get by" with poured babbit in the Buick and Chevrolet product line clear up to 1952. They discovered IF you had the rod machined so that the babbit layer was VERY VERY thin, it would work for a while. Again, they gave up and went to modern insert style bearings, and that is the industry practice today.

You need the services of a competent full service machine shop familiar with the oddities of old engines, to get you squared away. Tell us where you live, and one of our Packard buffs will give you some suggestions as to where to go from here.

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SafeTFlex is correct!!! Your engine is crying out for help!!! Do not ignore it's noise!!!

It is time to properly re-build your Packard 8

with modern bearings...

Don't forget to replace the pistons & wrist pins as these are normal replacement items in a proper re-build!!!

The rings & bearing shells used today are far better than those origionally used in the time your engine was first built....

Your re-build should be done SOON!!!

Don't wait until you throw a rod!!!

GOOD LUCK!!!

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