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Connecting Rod Bearings

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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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I agree with SafeTFlex... Your engine is crying out for some serious attention!!! I would not ignore it's warning noises... Over the years I have seen more rods punch their way through the crankcase of very rare & hard to replace engines... One such friend had his great old "Classic" on a tour several years back...

A restorer,who was also on this tour, was listening to his engine running and warned him about rod bearing noise. His reply "OH...It has been doing that for several years now...I am not worried!!!" Several weeks after the tour while out for a Sunday drive...Bang...Crunch...then a dead stop... The rod had let go punching it's way through the side of the crankcase... The engine was now just scrap metal... Total milage since noise started...250 !!!

I would do what safeTFlex told you to do + I would also check the pistons + wrist pins also!!! Pistons are a normal replacement item when doing a re-build on a high mile engine!!!

We Don't want to see you suffer the same fate!!!

Have the engine properly re-built...SOON!!!


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Is this as 'simple' as just machining the rods to accept the inserts? Or is there other work which needs to be done - ie to crank?? What about the mains - should they get insert bearings as well??

Is somebody making inserts for Packards , or do you just search for one that is going to fit and then machine to match.


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In answer to your questions:

You must measure you crankshaft bearing journals to make sure that they are not scored and are concentric . All rod bearing journals should be the same size. I would bet that after all the running time you have on this engine that they will have to be re-ground to a smaller undersize to bring them up to usable condition!!! Main bearing journals will be done in a similar manner. If some are worn too far they may be hard-chrome plated to add new metal to increase diameter.(a very specialzed process) The rods will then be machined for new insert bearings,and keyed, using new modern materials. Some are "Stock" engine bearing sizes, others are custom made for the old car hobby!

Main bearings are machined in a similar manner in the engine crankcase ( or block) and bearing caps. Simiar inserts are used.

Engine cylinder bores are checked for wear,roundness,and top to bottom taper. Most engines will require re-boring to an oversize...

Then engine must be properly honed to a proper cross-hatch pattern to allow new rings to seat and properly control oil useage!!! New oversise pistons + rings, along with new wrist pins are then fitted. In some cases small rod ends are drilled and bushings are insatlled.

A valve job is then done with new valve guides, possible new or re-ground valves and seats, seals,etc.

Timing gears(and chains) are checked + replaced as needed.

Cam should be checked + be re-ground or replaced as needed!!!

Cam bearings are checked + replaced as neeed...

All this is done after the engine is first stripped down, hot tanked, core plugs replaced etc and head surface checked to see if it, or block needs planeing.

Oil pump is checked +possibly replaced or re-machined and new pump gears found or made to return oil pump to as new condition!!!

All this work should done by a very good engine machine shop that specializes in vintage engines!!!

As you can see...to do a proper re-build on an old engine is not simple; and can be very very costly!!! Some parts may be found off the shelf...Others must be custom made to proper spec. It does not pay to "cut corners" and skip some of these steps...Don't do a "Half Job"!!!

If done properly the engine will now be BETTER than new!!! It will now out-last you,your children,and grandchildren!!!

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I do agree that insert bearings in the connecting rods are probably the best way to go. There are those in the professional business who disagree, however.

My own feeling is that babitt for bearings in older engines is fine provided that you do not run the engine too hard. There are businesses that specialize in replacing babitt -- "The Babitt Pot" in Glens Falls, NY is one.

If the babbitt is relativly new, then if run hard or under large clearances, it will "pound out" producing ever larger clearances. If the babbitt is ancient (from the 1920's or earlier) and the rods get too loose the babbitt will shatter like glass. Old babbitt has larger crystal grain sizes versus new babbitt, hence it looses its plasticity. Very much like bending a copper or aluminum rod back and forth until it breaks. The grain sizes on the fracture surface will be significantly larger than in the base metal.

It is true that with a five-inch stroke you are placing alot more stress on the rod bearings than a shorter stroke. The answer is to drive your anqique car as if it were an antique car. Slowly. If you do not have high speed gearing or an overdrive, keep speeds below 35 mph. On my Packard with an overdrive, I never get it above 45.

With babbitt connecting rods, you can "take up" the bearing a small amount if there is excessive clearance. This was standard procedure back in the old days. Again, I believe the key to driving a car with these kinds of repairs is to keep the rpm's down. If you need to get on the interstate occasionally or keep up with traffic on congested roads, then definitely do whatever it takes to upgrade the engine.

I do agree that the very best is to have your engine done fully by a profesional. That costs big bucks, though, that many of us don't have. You can drive and have a whole lot of fun by doing a few minor repairs on your own. Take off the pan and clean all the sludge and make sure the oil pump works well. I think it's OK to take up the rods. If you've got reasonable compression, it's OK to drive it without new pistons, rings, valves, etc. Once again, it's OK to do these things provided that you are vigalent about driving slowly and gently. Resist the popular temptation to rev up the engine, have jackrabbit accelleration and cruise at 60 in a largely original 80 year old engine.

My $0.02


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Scott hit the nail on the head here!!! Old bearing material can fail if over-stressed...

Also long-term exposure to older acidic engine oils that were used way in the past( from high sulfur in oils or acid by-products of combustion)...or left to sit in the engine bearings over many decades will , over many years time etch,pit and eat away the old style bearing material. I have seen many many bearings that were in very sad shape and heavily pitted.

Also as the bearings are subjected to the pounding of the crankshaft,rods, and other rotating mass, engine firing,high loads etc. they become "Work Hardened" and very brittle.

(See Scott's "Grain Structure" above.)

The Rod bearings are really the big problem here...more than the main bearings!!!

I would replace both...BUT...If I could only do part of the job...I would do the Rod Bearings!!!

You might be able to send these out yourself for machining for insert-type new shell bearings!!!

Most Failures are in the rod bearings...75%

The result is a thrown rod (through the crankcase or block),or seized engine,

Many people drive these cars in too high a gear; and put very high loads on the engine crank,rods,bearings,etc... The engines have so much torque that they don't think that they have to shift into a lower gear...while starting or going up-hill. This should NOT be done on an older non-rebuilt engine.

Installation of newer Higher Compression pistons in an older babbitt bearing engines should be avoided!!! (Unless shell-type modern bearings are also installed!!! (just ask my friend about his old Silver Ghost engine that failed!)

By the way... Do you have good oil pressure with your existing worn bearings???

What pressure do you see???

Does it change when the engine is under a high load??? Going up-hill & downhilletc..???(get readings only after engine warm-up is over!!!)

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Great info guys!!

I appreciate everyone's 2 cents worth, I'm taking up a collection though as all these two cents worth add up to major bucks to the re-builder. frown.gif

While I appreciate and accept all engine re-build thoughts, my question related specifically to replacing babbitts with inserts. I read or heard somewhere that hardening of the crank is also required when installing inserts??? I don't understand that one, but I'm not a rocket scientist!

Specifically with Packard engines I also noted an old topic (# 289780 on this forum) which discussed having to create side tabs in the inserts to reduce 'side play'. That scares me away from inserts.


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I see no real reason to harden the crankshaft!!! In theory you are really running on a very thin film of engine oil!!! There should not be any real metal to metal contact!!!

The tabs are there to properly locate the bearing shells and to keep them from rotating in place. The rear bearing shell also has a special thrust bearing surface to position the crankshaft...

I would not let this prevent you from having shell type beaings installed!!!

Folks that use these 70-90 y/o cars on extended tours almost always have their engine bearings up-dated to better cope with modern traffic speeds. I know of one lady in my area who drove her 1913 Rolls~Royce Silver Ghost 7000 miles across the USA and back last year!!! She had NO internal engine problems...only several tires,and a magneto points problem!!!

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This business of modifying an old engine to accept shell bearings has come up ten or a dozen times over the years of this forum. It has been the subject of debate, some heated.

I have my thoughts on the subject after much research and discussion with experienced VINTAGE engine builders. I urge anyone contemplating this operation to study the matter thoroughly. It is more complicated than it may may appear.

Sometimes, the advice you get here is worth what you pay for it.

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My oil pressure with the worn bearings was as high as I wanted it. In other words, I could increase the pressure to beyond 50 pounds, if I set the pump that high. When driving, the pressure would stay the same. Very little variation. After some experimenting I found that 40 pounds was a comforable pressure and left it there.

Older Packard connecting rod bearings have no side play on the crankshaft journal. The babbitted rod bearing sits quite confortably into a recessed space on the crankshaft eliminating side play. That could be why oil pressure remains high even if the rods are as much as 0.006".

The answer the other question about side tabs on inserts in such an engine may also have to do with the no-sideplay aspect of these engines. I do not know anything about putting inserts into older engine, so like the man said, advice here is worth what you paid for it.

We have had some good advocation for insert bearings on this thread. I thank everyone for their thoughts. Much appreciated. I would suggest that someone considering inserts on an older engine find a professional rebuilder of good reputation who does NOT like inserts and ask them why. I'll bet their answer will be very interesting. At the very least, it will allow you to make ann informed decision either way. (And please post their response here ! )


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If you use Babbitt or modern shell bearings it is not an easy job!!! Some older engines will take newer shells better than others!!! In others it is impractical or almost impossible to use them at all!!!

Modern shells have in fact been used with great success!!!

Re-Babbitted engines are fine used at moderate engine speeds!!!After -all they have lasted all these years!!!

One of the best things you can do is lower

the engines overall rotating mass. The use of lighter aluminum pistons over older cast-iron-steel can help lower the pounding loads on babitt bearings.

The use of an over-drive or change in the rear gear ratio can also improve high end speed while lowering engine speed!!! This is great for older style babitt bearings...But...an overdrive is a non-origional looking gearbox under your car! It's your choice!!!

It all depends on how you wish to use your car...and at what speed!!!

Any way you decide to go...

It is costly & not an easy job!!!

Some folks want to keep these cars all "Origional" and that is OK!!!

Others wish to tour for long distances at modern highway speeds...This will kill an older babbitt engine that was never origionally designed for such high engine speeds!!! Up-dating the engine & drive line might be the answer in this case!!!

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Scott, The rod bearing clearance would have been .0015". so .003" would be the point at which you would want to do a repair. If you drive at about 25-30 mph and ease off the throttle and hear a metallic rap or rattle sound that would be the rod bearings.

As for the oil pressure being ok, the conn. rod bearing clearance has little effect on that. The main bearing clearance is what will have the greatest effect on that as the oil from the pump goes to the mains first then to the rods. This would indicate that your mains are OK, it takes a lot of abuse to damage the mains in a Packard as they are huge.

If you decide to repair the rods I would recommend that you convert them to inserts. I have done that on a 32, standard 8 and a 35 standard 8. Cost wise it is toss up, babbitt's not cheap. The shop I used has done this for over 20 years and has it down pat. The last engine was done about 10 years ago and the cost was about $60 per rod.

The 35 is driven about 1000 miles a year, mostly on tours and I keep the speed to 55 mph as this is the speed that the car likes to settle in at. This is just under 3000 rpm.

Any way you look at it the repair is expensive, but it is less so now then later if one of the rods lets go.


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