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1920's Rod bearings


20Premier

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If your problem is to know how much to reduce the clearance on the axis of the conrods, (assuming that bearings and crankpins are otherwise satisfactory), you should be able to get compressible plastic gauge material . This is called Plastigauge here, and probably the sams where you are. There is a scale in the pack which gives you a direct reading of the diametral clearance according to the flattened width of the strip. You will need an honourable, economical, oriental 0-1 inch micrometer and shim stock if you need to make any new shims. Drilling holes in thin shim material is difficult unless you zero-angle sharpen a drill; and if you need to do this , ask me, and I will get my son to scan instructions for you to do this. More likely, the shims already there will suffice for adequate adjustment.

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I always worked on the assumption of .001" clearance per inch of diameter. I know that I read this somewhere in the last 55 years but cannot remember where. I have redone more than a few engines and have always found this satisfactory.

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Guest De Soto Frank

Splash-oiled motors like the Model A had a factory bearing "oil clearance" of 0.0015" on the rods.

Pressure lubricated insert bearing motors (steel shell / thin babbit lining) usually run about .001-.003"

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1920 Premier. See the car in Our Cars & Restorations, 20Premier

We tried plastigauge, and at .002 the rod would not turn at all. We have been experimenting with a method of putting the rod and piston at 20 degrees from the top and seeing if it will drop to the bottom without swinging. ?:rolleyes: Found this method in old books from the 20's.

post-54566-143138471034_thumb.jpg

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Are the crankshaft journals round?

When the rods are assembled on the bench, are the bearing inside diameters still round?

What rod side clearance do you have?

Also, possible bind between crankshaft journal fillet and rod bearing.

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Took a while to cross-check my reference books. I do not carry precise clearance data in my head, but always look it up when I have to do a job.

There can be abiguity and confusion on what is meant by bearing clearance. Logically this could mean either the difference between the journal diameter and the bore of the conrod bearing, OR the thickness of the oil film between the assembled two, (which you fondly believe should be the same all round.) This may not always be what you find when you measure with and accurate inside micrometer, (the screw barrel cliper type are least variable according to operator experience and technique.) Worn or heavily worked big-end bearings may become oversize on the axis of the rod. Ifyour big ends are slighly out of round and you plastigauged where the diameter was greatest, it could be hard to turn. Some makers apparently recommended almost zero clearance. In 1935 Chevrolet loosened their thinking a bit and specified 0.0005". Before that they said zero or it was too loose: If you could not turn the conrod on its journal by hand, but could turn it with a hammer that was what you needed! When I was a kid I used to wonder why garages would tow a car with a rebuilt engine in gear until it would start,-- that is, if the tyres had enough grip. Maybe the machined finish on their bearings was a bit rough. In the National manual the clearance listings were mostly one and a half to three thousandths, with most figures at the lower end. One maker's advice was that 6 thou worn was the maximum allowable.

When you think about it, the function of the oil is a) lubrication, B) cooling, and c) separation of the surfaces of the bearing, and d) distribution of oil spray to some other parts of the engine. The flow of oil through the bearing which is necessary for B) and d) is dependant upon the end clearance, Which seems to be mostly recommended to start at a lower limit of 4 Thou. If clearance and oil flow is excessive, the oil pump may be inadequate to maintain desired oil pressure. Obviously if the main bearings are worn but connecting rod clearances are correct, the latter may not get as much flow and pressure as you might hope.

Clearances listed for main bearings seem to be almost evenly half a thou greater clearance than the big ends.

The two technical reference books that I always find most useful are from Repco. You will remember it was they who developed one of you Detroit stock blocks into a very successful F1 championship winning engine. The Repco Engine Service Manual ISBN 0 909638 00 4 can be found on the internet (I have just checked) . The technical coordination, editing, and presentation of the knowledge of numerous Repco specialists was the work of Frank Hallam of Repco Reseach, who was later General Manager of Repco-Brabham Engines. It is a very good book.

The second book is the one that preceeded that one. Author is R Bertel, and the title is

Digest of Automotive Engine Re-conditioning. This is also available secondhand on the internet today; and its particular usefulness for antique automobiles is the most excellent section on re-metalling and machining tin-based babbit bearings and machining them.

If you want to get either of these books and cannot find them, I might be able to help if you send me a P.M.

One further old method which works if you do not have or have access to good inside and outside micrometers, is to apply "bearing blue", which is a light grease carrying strong Prussian blue die to the babbit, assemble it on its journal with ist bolts correctly tensioned, and turn it by hand if possible. This reveals the high spots that are dragging on the crankpin, and you can carefully relieve these with a bearing scraper. You repeat this until the bearing turns freely. This may not be ideal for purists, but it is an old art, and it does work.

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I have used plasti-gauge on both babbit and inserted bearings. I usually fit oil pressurized insert engines on the .001-.002 inches and babbit in the .002-.003 inch area. So far have not had any problems. I also use GM assembly lube when I put the engines together. It might also be called Engine Oil Supplement. The GM part number is #1052367.

Edited by Larry Schramm (see edit history)
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1920 Premier model 6D, engine by same manufacturer. Aluminum block with cast iron sleeves and aluminum pistons. 6 cylinder, 3-3/8" bore, 5-1/2" stroke. Pressurized oil, inserts bearings on both rods and mains. Overhead valves with roller tappets.

I like the idea of using the blue dye. Keep the info coming, it is interesting to learn how everyone has done this before.

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You have to "at least" check the crankshaft journals for oval and taper. If the crank is not true, all of the bearing blue, plastigauge and bearing scraping will net you nothing with rods that do not fit properly and a rod knock in the future.

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They are "insert" bearings but not the modern type we see on post 1930s cars. I haven't seen them but they are likely machined bronze inserts that fit into the rods and had the babbit poured into them rather than into the rod itself. The babbit is just as thick as it would have been were there no bronze inserts. They were originally fitted by scraping. Inserts like this were a common practice on high end cars before the invention of modern insert bearings, as the babbit stuck to bronze better than it did to steel.

From the descriptions above, it sounds to me as if the crank pins are slightly out of round - not surprising given its age etc. They probably should be ground and new babbit poured. While the babbit can be poured into the inserts, the rods have to be assembled with the big end bolts brought up to full torque before the babbit in the rods is final bored. They can then be disassembled and the finish scraping done. If the insert babbit is in good condition it might be possible to have the crank built up and reground sightly oversize while the rod bearings are bored out a few thousandths to meet the new dimension (This depends on how thick the babbit is. Many old cars have much thicker babbit than is really necessary)... I've never done this myself and I've only seen it done once but it did work. It was my car and I subsequently put a lot of miles on it. Today I would repour the bearings with the best quality tin based babbit metal.

Edited by JV Puleo
typos (see edit history)
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Guest De Soto Frank

1920Premier,

Any idea how many miles were on this engine when it was parked ?

A possible resource might be Kohnke Rebabbiting Services... Herm is a regular contributor on the Fordbarn website, and deals with many odd babbit engines...

I would go along with suggestions to thoroughly check the rod and main journals for "egg" and "taper"... if either condition shows-up, it will need to corrected...

Edited by De Soto Frank (see edit history)
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The odometer shows 21,373 miles. There is a lot of babbit material in the inserts. Do you know what type of tool to use to scrap the material with..

A connecting rod boring machine. Any automotive engine rebuilding shop will have one.

Using any other method will produce inferior results.

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Guest Backyardmechanic

Most connecting rod machin e of today is a sunnen grinder witch hone the rod without a bearing shell.Yes there are a machine that bores but these are getting rare.

Vern

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Be careful, a hone can leave grit embedded in the bearing and cause wear on the crankshaft.

The old way was to apply Prussian blue to the crankshaft, bolt the rod in place and move it back and forth, take it off and scrape down the high spots (with blue on them) until the whole bearing came in contact with the journal. A triangular scraper was used for this. If you don't have a triangular bearing scraper it is possible to make one by grinding down a triangular file. If you can find a triangular file these days.

Of course, you first have to check the journal to make sure it is perfectly round. You do this by taking multiple measurements with a micrometer. The journal should measure the same diameter no matter where you measure it. First inspect visually for wear or scoring but this is pretty obvious.

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Here is some info I dug up. Did a Google search for Babbitt bearing scraping and got 34500 hits. These are a couple of them. There are bound to be some good ones I did not look at too.

The art of scraping Babbitt - SmokStak

This thread even has some discussion of pouring and fitting Model A Ford bearings. Apparently there are different kinds of babbitt metal for different applications, car engines require a specific kind of babbitt.

CR4 - Thread: Re-babbitt bearing procedure

Fitting babbitt bearings, 1923 Star manual

http://www.machine-history.com/node/406

Edited by Rusty_OToole (see edit history)
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Guest marble1199

I have the 1920 edition of Automobile Engineering and it says that the clearance for the babbitt bearings should be 0.002 in total or 0.001 on both sides of the crank.

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Guest De Soto Frank

I would NOT go trying to scrape / otherwise fit your bearings before you've determined that the crank journals are round and straight.

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Yes there are a machine that bores but these are getting rare.

Vern

Yes, you are probably right about that, but they are around. I had my rods done on one. I would do whatever would be required to have the rods finished on a proper rod machine rather than getting out the blunt instruments and start scraping away!

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Babbit bearings should not be honed... boring is fine but, in the long run, not as accurate as hand scraping. None of these procedures are recommended by a first-time restorer with no prior serious machine tool experience. To this day the very best flat surface is achieved by hand scraping. It is used on bronze, iron and babbit but in all cases is a skilled undertaking.... and, scrapers aren't blunt, they're sharp.

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No... I'm not kidding. Haven't you ever seen scraper marks on the bed of a lathe or a milling machine... or hand scraped spindle bearings? It is all the same process. My late uncle scraped the spindle bearings in Brown & Sharpe surface grinders - a feat that demanded far more accuracy than is even achievable with machine tools. In period, babbit bearings would be poured and entirely fitted by scraping and it was a skill almost all general machinists had. One of the main reasons babbit was so popular was that it was so easy to work with. Those are largely lost skills now but that wasn't so in 1920. Hand scraped bearings were certainly good enough for Silver Ghost RRs. Scraping connecting rod bearings is time consuming and delicate work but it isn't hard to do and, with enough patience, doesn't even require all that much skill, although I still wouldn't recommend it as a "first time" undertaking since the potential for catastrophic failure is there. If these were relatively slow moving machine bearings, and failure would not result in a smashed block, it might be worth trying.

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Well, in this thread, I was trying to present practical ideas to help with the bearing problem.

Recognizing the make of car and its rarity, I suggested some measurements to take and how to make the most of a repair, if it was required. And that would mean a rod machine if the bearings needed attention. But, if scraping is the superior method, then that is the route to take and I accept my 100 lashes with a wet noodle...

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Look at it this way. A reamer is nothing but a sophisticated version of a hand scraper. With care, the hand scraper can fit bearings as accurately if not more accurately.

You will be surprised how close a fit you can get if you are careful. You will also be surprised how long it takes and how much material you have to take off. At least it seems like a lot but it really isn't that much, because if it is done correctly the shavings are very thin.

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Guest De Soto Frank

Usually, they are triangular, tapering towards the end, with a channel milled down the center of each face. Would think that the legnth of the straight edges of the tool should be a bit longer than the width of the bearing face that you are scraping.

Have you determined that the crank journals are perfectly round and true ?

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Dat's it!

Incidently babbitt is still routinely poured, scraped and fitted today on bearings and thrust blocks on large ships. I had an interesting chat with an engineer on the WW II Liberty Ship John W. Brown about their performing this work, also on the connecting rods. Bearings are huge, RPMs a low (in the range of 80 or so).

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  • 7 months later...

I don't know where to start here, and it is an old post, But there are a few things I can add here. First of all, our scenario is, if all babbitting, pouring, and or all machine work is done properly. Connecting rod lathes haven't been made, since around 1940, the rod lathes made today, are made to cut brass bushings, and resize rods for modern inserts. The R.P.M's are to fast, and the cutters have to be a totality different design. When you have a new rebuilt rod done on a rod machine, and the crank is ground right, you should not have to do any thing but bolt it in place.

This brings us to Plastigage, Plastigage will always work fine, IF you use it right. Instructions say clean all the dirt, and old oil off crank, and bearings. What it does not tell you, don't use it dry, you have to put 2 or 3 drops of oil on the crank, and bearing before the Plastigage, and the the cap. If you don't, a dry piece of Plastigage will not spread out right on a dry surface, as most of the time it smears, and will give you a false reading. Now, Always read the Plastigage on the hardest surface, which is the crankshaft, as the babbitt side can also be a little off. With out the oil, the Plastigage can press into the babbitt.

Now, on Scrapping, JV is right about the accuracy of hand Scrapping, as a machine, that mills, or in most cases surface grinds, on say lathe ways, the close as it can come is .003 thousandths, that is not good enough, as most go for .00150, or less in the full length of the bed. On babbitt, scrapping most scrapping in done on old wore babbitt, rather then New machined Babbitt. Now on the oil Blue that can be used, you are better off, if without experience. or with, not to use it at all. The best way is to tighten the cap down, so it drags a little, and the crank will leave Black marks, and that is what you take off. With that said, you should only take off , about .00020 of a thousandths at a time. The scrapper should be laid flat, and the scrapper pulled towards you, and very lightly, and keep them very sharp.

This brings us to bearing clearance, which at best can be confusing. In bearing books like Federal Mogel, lets use a 2" crank for this purpose. A bearing book can say .00050 to .00350 thousandths clearance on the shaft. What that means, in even a modern insert motor, you do not have any shim adjustment, so on an engine out in the boonies, you might have to settle for what you can get. Ok, here's the deal, a 2" shaft when hot, will enlarge by 1-1/2, or 2 thousandths, if you gave it 1/2 thousandths, where is the oil clearance going to come from. You need a shaft clearance of .002 Minimum, to 2-1/2 Maximum, so the crank has to strugle to get its self free, and in doing so, where does the extra clearance come from, your right, it is not the crank, it is the softer bearing, and that don't do them any good.

The biggest reason for rebabbitt bearing failure is the babbitt not stuck to the brass, or steel shells, in which one, is NOT any harder than the other to pour. When your babbitt job lasts 1000 miles or less, its the bearing rebuilders fault. All I have to do is look at a failed bearing, and easy to tell what happened. About Tinning a bearing, you can have the best job of that, and still not have a good bond, as you only have 40 degrees + or - of temperature to go, or no go. If you don't have two Babbitt Pots that are regulated, it is just about impossable to pour a bearing, and not have it fail, just a matter of fact.

Edited by Herman Kohnke (see edit history)
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