I have a question about shimming the bearings on an engine. I am starting to install the components on my A engine. All machine work is done and I have used Timesaver to lap the main and rod bearings, which seemed to work well. The question or problem I see is it appears that each shim is .003 thick, so that is the smallest granularity available. Bratton's model A catalog state the shims are .003 thick with several of them in stacks of 4 or 5. I put a shim stack on the rear main to start with and with plasti-gauge it read .002. The ideal clearance seems to be .001 to .0015. I want to have no or minimal oil dripping from the engine. In Les Andrew's mechanics handbook, it says to remove or add a equal number of shims from each side when adjusting the bearings. If I did it that way, I would lock up the crankshaft. Les must of had .001 shims available. I guess you could install shims in an unbalanced configuration, i.e. 4 on one side and 3 on the other. With .002 clearance, if I remove a .003 shim on one side, it seems like the clearance will be too small. Is it better to have a little more or a little less clearance on the bearings, since getting .001 to .0015 is very hard to archive with .003 shims. Thanks for any help.
Engine shim question
- Thread starter wrndln
- Start date
Glenn C.
Well-known member
In my opinion, it is quite difficult to get a accurate clearance reading of .001 - .0015" using plastigage, due to the softness of the babbit. You are better off using either .002" thou shim stock or a equal thickness of kitchen tin foil, cut in a narrow 5/16" strip across the journal, doing each bearing one at a time, using the lock-up method. Make sure you have lapped the rear main on a piece of ground granite or heavy plate glass, to ensure flatness and a good seal joint, before doing the final clearance check on the rear main.
Had the same problem trying to an "in between" shim thickness. You can actually sand them down. Get a really flat surface (a piece of window pane or lucite from the plastic store) use 400-600 grit paper and sand them down. If you have a 0-1" micrometer it is easier to measure, but you can do it trial and error until it is right. Clean the shim with low viscosity oil (or 3 in 1) and toilet tissue paper or paper towels until absolutely clean before putting them in. Do not use cloth. Any minute pieces of paper will break down and dissolve, cloth particles wont. You can lap the rear main as Glenn C says the same way.
Kohnke Rebabbitting
Well-known member
Wrndln, there are two places where they make shims, one makes .003 thousand shim pieces, and the other makes .002 thousands pieces. The reason there is, and always has been .003 thousand pieces, is that they are cheaper to make. we only use the two, as there in not as much messing around. Always alternate from side to side. Taking out a .003 piece, as you already know is way to coarse of an adjustment. If your .003 is to much in, and the .003 is to much out, get sheets, or roll of .002 thousand, and .001 thousand Brass. We get ours from the factory, and you have to buy a bunch at a time. It can be had from some machine shops, or McMaster Car, M.S.C. ect. Now then, if say the .003 is to tight, lay your shim pack on a .001 thousand sheet, and take a very sharp hobby scalpel, and trace around the shim,( I lay it on a piece of OAK),very carefully, and also the hole. Just doing that should have it cut out. Put the .001 in, if still to tight, put a .002 thousands in, If you need to go in between that, go to the other side, and take out .003, and put back in a .002. One last thing, for the guys that cant get Plastigage to work for them. Take the strip, about 3/4 inch cut, cut that piece in half, and place on each side of the crank, divide in 1/3's, if the motor is out of the car, in the cap if you are under the car. The most important PART, or PLASTIGAGE WILL NOT WORK. Wipe any, and all old oil, dirt, ect., from the cap, and shaft, I use lacquer thinner, also in wiping off used Plastigage. YOU have to use oil, or the Plastigage will not spread out, and give you a false reading, and WILL go into the babbitt, I use 30 weight, as it will hold the Plastigage better, and will let it spread out better. When you pull the cap again, DO NOT READ THE PLASTIGAGE ON THE CAP, you have to read it on the hard surface, THE CRANK. If you don't use the oil, it just won't give you an accurate reading. Thanks Herm.
Mike V. Florida
Well-known member
Would you not need to remove a .003 shim from each side to decrease the opening by .003? Would not removing only one shim from one side decrease the opening by about .015?
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1931 flamingo
Well-known member
Mile V. Florida; Maybe I don't understand, haven't done it myself, but would you not be DEcreasing the clearance by REmoving shims?
Paul in CT
Paul in CT
Mike V. Florida
Well-known member
Mike V,
If I have .002" clearance now and I remove one .003" shim on one side, by my calculation I would only have .0005" left (.002" - .0015) - to little. Where am I wrong? I think the solution is to buy some .002" shims or make some my self like Herm suggested. I didn't know where to buy shim stock, but Herm said McMaster-Carr sold it. Something else Herm mentioned, use a little oil before laying down the the Plasti-gauge. I had never heard of that, but he should know being a babbitt shop.
Thanks to all to responses.
Rusty Nelson
If I have .002" clearance now and I remove one .003" shim on one side, by my calculation I would only have .0005" left (.002" - .0015) - to little. Where am I wrong? I think the solution is to buy some .002" shims or make some my self like Herm suggested. I didn't know where to buy shim stock, but Herm said McMaster-Carr sold it. Something else Herm mentioned, use a little oil before laying down the the Plasti-gauge. I had never heard of that, but he should know being a babbitt shop.
Thanks to all to responses.
Rusty Nelson
Kohnke Rebabbitting
Well-known member
You are right Mike, if the shims are gone, there is no other choice, and if you go to far, you can put a shim back in. Thinks, Herm.
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Kohnke Rebabbitting
Well-known member
Yes, very true,Mike, if you need to, but when you get down to the fine setting, that is when, you use the .002, and .001 thousands shims for adjusting. Taking out a .001 shim space out of one side, drops the crank .000-500 thousandths. Thanks Herm.
Kohnke Rebabbitting
Well-known member
If you have .002 thousand clearance, leave it, you are on the money, any, and all engine bearings should be set .001 per inch of shaft minimum, or plus .000-500 maximum. Say a 2" shaft, would have .002, or plus a 1/2, maximum, which would be 2.000-50. in the case of model A mains, shaft size 1.625, or 1-5/8's, or 1 and 6/10's, there for we set the bearings at .002 thousands, or 1/10 over .002. If you were to say set a Model A main at .001, you are going to take a chance of burning the mains, or rods out. The shaft on a Model A main will grow, at least 1-1/2 thousands larger when hot, take any crank and mic. it, and then put in the sun, or under a heat lamp, so it in hot to the touch, like it would come out of an engine, then Mic. it again, and see how it has grown. So if you set the mains to .001 thousands, where is the room for the oil, and where does the to small of babbitt room go. If you don't burn the mains out in the first 15 minutes, because you will smear the babbitt, (( The small pieces of babbitt fill the oil groves, and oil wells, to haunt you latter)), and if the bearing can recover long enough, the smeared particles of babbitt, now hard, and the size of sand, now break off, and go through the bearing, the take it out, or really drops the longevity of the bearing. In modern bearing books, modern engines will have there shaft size listed, say a Ford 312 engine, standard main shaft size is 2.6235, to 2.6243, they list bearing clearance at .0005 min. to .0029 max. You have to keep in mind there are no shims in these bearings, so they list what you can get by with. So we got a crank that is 2 and 5/8, and some set at the .0005, how much is that crank, when hot going to swell. It will have oil pressure, which is better, but the bearing hole is to small, so what will happen??? So if we take the .001 per inch, plus a 1/2 it will be set to 2 and 6/10ths min. or 1/10 of a thousandths under .003. Thanks Herm.