Adjusting Model T Rod Bearings


by Glenn Storck

For folks who often drive their Model Ts, a strange “knocking” noise coming from the engine is inevitable. This noise will sound like a distant tapping of steel with a hammer. It is most evident with the engine running slightly above idle and traveling on a slight downhill grade at about twenty miles per hour. When this happens, it is time to take up the rod bearings. It is not a very complex job; it can be accomplished in a few hours’ time. No special tools are required. I have had to do this while on tour, once even on the banks of the Mississippi River in Natchez, Miss.

I have found it very helpful to remove the spark plugs to make it easier to turn the engine with the hand crank (don’t miss this opportunity to clean and gap your plugs). It is not necessary to drain the engine oil, but I don’t usually pass on the chance to change my oil either.

Next, remove the bolts holding the pan in place at the bottom of the engine block There are fourteen bolts holding the pan in place, so I use a socket and speed handle to remove them; you may have screws. When removing the pan, try to save the old gasket if you do not have a replacement available, and be prepared to catch the oil remaining in the dip troughs in the pan. There are two “U” shaped metal bars with threaded holes lying on the up side of the flanged engine opening. These holes line up with the pan holes to accommodate the pan mounting hardware. These strips will be loose and are used as a nut plate to hold the cover in place. Remove these strips and put with the cover to be cleaned before re-installing.

Now you are able to see the connecting rods by looking up inside the engine. To determine which rod bearing is loose, turn the engine over using the hand crank until two rod bearings are at the bottom of the stroke. You should not be able to move the connecting rod forward or backward by hand, so a light tapping with a small hammer may be necessary to move a properly adjusted bearing. If you can readily move the rod forward or backward, your bearing needs attention. Continue the inspection by bringing the remaining two connecting rods to the bottom of their strokes using the hand crank to rotate the engine. In most engines the number four cylinder will not be easily reachable because it is tucked under the front part of the flywheel housing. Do not despair because you will have enough room to feel the connecting rod for clearance. It will just require a slightly different method of adjusting the rod bearing.

After you have determined which rod bearing needs attention, the fun begins. The rod bearing cap is held in place with two bolts. Removing these two bolts will allow you to remove the rod cap; the lower part of the bearing. A word of caution before removing the two bolts and cap, make sure the cap is marked so that you will replace it in its correct original position. Usually, when an engine is rebuilt, each cap is identified with the cylinder number or marks made with a center punch or chisel. It does not hurt to put a mark indicating the bearing front.

Next, remove the cotter pins, or in some cases safety wire, from the cap bolts. If your engine is equipped with sheet metal oil dippers on the rod caps, you will need a thin walled 9⁄16 socket wrench to remove the nuts. Next you can remove the two nuts from the cap bolts. The cap may need a light tap to free it from the crank shaft. Remove the cap and two brass shims. Inspect the bearing material (babbitt) to make sure it is in good shape and there is sufficient material left. If your engine does not have brass shims you will have to file material from the cap until you obtain proper clearance. This is done by laying a file on a flat surface and drawing the cap across the file removing a small amount of material, then trial fitting the cap for clearance. Note: most engines that have been rebuilt have shims installed. The shims are laminated in layers, so it is a matter of peeling off one layer at a time. I use a sharp knife for this job. One layer is .002 inch, which you should verify using a micrometer. After you have done this a few times you will know what .002 inch feels like. This action of peeling off a shim layer to me is the most difficult part and I do not have a preferred method, so it is trial and error. Do this to only one shim before trial fitting the cap back on the connecting rod.

To assemble the cap with shims is difficult, so I have found a technique that works. Install the cap with one bolt and nut, with a couple of threads engaged. Before doing this put a thin film of motor oil on the rod bearing surface (rod journal). Then slip the shim into the other space while holding the bolt up until the shim is lined up with the bolt hole. Let the bolt come thru the holes and fasten it with a nut engaging a couple of threads. Do this to the remaining shim and bolt before tightening both nuts. Note: the bolt heads have a flat spot machined on their heads which will fit against the connecting rod and keep the bolt from turning while tightening. Now check the rod again for movement back and forth. If it is still loose remove one bolt and shim, peel off another shim layer before replacing it. Once you feel that the cap is adjusted properly, try turning the engine over using the hand crank. It should turn over without much effort. If you really have to put a lot of effort to do this, you probably have the rod bearing too tight and have to go back and replace a shim. If the fit seems satisfactory, it’s time to replace the oil dippers (if you have them) and tighten the bolts. After tightening, I use an aircraft stainless steel safety wire instead of cotter pins.

A word about the number four cylinder; because of its position and lack of clearance it will be necessary to position the connecting rod to provide access to the cap. To do this, rotate the engine by using the hand crank until the number four cap is at a 30º angle from the vertical when looking up at the engine. Remove the cap bolt and rotate the engine until the remaining bolt is at the 30º position opposite the first position (you should have rotated the engine 60º to get to this position). Now you can remove the last bolt and cap. A word of caution: Do not rotate the engine with a cap removed. Now continue as before with shims and continuing to check clearances before final tightening.

The next step in replacing the pan is to place two U-shaped nut plates in place. One will have a dip or bend at the bottom of the U; this is the rear plate. A center punch or drift is useful to position the pan cover, nut plate, and gasket before placing the screws. I have found a little silicone placed under the pan screw heads helps to prevent oil leaks. Now it’s time to replace the oil and spark plugs.

Start the engine, check for oil leaks, and go for a drive. It is not necessary to have a break-in period, so drive normally and have fun!