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Learn How Replica Engine Bearings are Machined and Babbitted

This is Part III in a series of articles covering mechanical rebuilding of a 1914 Simplex 50 h.p. Model “F” long-stroke Speed Car from the Collier Collection at the The Revs Institute, in Naples, FL. This installment covers: align boring the crankcase and bearing caps, machining and babbitting new bearing inserts, align boring the bearings, and fitting the crankshaft.

The lead photo shows the 39-inch long 80-pound crankshaft after straightening and regrinding. Next to it are three new main bearings after machining, babbitting, semi-finishing and cutting the oil grooves. At the top on the far-left are 10-newly machined camshaft bearings, in the center are four new connecting rod bearing inserts before being cut in half. We will cover both in a later installment.

Center main bearing

  • An original main bearing cap (bottom), and bearing insert (top-left) tightened up with shim stock (top-right).

When this engine was last given an overhaul in the early-1950s, the worn main and rod bearing inserts were hand filed at the parting lines. This was done to make the i.d. smaller and take up the excessive bearing clearance. Finally, the shims as seen on the top-right (above) were fitted behind the bearing halves in an attempt to keep the inserts tight in the crankcase and cap.

This old-fashioned patch job if done well may last for a few thousand miles, but the bearings are not held in the bore tight enough to keep them from moving around during operation. Note the shiny areas on the back of the insert that are the same exact shape as the brass and steel shim stock. This wear is caused by the bearing moving back and forth in the crankcase and cap.

In use the bearings not only suffer continued wear of the babbitt, but the outsides of the inserts and the bore also wear causing the whole assembly to loosen up, eventually knock and cause even more damage. One other drawback is the heat generated in the bearing cannot transfer easily to the cooler crankcase and bearing caps. It in turn causes the bearings to run hot and sometimes burnt out (melt the babbitt) in use.

Align Bore Crankcase

  • Simplex crankcase and main bearing caps after align boring the inside diameter 15-thousandths of an inch oversize (.015″) to clean up the earlier damage and end up with a round bores in correct alignment .

The best way to repair this situation is to first align bore the inside diameter of the ductile iron main bearing caps and the aluminum crankcase bearing bores slightly oversize (in this case .015-inch) for a new round and true surface. This view shows the boring bar and a cutter that rotate and are fed forward at about .002″ a revolution to machine one bearing bore at a time.

Before this is accomplished, the distance from the centers of the front and rear main bearing bores to the top of the crankcase are checked. The correct center-to-center distance from the crankshaft gear to the timing gears are also calculated for proper gear mesh. The crankcase is then adjusted in the machine so that the main bearing bores will be the correct distance from and also parallel to the top of the case and the timing gear mesh will also be correct.

Turning Shell

  • The outside of a new bearing insert being machined in the lathe from cored bronze stock.

After the crankcase and bearing caps have been machined, calculations are then made to determine the correct dimensions for machining the new main bearing inserts. A special mathematical formula is used that was obtained from an engine bearing manufacturer’s engineering department. It gives the correct sizes to machine the inside and outside diameters of the new bearing inserts to in a lathe that will fit in the bore tightly even after it is cut in half.

insert bearing bore

  • The inside bore of a bearing being machined. The rough screw thread finish surface gives the new babbitt bearing a better grip and a bond that will last for the life of the bearing.

Cutting Inserts

  • Finally the bearing insert is cut in the middle at the desired location in a milling machine.

After a bearing insert is finished machined it is then spilt in the middle. When this two-piece bearing is put in place in the crankcase, and bearing cap and the fasteners are tightened, it ends up being perfectly round inside and out. The two halves will also have the correct bearing crush (the o.d. of the bearing is .001″ larger than the bore) that will keep it tight in its location and prevent it from ever loosing up.

Half of this process has been covered here earlier in detail and images on The Old Motor in a post, Align Boring and Machining a New Insert Bearing. You can also view an earlier detailed article, Cutting Insert Bearings complete with photos showing exactly how the bearing inserts are cut in half. 


  • Two bearing halves (L and R) after being babbitted with a third in the heated mold ready to have 950-degree molten babbitt poured into it. A thermometer with a long shank is located in the center of the mandrel for monitoring the temperature which is very important.  

After all of the bearing inserts have been cut in half, the sharp edges are de-burred, and all of the bearing halves are prepared for babbitting. This consists of covering the edges of the inserts with an anti-tinning agent and then coating the area to be babbitted with a specially prepared tinning semi-fluid mix consisting of flux, tin, and lead.

Two bearing halves can be viewed, one with the tinning compound applied, and another showing it after heating and tinning the bearing. The bearing inserts are then placed in the pouring fixture, clamped in place and then heated to the correct temperature so the tinning compound melts to provide a perfect bond. The molten babbitt is then poured into the shell and allowed to become solid, at that point it is removed for the fixture.

You can view an earlier post that covers the entire tinning and babbitt pouring process here.   

semi finishing

  • A newly babbitted bearing half after being semi finished in a special bearing boring machine.

The next part of the process after pouring the bearings is to semi-finish the babbitt to .040″ underside of the finished diameter. Next the oil pockets are cut at the parting faces of the bearing on a milling machine and the oil grooves in this case are cut by hand with a grooving tool. The entire process can be viewed in detail here in an earlier post.

Simplex Crankshaft

  • This image shows a close up of the lead photo of the reground crankshaft with half of the front and center semi finished inserts next to it that are ready for align boring.

The next operation leading up to the align boring the main bearings is to place the semi-finished inserts into the crankcase and main bearing caps and tighten all of the fasteners to the correct torque. The bearing bores are then one again checked for the correct alignment to the machine and adjusted as needed. The three main bearings are then finished bored to a size .002″ larger than the reground 2.240″ crankshaft journals.

crankshaft spinning

  • The crankshaft after being installed in the new main bearings and run at 550 r.p.m. during the fitting process.

After the job has been completed the bearings are run in on the machine, and any high spots on the bearings are hand scraped to a precision fit. This is needed to deal with small variations in the finished size of the extremely long crankshaft journals. By taking this extra step, when finished the crankshaft is a free and smooth fit in the new bearings and will not need to be broken in. You can view this entire process in an earlier post titled Main Bearings Align-Bored and Fit.

  • The crankshaft after the final assembly complete with the new camshaft bearings. In the next post the machining of the camshaft bearings and align boring of the camshaft tunnels will be covered.

Bottom End

24 responses to “Learn How Replica Engine Bearings are Machined and Babbitted

  1. Brilliant job David! Is that the original crankshaft? Aside from RR, I’ve run into very few “machined all over” cranks but, of course, I don’t think I’ve ever taken a really high end brass-era engine completely apart. I’m rapidly (for me) approaching this part of my job so this article is extremely timely. Thanks.

    Joe P

  2. Thanks, to all of you for the kind words.

    Vincent, I met Peter Helck when I was five and he arranged a ride in Old 16 – my life’s path was set that day.

    Later on I met Edgar Roy, who has also served as an inspiration.

  3. It’s wonderful to see the work of a true craftsman. Attention to detail is what makes the difference in a motor lasting for the lifetime of the client. Thanks for you great posting on Babbitt and you great website! Ed

  4. I assume there will be shim stacks to compensate for the kerf of the slitting saw splitting the bearing halves. I don’t recall any mention of that.

  5. A great reminder of visiting MOTOR BEARING SERVICE, THE place to go for accurate POURED Babbit BEARINGS, in Los Angeles , —when I was first involved with early engines. Thank- you, Edwin – 30 –

  6. Excellent story and phtos. I was in the engine bearing business for many years as a Product Manager for Federal Mogul. While doing the engine bearing manuals it would have been nice to have a digital camera as you have. Thanks.

  7. I am surprised to see the finish on the inside of the bearings , I have over the years seen many babbited bearing shells made , mostly for locomotive and waggon axles but many of car size and all were dead smooth on the inside . If the tinning is done right there should be no need for a rough surface which needs a deeper layer of babbitte which is weaker than a thin layer . Also there is the consideration of what happens if the babbitte seizes and melts ,a smoother surface would save the crank better.

    • Manufactured bearing shells in the early days for the more part had a surface similar to what I duplicated here. With use and the oil containing dirt that gets ingested into these engines the bearing wore relatively quickly and unless adjusted soon would stat the pounding (knocking) and break the the babbitt bond to the shell. The rough surface stops this from happening.

      If the babbitt does melt or break up the bronze used here is a bearing grade of bronze containing tin and it will not harm a shaft journal at all.

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