MTFCA Forum

In a perfect world, how thick should the babbitt be ? talking about main bearings
I understand that more is not better, but how thin can you go,.... without losing sleep, in my case I have measured .010 - .012"


Dave

How is it that you've come to have so little? Whether or not .010/.012 is enough kind of depends upon what your situation is. Are you pouring boring babbitt for a special crankshaft, with enlarged journals? Do you have a stock T with rod & main caps that have been filed so much that only .010/.012 babbitt remains?

This a modification, installing a Chevrolet crankshaft int a T block

Take a block knock out the Babbitt. Measure the hole in block. Subtract the standard size of T crank and you got it.
Lot more than what you are saying you have.

Very important that the bearings in the block are in line with each other. Often the downward of the transmission tends to cause the top of the center main in the block to wear. When that happens the crankshaft flexes with each rotation and eventually breaks. So the bearings should be poured and line bored to keep the crankshaft straight. The crankcase must also be aligned on an alignment jig to keep it straight from front to back. The 4th main should also be fitted at the back of the transmission and when the engine and transmission is placed nose down, the 4th main should with bolts part way down should have a snug fit on the tail shaft but loose enough to move up and down in the tail shaft.
Norm

Let me explain , I have a crankshaft grinder as well an align boring machine.
The mains saddles in the block have been sized to accommodate the Chevy main journals,
I am just asking if .010” - .012” is too thin, hoping some engine builders will chime in.

As far as thickness go's the 10 would be plenty but the downside would be in cutting the oil channel for lubrication and more often than not, the crank is not in the centre line of the saddles for the crank to cam distance, you need extra thick babbitt to allow for that or you will hit the block trying to cut, if you are lucky enough to be in the centre line, then the problem of cutting the oil channel, it will hit the block making a weak bearing being nearly cut in half.

Kerry wrote: ↑

Tue Jan 14, 2025 3:16 pm

As far as thickness go's the 10 would be plenty but the downside would be in cutting the oil channel for lubrication and more often than not, the crank is not in the centre line of the saddles for the crank to cam distance, you need extra thick babbitt to allow for that or you will hit the block trying to cut, if you are lucky enough to be in the centre line, then the problem of cutting the oil channel, it will hit the block making a weak bearing being nearly cut in half.

Maybe he's got a drilled crank for pressurized oil and won't need oil grooves.

Dave1 wrote: ↑

Tue Jan 14, 2025 3:15 pm

I am just asking if .010” - .012” is too thin, hoping some engine builders will chime in.

That's how the forum works. You get answers to every question you didn't ask...

Jerry, I like that quote!

Kerry , you are correct in that the mains are not centred to the bolt holes, I referenced the camshaft when setting up the align boring machine. The thinnest Babbitt is up near the mating surface of the main right next to the bolt hole.
At the bottom where the oil channel will cut it is a minimum of .023”
I had planned on cutting the oil channel on a milling machine, but I understand it’s not good to cut through the Babbitt.
Have to ponder this a bit, thinking I could use a ball end mill, or cut it by hand.
Thanks again.

Dave

No pressured oil system, splash type

Dave1 wrote: ↑

Tue Jan 14, 2025 3:15 pm

Let me explain , I have a crankshaft grinder as well an align boring machine.
The mains saddles in the block have been sized to accommodate the Chevy main journals,
I am just asking if .010” - .012” is too thin, hoping some engine builders will chime in.

My thoughts (-as an engine machinist) is about 0.030 thickness is the minimum I would want on the web side. Why I say this is I would think you risk fracturing the area around the locking pins when the casting is peined into the block.

Depending on the amount of shims you put under the caps when you are boring (-I would put around 0.010-0.012), that effectively gives you around 0.040" for wear. Not sure if you have ground the crank yet, -and if so, what your finish size was, but a 0.010 under journal pin effectively gets you another 5 thou in the cap and block. Maybe as Dan suggested above, line bore the block to gain another 0.010 - 0.020 per side.

Just for FWIW comparison, Ford specified about 0.060" - 0.070" thickness in the 'A' block.

If you tin the block so the babbit is fused to it, the babbitt could be quite thin, assuming the crankshaft is straight and accurately sized and the whole thing is very closely aligned from the front main to the 4th main. Thicker babbitt is more forgiving, up to a point.

If your crank is already ground to size, line bore the block to except the larger journals, .010 is to thin.

Listen to Joe.

Allan from down under.

I read once upon a time that .050” radial thickness was ideal for imbeddability and fracture resistance. Has been working for me so far. I wouldn’t drop down to .010” unless the bearing is bonded to the shell, as in the case of a bronze insert

Gotcha, not a huge job to mill the saddles out a tad.
I know…..”How many thousands are there in tad?”

I always bore the blocks out for bigger cranks on my Tobin Arp Ta 14.