Insert Main Bearings

[some mech 3]

Since expert Babbitt pourers-scrapers are extremely rare and/or expensive, Snyder's offers insert main bearing sets as an alternative to the Babbitt bearings. They say that the block must be machined in order to use the inserts. Could someone describe the machining and if this is something that a Model A owner can do? Also, is it wise to make the switch? Thank you.


some mech 3

[nick c]

Most model A owners don't have a line boring fixture that bolts to the block and has a motor for power, there will be a boring bar that is longer than the block, held in place by 3 support bearings, it must be inline with the center line of the engine. the bar will have tool bits like used on a lathe to cut the cast iron, you would need inside and outside micrometers.
there might be a video on you tube.
carry on

[1930artdeco]

From what little I know.....there are pluses and minuses to both set ups. Plus, tighter tolerance and much longer engine life. Minus, a little piece of metal destroys the insert bearing and you rebuild. Babbit, is a little looser and you don’t get the same number of miles BUT, it is softer and any metal gets pushed into the bearing and you can keep going. That is about all I know so wait for the rebuilders to chime in.

Mike

[Purdy Swoft]

I'm not a professional engine builder . Babbit bearings can be adjusted to some degree by removing or adding shims , not so with insert bearings . Usually when insert bearings fail the crankshaft will need to be reground and an undersized bearing will have to be used .

[BRENT in 10-uh-C]

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Originally Posted by some mech 3

Since expert Babbitt pourers-scrapers are extremely rare and/or expensive, Snyder's offers insert main bearing sets as an alternative to the Babbitt bearings. They say that the block must be machined in order to use the inserts. Could someone describe the machining and if this is something that a Model A owner can do? Also, is it wise to make the switch? Thank you.

As a professional Model-A engine rebuilder that does both insert conversions and casts bearings also, my opinion is the short answer for you is this is NOT something a typical home hobbyist can do unless they have some type of line-boring ability.


Since you mentioned "...expert Babbitt pourers-scrapers are extremely rare and/or expensive" let me offer to you why I feel that is not truly an accurate statement. There are quite a few quality shops around the country that can still cast alloy bearings. By the same token, there are shops who will provide what I like to call the WalMart quality of engine rebuilds. In that same breath, there are quality shops around the country that have the equipment and expertise to do a good job converting to inserts, -and there are also many that don't do a good job.


It is my opinion that a shop is only as good as their equipment and their attention to detail. Many shops around the country are using old, worn-out equipment that cannot hold good tolerances to do the work. Good quality machines cost lots of money, -and the machinist must charge more to pay for the quality. Secondly, to some shops it is simply a job for them to rebuild engines, and to others it is a passion. Their attention to detail typically equates to longevity of your engine however these rebuilders must take more time to attend to those details. So ultimately, it is you that must decide what is expensive and what does a value cost.


And, to answer your question regarding what must be done to convert a block to inserts, there is way more to the process than what I am willing to type here, but the basic outline is the block must be set-up to have the main bearing caps & journals bored to a size of 1.7705" with a tolerance of five ten-thousandths of an inch. These three main bearing bores must be within 0.0002 (two ten-thousandths) of an inch or less of parallel with each other and still maintain correct crankshaft centerline to camshaft spacing. In this area you have about 0.003" of backlash variance or less. Then the block and rear main cap must be machined for a counterbore of approximately 0.125" for controlling the thrust of the crankshaft. Additionally, the block and caps must be machined for the tangs on the insert shells, and the caps should be machined for pinning to allow them to be located in the same position each time they are installed. There are other minor machining ops that must/should be done, but this gives you the basics to understand if this conversion is right for your project. If you have any further questions, feel free to ask here or in a PM.