Connecting rod bearing tolerances

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Model T Ford Forum: Forum 2017: Connecting rod bearing tolerances
Top of pagePrevious messageNext messageBottom of page Link to this message  By Chris Barker, Somerset, England on Monday, August 14, 2017 - 09:25 am:

My spare engine's crankshaft rod journals all measure between 1.2262" and 1.2268" at all positions.
Will reconditioned rods at 0.020" undersize work (maybe with one shim removed), or are they designed to have the correct clearance on journals of 1.228" please?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tony Bowker, Ramona, CA on Monday, August 14, 2017 - 10:33 am:

Hi Chris, I have pondered this on occasion. Rods that are 0.020 undersize will be 1.230. If your crank is 1.228, there will be 0.002 clearance, so removing a 0.0015 shim on one side should bring overall clearance to about 0.001, which in my simple mind it perfect.
In my experience things rarely work out perfectly but if you approach slowly and carefully you can get clearances between one and 1 1/2 thou. Don't rush.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kohnke Rebabbitting, Clare, Iowa on Monday, August 14, 2017 - 11:52 am:

If your going to buy a new set of rebuilt rods, have a set built that will fit.

The rod hole size should be 1.228. If you go with a standard .030, you will have .004 thousandths to start out with, and have a head start with an egg shaped hole.

Herm.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Chris Barker, Somerset, England on Monday, August 14, 2017 - 12:18 pm:

Thanks Herm. That's what I feared. Taking out a shim only fixes up-and-down. Still a big gap at the sides.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tony Bowker, Ramona, CA on Monday, August 14, 2017 - 01:25 pm:

Maybe I don't understand?????
If the rods were 0.030 undersize then they would not sit on the crank at the top and bottom, as the rod diameter is less than the crank diameter. To fix that you would need to scrape the bearing at the sides, which for a novice, is tough.
With the 0.020 undersized bearings on a 1.228 crank there would be 0.002 clearance at the top and side. One shim on one side of 0.0015 would certainly fix the vertical space and the 0.001 each sounds just about right to me.
As an aside, most new rod internal diameter I have measured seem to be exactly the nominal size where the crank is often 0.0015 to 0.002 under. Is this industry standard practice?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kohnke Rebabbitting, Clare, Iowa on Monday, August 14, 2017 - 04:02 pm:

Tony, you may have just misread the crank size.

It is 1.226-20, to 1.226-80.

Yup, if the crank was 1.228-00 a .020 would work fine.

Yes, T rods should be cut on every .010, using a standard of 1.250-00.

Herm.


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