How much is too much.. con rod shims?

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Model T Ford Forum: Forum 2008: How much is too much.. con rod shims?
Top of pagePrevious messageNext messageBottom of page Link to this message  By Chuck Martin on Friday, January 23, 2009 - 11:30 pm:

Putting together a motor with new con rods.
Got the first one scraped in and it looks like it will take about .050" in shims to set it up right.
This seems like quite a lot of shim under the cap.
Any advice? or is this OK?


Top of pagePrevious messageNext messageBottom of page Link to this message  By jack daron-Indy. on Saturday, January 24, 2009 - 12:24 am:

That is a lot. Did you turn the crank? Did new rods come with shims already in them?


Top of pagePrevious messageNext messageBottom of page Link to this message  By John H on Saturday, January 24, 2009 - 06:12 am:

I agree it is a lot. Would be nice if my conrods had that amount of shims...long time since they had any shims at all (well into the filing of caps stage). However, it does mean you'll probably never have to have them rebabbitted. It sounds like new conrods come prepared for worn or ground down crankshafts.
Provided you've got 80% or more contact on the bearing surface and the conrods have just a tiny bit of resistance, then all is well.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Norman T. Kling on Saturday, January 24, 2009 - 10:38 pm:

If the caps have been filed the rod might need more shims when you install the new babbit. Many times when all the shims are gone, the caps are filed to take up the bearing. If one files it too much, there will not be enough space for the babbit unless additional shims are installed.
Norm


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kenny Edmondson on Sunday, January 25, 2009 - 10:54 am:

I'm thinking whoever rebabbited the connecting rods would have the correct shims installed when the rods were sized. Norman you may be correct, but Chuck has never answered Jack's original question. How were the rods shims set up when he got them?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Chuck Martin on Sunday, January 25, 2009 - 11:10 am:

The rebabbited rods came with a .035 shim. Don't know where they came from, they were part of a trailer load of parts I picked up. They measured out to be about .010" undersize. I didn't want to grind the crank to fit the rods....


Top of pagePrevious messageNext messageBottom of page Link to this message  By Seth Harbuck on Sunday, January 25, 2009 - 11:47 am:

Chuck,

If those rods are 0.010" undersized they are going to fit very tight on the sides on a standard crank unless you do some fitting with scraping and Timesaver.

If mine, I'd trade for some rods machined to standard and eliminate all that work fitting them.

Seth


Top of pagePrevious messageNext messageBottom of page Link to this message  By gene k. french on Sunday, January 25, 2009 - 12:28 pm:

Chuck:
since you indicate that you have already scraped in atleast one rod ,the best solution may be to re-bore all rods with a normal set of rod shims inplace ...i doubt that anyone would trade or exchange rods that have been modified ...
Seth is right about rods fitting very tight on sides only ...you likely have only a 2 point contact and excessive clearance at top and bottom of rod due to differance in radius of the two components
i would fixture the rods in a mill and indicate rod bore for position ...rebore to fit your crank size ...(if you bore to .001 over crank journal size,then use timesaver lapping compound you can quickly fit rods with necessary operating clearance of .0015 )
i also like to cut "X" oil grooves in rods and add dippers...if you cut oil grooves and drill caps,you should lap rods after grooving operation to eliminate any burrs that would interfere with "feel" of assembled components
ALSO: rods should be checked for ALIGNMENT before assembly
hope this helps
Best regards
gene french


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry VanOoteghem on Monday, January 26, 2009 - 10:27 am:

Chuck,

.010 may not seem like a lot but to remove it by hand scraping so that it will fit a standard crank would take many hours per rod and would result in junk. "Paint" the entire bearing surface of the rod with a magic marker. Put the rod back on the crank, fitted up snuggly. Rotate the crank a few turns in the rod. Remove the rod and see where the magic marker has rubbed away. You will most likely find, as others have suggested, that the babbitt is contacting on two very small areas with the remainder of the bearing gapping greatly from the crank journal.

The reason I say that to even take the time to scrape away .010 will result in junk, is that a connecting rod is a highly precise article, machined to very tight tolerances, such that the centerlines of both ends are parallel to within one or two thousandths of an inch. After working by hand to remove that much material, such tolerances can not be maintained. Hand scraping is a legitimate, accurate way of fitting bearings but is only for final fitting to remove high spots, not to totally resize a bearing.


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