Question re rod bearing clearance with A crank and pressure lube...

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Model T Ford Forum: Forum 2012: Question re rod bearing clearance with A crank and pressure lube...
Top of pagePrevious messageNext messageBottom of page Link to this message  By Chris Bamford, Edmonton AB on Sunday, December 30, 2012 - 05:03 pm:

Thread title pretty much says it a friend is re-assembling a '26-27 block that was fitted out with a counterbalanced A crank and VW oil pump supplying rods and mains (was torn down for a cracked drum replacement).

The MTFCA Engine book suggests 0.0015 for splash-lubed rods but does not discuss pressure-fed. My 1943 Dyke's suggests 0.001 per inch diameter for pressure-fed bearings. On the next page it specifies 0.0005 to 0.0020 with 0.001 preferred for splash, and 0.00075 to 0.0025 with 0.0015 preferred for pressure-fed.

We have one rod bearing now at 0.0015-0.002 (tiny bit of taper in the crank) and it turns but seems awfully snug.

Suggestions? And for the mains?

Thanks.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Matthew David Maiers on Sunday, December 30, 2012 - 05:16 pm:

I would think a .0015 would be fine, .002 would be a bit on loose side i would think for that journal diameter.

.0015 is a fairly commonly used clearence in many engines for mains/rods.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kohnke Rebabbitting on Sunday, December 30, 2012 - 06:03 pm:

We have one rod bearing now at 0.0015-0.002 (tiny bit of taper in the crank) and it turns but seems awfully snug. "END QUOTE"

If you have 1-1/2, to .002, and it is snug, you better recheck the accuracy of how your measuring the clearance, as with those clearances won't be that snug.

How much taper do you call a little? On a new grind there shouldn't be any.

On clearances, .000-50 will smear the bearing, and .002 is a waste of clearance. although when it is broke in, it may be .002, or a little over, but you don't want to start there, .001-25, is where you want to be.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Chris Bamford, Edmonton AB on Sunday, December 30, 2012 - 07:51 pm:

We used Plastigage Green to measure the clearance. On the last two engines I assembled, I used TimeSaver to lap in the bearings and Plastigage to measure the clearance. 0.0015 felt considerably easier to turn than it does on this engine.

The taper, as measured with the strip of Plastigage, is 0.0005. The babbitt is new but the crank was not reground.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kohnke Rebabbitting on Sunday, December 30, 2012 - 09:10 pm:

Are you using the Plastigage Dry?

and a 1/2 thousandths taper is a lot.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Chris Bamford, Edmonton AB on Sunday, December 30, 2012 - 09:46 pm:

Plastigage with oil. I recall reading the manufacturer's instructions, online, that specified lubing the surface... interestingly, the printed instructions that came in the Plastigage pack made no mention of lubrication.

0.0005 taper may be a lot but that's what it is. He is unlikely at this point to have the crank re-ground and another round of re-babbitting.


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