Floating Rod Bearings

Ken/Alabama

Ken/Alabama

Well-known member
Over the years I’ve rebuilt several flatheads using the floating bearings. I’m running into an issue I haven’t encountered before and need some guidance. 37 engine, crank ground .060 under. Rods sized to 2.220 , bearings measure .140 thick. Adjusted the spread on the bearings to 2.219 . With the crank on the workbench I oiled the crank and rod and bolted it up and it locked to the crank. Pulled it back off , made sure the bearings were not hanging up at the radius and they weren’t. Put the micrometer on the crank and it measures 1.9394. So there’s the issue, the crank is a little on the fat side. So what’s the recommended solution for this. Can the crank grinder take this small amount off to get to 1.938? Or can I open up the rods on my rod hone to make this work? Thanks
 
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Bit difficult for the crank grinder to grind .0014 off of the journals, but he could polish that amount off, just take a while. Preferable to opening up your rods by about that much...
 
If I have a problem I find it’s usually better to fix the problem than create another to go along with it. Have the crank grinder polish it to proper size.
 
alchemy said:
If I have a problem I find it’s usually better to fix the problem than create another to go along with it. Have the crank grinder polish it to proper size.
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That was my line of thinking also. Just didn’t know if that much could be polished and keeping it straight.
 
Ken/Alabama said:
That was my line of thinking also. Just didn’t know if that much could be polished and keeping it straight.
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Exactly. Polishing .0014" and keeping it round with no taper, barrel, or hourglass isn't simple. I've tried it and ended up in the grinder again. A sharp guy with a careful setup on a good machine can do it but it'll take a bit to dial it in for sure.
 
cadillac512 said:
Exactly. Polishing .0014" and keeping it round with no taper, barrel, or hourglass isn't simple. I've tried it and ended up in the grinder again. A sharp guy with a careful setup on a good machine can do it but it'll take a bit to dial it in for sure.
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I have the book that ol Ron put out so I dug it out and he talks about measuring the thickness of the bearings and doing the math that the ID of the bearing should come up to around 1.999. So this leaves no clearance for oil.This is where I’m at but plus .0004. So he talks about opening the rods to 2.221. I’m thinking if they can polish the extra .0004 it will get me to to 1.939. ( I’m dealing with.060 undersize) I could go the .001 on the rods which should put me at a little over .002 total clearance and they have to polish so much on the crank. My rods are sized at 2.220 and the bearing will turn in the rods untill they’re bolted to the journal. Bearings are .140 thick. Thanks for your input
 
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Tony is correct . . . an experienced crank grinder with top-notch equipment can easily handle this. Also, it doesn't hurt to open up the rods a bit for additional "floating clearance", but the crank journals need to be correctly sized regardless.

I had a superb crank grinder give me an additional .001 clearance on a new $6000 billet crankshaft. The crank came to me with journals that were not correctly clearanced for racing (dugh!) - so it needed to be fixed.

Polishing is NOT the way to solve this issue as it is not meant to remove hardly any material and if you attempt to do so, you'll end up with issues like taper, out of round, incorrect fillets, etc..
 
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