0.0003 Clearance for Connecting Rods?

BigSkyBigSky Posts: 486Senior Contributor
edited October 2019 in HUDSON
Is this for real?  I’ve seen it multiple places that the connecting rod clearance (big end of rod) on my 37’s 212ci 6 cyl engine is 0.0003 to 0.0006”.

At first I thought it was a misprint but then a local Hudson expert told me it was correct & not a misprint.  So if that is correct, how the heck do I check this thickness?  Plastigauge doesn’t go that thin that I could find, so I’m really stumped!


  • lostmindlostmind Posts: 1,273Expert Adviser
    I was taught to remove shims until the rod won't turn , add shims until it does.

    you need to oil the crank when doing it.

    Tedious job , but I never had a problem.
  • Ric West INRic West IN Posts: 473Senior Contributor
    Hudson shop manual for 1942-1947 states radial clearance on big end of rod is .001.
  • BigSkyBigSky Posts: 486Senior Contributor
    The 1937 Hudson procedural manual showed this:  
    - connecting rod:  lower end (crank journal l) clearance 0.001” &  Upper end (piston pin) 0.0003”.

    Measuring with plastic gauge on one of the crank journals gave me 0.0015”.  This is how it was set up before the engine was disassembled and there was no knocking.  I’m inclined to think 0.001 is the correct clearance for the crank end (big end) of the connecting rod.  

    Anyone with confirming knowledge of this?

  • BigSkyBigSky Posts: 486Senior Contributor
    “Thank you” to the gentleman who responded to my inquiry and yet I found it surprising no one else had knowledge or experiences to post on this. 

    For future individuals who have the question of “what is the correct crank rod clearance?”....  go with the Hudson Manual of 0.001” for the connecting rods crank end (big end).  The “Motors” manual which showed 0.0003-0.0006” is definitely wrong because they are referencing the rod pin end (small end) clearance, as the rods crank end (big end) clearance.  Again go with the Hudson Mechanical Procedural Manual specs: 
    -Upper Connecting Rod End (rod small end for piston pin) Clearance is 0.0003“ and 
    -Lower Connecting Rod End (rod big end, attaches to crank journal) Clearance is 0.001”.

    This is how I came to that determination:
    I used ”plastic gauge” on the clearance of the rod to the crank journal and found it to be 0.0015” or 1/2 a thousands greater than 0.001”.  I then removed only one of the thinnest shims (approximately 0.001” thickness as I measured) and reassembled it to find that while I could turn the rod on the crank journal, it was definitely too tight.  I put the shim back in place, reassembled it & it turned fine.  

    Since there was no knocking previously, I’ll be reassembling the connecting rods as they were when I took them apart.   In the future, If I do need to tighten them up, then at that time I can again try removing a shim & see what the results turn out to be.  

    From the 1937 Hudson manual:

    The photo below shows how removing one shim made the clearance too tight.  In the photo one can see the rod connected From below the block as it sits in the car.  In the position it is in, the rod is in the horizontal plane & it wouldn’t drop any distance on its own.  Instead I had to reach up & pull it down to get any movement & it took a large amount of pull to do so.  

    In this photo one can see how far the rod/piston fell on its own without any assistance with the shim returned.  

    Hope this will be helpful to someone in the future!
  • BigSkyBigSky Posts: 486Senior Contributor
    I reached out to Hudson Expert Al Saffrahn, in Arizona on the rod clearance at the crank journal.  He said “do not go less than 0.001” and preferably set it up for 0.001 to 0.0015 clearance, you need enough room to get oil in there”.  
    He also said to not use Plastic gauge on the bearings because the Hudson babbitt was very soft & one could end up marring the babbitt. 
  • 35 Terraplane35 Terraplane Posts: 255Senior Contributor
    Would the clearance be measured by bolting the rod cap w/shims in place to the rod then measuring the ID bore? Then checking that dimension against the OD of the crank?
  • GeoffGeoff Posts: 3,819Senior Contributor
    The foil shim method is the best.   Insert a folded over cooking foil shim into the bearing, about 1/4" wide, the length of the journal.   Normal cooking foil is half a thou' thick, so fold it over twice to give four thickness.   The engine should not turn..   the other method is to take a thin shim from each side, bolt up and check the turning resistance.
  • BigSkyBigSky Posts: 486Senior Contributor
    edited October 2019
    35 Terraplane, I think your on the more precise track which I’d be leaning towards on a full rebuild.   I just don’t trust my micrometer readings so I’m trying to figure out the number and the feeling of “just right”!

    Geoff,  Thanks for sharing your method.  If I understand correctly what your trying to find out is if the clearance is greater than about 0.002 thickness of tin foil?  If it turns then clearance is greater & shim(s) need to be removed but if it doesn’t turn then one is good on the clearance? 

    Ok, with that said what is the range of clearance that your shooting for on the splasher engines; 0.001 to 0.002?   Can you also describe the turning resistance?   When testing this is the engine assembled & one is just pulling one rod cap at a time?
  • GeoffGeoff Posts: 3,819Senior Contributor
    You can do it with the engine assembled, plugs out, and turning the flywheel, but it is more accurate to do it with the rods out, one at a time.  1 to 1-1/2 thou is the recommended clearance.  you wont hear them rattle at 1-1/2', but you will at 2.   
  • BigSkyBigSky Posts: 486Senior Contributor
    Geoff, when I measured the thinnest shims on my rod ends I was getting around 0.001” is this what you have seen as well for the thinnest shims?  As I described above, when I pulled one shim on one side only it became very hard to turn & to me it appeared to hard so I put the shim back.  Did that sound like a reasonable plan or is there some other way to take up half the shim thickness I pulled out, to fine tune it?
  • GeoffGeoff Posts: 3,819Senior Contributor
    You can take out a .003 shim and put a .002 one back in to get the required clearance.   It sounds to me like you have the clearance correct.  
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