A Public service announcement for 4-speed owners

Engine, Trans & Driveline

  1. Richard Cranium

    Richard Cranium FBBO Gold Member FBBO Gold Member

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    I've been doing a some P/M on my 69 Road Runner survivor over the winter & while the radiator was out, I figured that I'd pull the water pump, balancer & timing cover off to replace the factory timing gears and chain. Of concern to me was the plastic coated camshaft gear. In spite of the car's low mileage (33,000 mils), I'm sure that plastic wasn't meant to last 52 years. No teeth were broken, which was a relief, but when I pried the crank gear off, I noticed that there was a strong 1/16" - 3/32' play in the crank, front to rear which isn't good. The next step was to pull the oil pan and #3 main bearing. Once I did that, this is what I found....

    Front face of the thrust of bearing...

    PXL_20210317_080441666.jpg


    Rear face of the thrust bearing...

    PXL_20210317_080459083.jpg

    The actual bearing journal surface of the crank & bearing are fine with no scratches or debris imbedded in the bearing. I started thinking about what could have caused this & the only thing that I can come up with is the previous owner cranking a cold engine that sits for long periods of time while pushing the clutch in. With the clutch pedal is depressed, it pushes the crankshaft forward which in turn puts pressure on the rear of the thrust bearing & if it's dry (from sitting), this could be the end result.

    Seems to make sense to me, but if anyone else has another theory, please chime in.

    When I bought this car, I also bought a bunch of NOS parts & included were new main & rod bearings, so I put a complete new thrust bearing in which tightened the clearances up. There was a slight amount of wear on the side journal which made the end play slightly over spec, but I'm sure that it'll be fine. I'll be buttoning things up as soon as my cam gear set comes in & will report on things once I restart the engine.

    We all know that it takes some cranking to get the gas up to the carb when th car has been sitting for a while & my advice now is to always start a cold 4 speed equipped car without depressing the clutch.
     
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    • Road Rat

      Road Rat FBBO Gold Member FBBO Gold Member

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      Never gave much thought to cranking them over with the clutch depressed but definitely sounds logical to me. Thanks for sharing your findings.
       
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      • moparedtn

        moparedtn Ed on the Ridge FBBO Gold Member

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        Yup, I've always been that way with anything with a manual transmission.
        Just common sense, really - I want everything to have as little "preload" or tension
        on it as possible whilst bringing an engine up to temperature.
        Letting the crank hang natural while turning the engine over just makes sense.
         
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        • 493 Mike

          493 Mike FBBO Gold Member FBBO Gold Member

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          And if your car has a "safety" switch that prevents cranking wo/depressing the clutch pedal, bypass the switch so you can crank wo/loading that crank thrust bearing.
          Mike
           
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          • Big Bad Dad

            Big Bad Dad Well-Known Member

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            Yes. My Dad was a long time mechanic and Auto Shop teacher. He taught me about this when I had my first 4 speed car in the 1970's. I have been cranking in neutral with the clutch released since I was a teen.
             
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            • Outlawd

              Outlawd Well-Known Member

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              Yep, I agree 100% never start cold with clutch depressed.
               
            • hunt2elk

              hunt2elk FBBO Gold Member FBBO Gold Member

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              Dang, I have always pushed in the clutch before starting a car. That is going to be a hard habit for me to break, but it looks like I better work on it. Especially since none of our newer stick shift cars will turn over without the clutch pressed in.
               
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              • 67charger383

                67charger383 Well-Known Member

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                Thanks RC.
                Don't wanna break anything.

                Screenshot_20201019-155013~2.png
                 
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                • yella71

                  yella71 Well-Known Member

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                  good argument to get rid of the clutch switch.
                   
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                  • 64BEL

                    64BEL Well-Known Member

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                    I stay off the clutch as much as possible. Waiting at a light, etc...
                     
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                    • PRND21

                      PRND21 FBBO Gold Member FBBO Gold Member

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                      If at all possible, I never push in the clutch while starting an engine, at a stoplight, take it out of gear, take my foot off the clutch, and never use the pedal as a footrest. Saves the thrust bearing and the release bearing.
                       
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                      • 69Bee

                        69Bee FBBO Gold Member FBBO Gold Member

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                        When you go thru the engine, you can get........

                        Well, it seems you have 3 choices:
                        1) Find a main bearing set with an oversize flange (MB2456C-60 which is 0.010" oversize length)
                        2) Get a reground crank (or another from another source to be ground)
                        3) Have the flange welded and reground std. and the R/M to 0.010" undersize

                        Each year, it gets harder and harder to find parts for Mopar that used to cover the earth...
                         
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                        • vance.dykes

                          vance.dykes Well-Known Member

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                          you are right the other thing is the oil that is used and the oil filter / change intervals did it get flooded with fuel. and last don't sit at a light with the clutch in for 2 or more minutes lot of force up against crank pushing forward.
                           
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                          • zyzzyx

                            zyzzyx FBBO Gold Member FBBO Gold Member

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                            Just my 2 cents, but now that the crankshaft thrust face is no longer flat, the new "Flat" thrust
                            bearing will point load and wear fast because there will not be an oil film between the parts.
                            Those two notches in the face are for oil to lubricate the thrust face while the clutch is depressed
                            and now they won't be able to do their job. I would pull the crank and have it welded or replace
                            crank if your engine makes some serious HP. It will restore the end play and the flat face of the
                            crank thrust. The same thing happens on automatic cars if the end play isn't set right. It'll chew
                            through the thrust bearing and into the block! The reason I say to replace the crank is because
                            when they weld up a crankshaft, they don't stress relieve the heat affected zone, so cracks can
                            start in this area and you'll have a two-piece crank.
                             
                          • zyzzyx

                            zyzzyx FBBO Gold Member FBBO Gold Member

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                            • Johnpat

                              Johnpat Well-Known Member

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                              Very good advise,thanks
                               
                            • bm02tj

                              bm02tj Well-Known Member

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                              A shop I worked for had a crank chromed at an industrial shop that did hydraulic shafts and stuff as it was something hard to replace
                               
                            • 4mulas

                              4mulas Well-Known Member

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                              I don’t think that if a car has a clutch in safety switch for starting that it’s going to hurt it by starting it with the clutch in. Just don’t sit there and hold it in afterward, or for long periods at stop lights etc.... excessively heavy clutches could possibly cause premature wear, or release bearings not releasing all the way and constantly applying pressure. Maybe bent clutch fork cause constant light engagement... all just thoughts...

                              Might be a good idea to look at the clutch and flywheel to make sure they aren’t blue....

                              I think the car in question here is low miles, so maybe there’s more to it than someone holding in the clutch all the time. Borg and Beck clutches aren’t all that light and for most, I couldn’t see them just sitting there and holding it in for several minutes at an intersection etc... I know it would make my leg wobble after a minute or so when I did it back when I was 18!
                               
                            • F4R/T

                              F4R/T Well-Known Member

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                              Wonder if you could spray weld one being it’s just the thrust area worn. I’ve had it done on shafts at work but I’m not familiar if the material they use would be suitable , just an idea.
                               
                            • 69Bee

                              69Bee FBBO Gold Member FBBO Gold Member

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                              Steel cranks are welded all the time with no failures when done correctly on a crankshaft welding machine.
                               
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