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Thread: 1969 GS 350 connecting rod stampings

  1. #1
    Join Date
    Sep 2017
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    Kearney, N
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    1969 GS 350 connecting rod stampings

    I got into the engine of my resto, it was tight as hell and supposedly rebuilt. The thing is tough as hell to turn over with a breaker bar, so I loosened up the connecting rod caps and I was able to turn it with one hand. After tightening them down one by one and turning, it became tight again when the #6 and #7 caps were torqued back to 35 ft lbs. Looking at the caps, the stampings say 8 and 4 on them. I inspected all the stamps and they are all screwed up. I have two 8's 4's and 2's.

    Am I reading the stampings correct? Shouldn't there be 1-8? The only other markings are 0 l 0 and other numbers. Can anyone help me tell what the markings mean, and as of now its looking like the previous owner placed the pistons in the wrong places. Some of the oil holes are facing the block and some to the center.

    Any advise would be greatly appreciated!

    connecting rod cap.jpg

    connecting rod cap.jpg

  2. #2
    Join Date
    Sep 1999
    Location
    Dbl Oak,TX
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    745
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    The oil spurt holes should face up (towards camshaft). The rib on the rod and the boss on the cap face rearward on the left bank. These face forward on the right bank. There is a notch on the top of the piston that faces forward. Caps and rods should not be interchanged. See attached pix from shop manual.

    Piston, rod, cap have to be marked for the cylinder number before disassembly. There is only one part number for a rod, although it changed through the years. First part number used was 1385362. This was superceded by 1240016 in '74. That number became 1241938 in '75. These may be cast into the rod or also the number 1374563.

    So stamped numerals on rods do not indicate cylinder number. There may be forging numbers on the rods like 561 or 942. Meaning of the 2, 4, 8 marks is unknown.

    Clearance between the rod and the crankpin can can be checked in place with Plastigage. Clearance for a new bearing should be between 0.0002 and 0.0023". More is permissible on a used bearing. With bolts tightened down, rod should move freely back and forth on crankpin as allowed by end clearance.
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    What has been, can be again. (Bob Wills, 1942)

  3. #3
    Normally, when an engine is disassembled for the first time the disassembler should stamp the rods and caps before removal so that they stay as a matched set. The stampings can be done with whatever the person has at hand. Number punch sets are the norm, but they could be marked like dice with a center punch. If a rod has been replaced in an engine, it is common to find two with the same numbers. They do not have to be in the numbered position, a second set marked 2 could be found in number 5 cylinder.

    It sounds like your rod and caps have been mixed up, the engine should turn in the manner you are attempting and your check is good. The simple fix is to take them to a machine shop and let them deal with it. They will resize the big ends. Even doing this, it is desirable to match the caps and rods as well as possible.

    If the caps and rods have been removed from the engine before marking, they can be mixed up. Sometimes they can be matched back up by carefully examining the honing marks. After matching the best you can, they can be check by torquing the caps on and then checking the dimensions for roundness with a micrometer. If they pass the mic test you are good to go.

    Finding a problem like that with the assembly, you may want to completely disassemble the engine and check everything.
    Last edited by Bob; 09-16-2017 at 07:35 AM.


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  4. #4
    When I re-freshed the engine on my dad's '72 Skylark, he was the original owner & the engine was NEVER apart by ANYONE, I had the same problem. The 1st. assembly I installed was #2. Torqued down & engine was seized. Pulled it apart MANY times looking for a problem. Installed the other 7 no problems, turned over like it should. Took #2 to the machine shop I used. He checked the rod all over. One of the last things he looked at was the alignment of the cap to the rod. Like I said the engine was NEVER apart since new. Turns out the rod was drilled wrong from the factory & the factory re-drilled it. I don't know how it turned over & lasted so MANY yrs. with no problems. Ended up getting another used rod. There was NO ABNORMAL wear on the crank or bearing.
    I still think about it today.


    Tom T.
    Tom Telesco
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