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Thread: main bearing tunnel bore sizes for 248ci rebuild

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    main bearing tunnel bore sizes for 248ci rebuild

    Hi, I sent my engine from my 1940 Buick Super to an engine machinist for a rebuild over a year ago and the machinist is stuck at tunnel boring the crankcase. Can anyone advise machining sizes to tunnel bore main bearing housing in crankcase? I have a Buick workshop manual and a Chiltons manual but can not find this information. Any help greatly appreciated. Matthew

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    Tunnel Boring

    You may have a bigger issue than you think. I'm not sure if your '40 model has it, but my 1946 248ci certainly has main bearing tunnels that get progressively larger towards the transmission. Apparently this was done to either provide more strength or try take out the whip that such a long crank can develop. Unless you have damaged the block, shouldn't your machinist be working on the crank itself ?

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    Quote Originally Posted by markyboy64 View Post
    You may have a bigger issue than you think. I'm not sure if your '40 model has it, but my 1946 248ci certainly has main bearing tunnels that get progressively larger towards the transmission. Apparently this was done to either provide more strength or try take out the whip that such a long crank can develop. Unless you have damaged the block, shouldn't your machinist be working on the crank itself ?
    Thanks for your input. You are right in saying that they are they same engine with the main bearings stepping up in size. I think the machinist has already machined the crank. The block is fine. I think what he wants to do is to machine the tunnels in the crankcase back to perfect diameters and clearances. He could work it out but it will take a while seeing as he needs to do it 5 times over for each bearing.

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    Here are your housing bore specs for the 248:

    #1- 2.5010"/2.5030"
    #2- 2.5640"/2.5660"
    #3- 2.6260"/2.6280"
    #4- 2.6890"/2.6910"
    #5- 2.7510"/2.7530"
    There's no such thing as too much cam....only not enough engine!

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    Quote Originally Posted by CNC-Dude View Post
    Here are your housing bore specs for the 248:

    #1- 2.5010"/2.5030"
    #2- 2.5640"/2.5660"
    #3- 2.6260"/2.6280"
    #4- 2.6890"/2.6910"
    #5- 2.7510"/2.7530"
    Wow ! From where do you have this measures ?
    I will write it direktly in my 38 Manual.
    Maybe,You have this also for the Rods ?

    Jenz
    '38 Special Coupe, pimped 263 cui
    ---- LIFTERS CC GERMANY ----

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    Glad to help out Jenz, I have a lot of old bearing books from when I worked in a race engine shop. Here is the 248 rod housing bore specs:

    248 Connecting Rod Housing Bore: 2.1240"/2.1245"
    There's no such thing as too much cam....only not enough engine!

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    Thanks Dude
    Makes me feel like my Manual are nearly complete now.

    Jenz
    '38 Special Coupe, pimped 263 cui
    ---- LIFTERS CC GERMANY ----

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    Good deal. I have the bearing journal sizes also if you need them.
    There's no such thing as too much cam....only not enough engine!

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    Sounds good but thanks Dude,i think thats stand in my Manual.
    If I need some Specialmeasures it woul be glad to come back to your Offer.
    Since a couple of Years I have a 263 in my 38 I know its cmpletely different.
    but two 248 ,my origin and a spare One, for that is good to have your Datas.
    Years ago ,as i has fixed my origin 248 with the (newer )Insert bearing Rods,the new Inserts dont has match into the Rods,and Nobody could tell me why. The Rod inner Diameter was too small. At that Time and I had need this Measures.
    It was lot of carefully trial work for the MashineShop o get the needed correct Diameter.

    Jenz
    '38 Special Coupe, pimped 263 cui
    ---- LIFTERS CC GERMANY ----

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