360 LA Crank vs 360 Magnum Crank

Engines, Exhaust and Fuel Systems

  1. Bruceynz

    Bruceynz Well-Known Member

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    Hi Guys,

    The 360LA and Mag had different balance because of the lighter 470g pistons used in the mag, but did this change the crank balance as well or can you used LA 360 pistons and rods on a Mag crank as long as you have the right harmonic damper and correct Torque converter weight?

    Thanks
    Bruce
     
  2. XfbodyX

    XfbodyX Well-Known Member

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    Nope, crank is balanced to the mag motor bobweight for the sum of the parts.

    Your la parts have a different bobweight the crank is balanced to.

    SO, NO to what your asking.
     
  3. Bruceynz

    Bruceynz Well-Known Member

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    Sweet! I thought as much!!!
     
  4. AJ/FormS

    AJ/FormS Well-Known Member

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    But I don't see why you couldn't toss the entire Magnum rotating assembly into the LA block. I mean if that was what a guy had laying around looking for a home and how much are those KB107s/ 502 grams? Lemmee check. HYUP
     
    Last edited: Jan 28, 2020
  5. Bruceynz

    Bruceynz Well-Known Member

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    AJ I have a 360 Mag crank cut 10/20 and a set of KB107 pistons I was just thinking and thinking is dangerous lol
     
  6. Bruceynz

    Bruceynz Well-Known Member

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    To be honest I won the pistons at auction by fluke! Probably shouldn't have bid but the price was so cheap!
     
  7. AJ/FormS

    AJ/FormS Well-Known Member

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    And a block?

    I'll tell you a secret; My KB107s, running at 205*F minimum coolant temp, liked a lil more clearance than the KB catalog said..... and the ring gaps needed to be opened up too.
    It seems to me you have a stock-bore 5.9 kicking around that might clean up easy........................
     
    Last edited: Jan 28, 2020
  8. Bruceynz

    Bruceynz Well-Known Member

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    Ok I have been thinking here and thats a bad thing, the piston weight is the piston weight, the webbing weight is there to balance the part of the con rod that is in the webbing, I don't think you balance the pistons to the crank, aren't sort of just balancing that bit in the red to the crank, don't they some how weigh each end of the conrod to work out the bob weight, they don't stick the whole piston and rod weight for the bob weight. All they want to do is make sure the pistons are all the same weight and the rods are the same weight then they set the crank up to balance with a part of the conrod??

    upload_2020-1-30_16-42-16.png
     
  9. XfbodyX

    XfbodyX Well-Known Member

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  10. BudW

    BudW Moderator Staff Member

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    @XfbodyX that first link is not 100% working for me, but that might because of my work limiting it (maybe).

    Bruce, there is an art to engine balancing. My brother used to work in a huge race car engine builder (GM engines), back in the ‘80’s, and some of their engines would turn 8-9k RPMs, mostly for sprint cars and for other high speed circle track cars. My brother knows more of the things they do more than I do (I know the basics). They would also purchase pistons and rods by the hundreds, weigh (and mark) them then sort them into groups by weight. This was before a lot of the lightweight parts came out. I’ve heard them say when they get an engine balanced exactly to factory specifications, then they are very happy.

    What gets me is if the factory specifies certain specifications for parts (weights, sizes, clearances, and so forth) – then their engines wouldn’t it leave the factory that way. Sizes and some clearances, yes - but weights and balancing are definitely not at specs.

    Even taking a brand-new engine and blueprinting it (ie: getting it to factory specifications) eliminates vibrations, blowby and uneven wear.

    I have two new 400 short blocks in my garage. You can see numbers hand painted on the sides of the cast iron blocks:
    20141121_220233.jpg

    20160930_095909.jpg

    After the factory builders bore/hone the cylinders, they measure the holes and mark the size as a letter. In this case “C”, “D” or “E” as shown. Then they go a roomful of pistons (with pallets marked “C”, “D” or “E” and so forth), retrieve and place the pistons on a tray (in order) so the rods can get pressed in.
    It is possible the pistons gathered might be made a month apart or even from a couple different venders, sometimes. I have removed pistons from these engines (back in the day) that are evident there were 2-3 different style pistons used – so there might have even been compression differences as well (I didn’t look that hard for I was getting paid by the job, not by the hour). I would say most factory engines are a long way away from being close to factory specifications.

    If you look closely, you can see “D”s handwritten on the piston tops (but covered with cosmoline) and one notch indicating front of engine.
    400 Block 5.jpg
    Now look at this (the other block), This one has two notches indicating front of engine. The lower piston you can see a “D” (or maybe a "C"). The top piston might be a weird “D”, maybe (not sure).
    20160930_095837.jpg

    What I’m trying to get to are both engines are using different (brand, maybe) low compression pistons. Both are brand new short blocks. The blocks cast within a day of each other (one on 3-13-78 (see top picture, above) and other on 3-14-78) – but not sure of machining date or assembly date (not looked). There might have been a couple of weeks of the blocks being machined and/or assembled. It is hard to have two different pistons weight in at same amount – even from the same casting. Even notching the pistons will make them weight in differently.
    Also, one block has a heavy coat of cosmoline on it and the other, not so much.
    400 2 Block 4.jpg
    Block 2 - I wonder if it was cast on a Friday?

    The good news is I have new stroker crank kits for both engines, so the cast cranks and these low compression, um, pencil holders/paper weights won’t have to worry about balancing.

    On the underside, there is all kinds of paint markings on it. Each main cap bolt and rod cap nut has a dab of paint – which I would assume verified it is at correct torque. The crank has a lot of blue, which I suspect means a 400 with cast crank with normal rod/main bearing sizes. The connecting rods have orange paint on them – which I assume is standard size bearing inserts installed. I think it is interesting on the story each of these engines have – but once you paint the outside of the blocks and coverup the pan – that story is gone.
    20160930_095943.jpg

    When I get ready to rebuild both engines, I’ll take the old pistons/rods out and weight them for giggles. The builds will be listed here – just not yet ready for that, yet.
    BudW
     
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  11. M_Body_Coupe

    M_Body_Coupe Well-Known Member

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    Look, if you must use that crank, you could always add metal to it, the balance shop would be able to do that for you.

    As to that approach being worth it? Well, mallory metal is expensive to start off with, and I have no idea how much of it you'd need to balance out the extra weight of the LA motor piston/rod.
     
  12. Duke5A

    Duke5A Well-Known Member

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    I think he is trying to make due with what he has. Importing parts there has to be stupid expensive.