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  1. #1

    Default 73 shovelhead looks to have 80" jugs? how to tell if it's stroked?

    welp, i ordered some parts before i fully tore into my engine rebuild like a ding dong

    it's a 73 and should be a 74" so i ordered some pistons and wouldnt ya know they aren't the right size.
    it's got 3.49 bore cylinders on the thing one has the part number 16587-78 I'm guessing someone put jugs on it from a 78.

    my actual question here is, to step up displacement on the 74" shovels was it as easy as just putting 80" jugs on? or was it necessary to stroke it? I'm not really sure how that works and if someone were going to go to the effort to stroke it why only go to 80"

    any thoughts would be appreciated

  2. #2
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    it's got 3.49 bore cylinders...
    3.438"is stock 74" bore;
    3.487" is +0.050"
    3.497" bore is +0.060" for a 74"
    3.497" bore is stock 80" bore
    http://www.wiseco.com/Catalogs/VTwin/PanShovelHead.pdf

    2011 Thread: identify my shovelhead
    "...they are late 78 80"..."
    http://www.chopcult.com/forum/showthread.php?t=15277
    Last edited by TriNortchopz; 04-11-2019 at 10:26 PM. Reason: 80"

  3. #3

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    To turn a 74cu.in. into an 80, the bore is made 1/16 bigger, but also the stroke is increased from 3.968 to 4.250.
    As said above, you can use 80cu.in. cylinders on a 74 by using plus .060 pistons for a 74.
    Keep in mind there were also late style cylinders like you apparently have that were originally bored for a 74.

    The lesson here is DON'T order pistons until you or your machinist determine the bore size the cylinders will finish to. These motors are now OLD. Do not ASSUME anything, as these things have probably been worked on several times during the DECADES, not YEARS.

    To measure your stroke with the cylinders on, measure how far the piston moves using the top of the cylinder fire ring as a reference point. With cylinders off, measure from the case deck to a point on the rod (like the top of the wristpin hole) with the rod at the top and the bottom of the stroke, and the rod held centered in the case mouth. The difference in measurements is the stroke.

    Jim

  4. #4

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    Quote Originally Posted by JBinNC View Post

    The lesson here is DON'T order pistons until you or your machinist determine the bore size the cylinders will finish to. These motors are now OLD. Do not ASSUME anything, as these things have probably been worked on several times during the DECADES, not YEARS.
    You’re not wrong i got a little ahead of myself on this one but let me ask you one more thing

    the idea was to put in hi comp pistons the ones already in it were stamped .010 and the cylinder walls were in great shape so i just wanted to put the same in but hi comp.

    what i'm really driving at here is could it be possible that some yahoo just put 80" jugs in without stroking? can they really even run that way? i only measured the piston travel in one cyl just using a tape measure (and i need to do a more precise measurement tonight) but i seemed like the stroke was about 4" on the nose

    if thats the case, should i look to find 74" jugs and keep the new pistons

  5. #5

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    An oft repeated mistake is putting 80 pistons with a 74 crank. That results at best in about 6.5:1 compression and the motor is a dog.

    Measure the cylinders first with telescoping gauges or better, a dial bore gauge to determine taper and out of round. If the cylinders are used, you will probably find they need reboring up to the next oversize. Plus .070 pistons for 74s are readily available, but plus .080 may be harder to find. The late, thick base cylinders you have will safely accept a plus .040, 80 piston, which would be a plus .100 74 piston.

    Fitting new pistons to used cylinders is usually an exercise in futility. I just said the same thing on Shovelhead Forum.

    Jim

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    Fitting new pistons to used cylinders is usually an exercise in futility.
    Truth! I don't do that either.

    I have snap gauges and internal mics but I take motorcycle cylinders to my machinist because he's got the correct dial bore gauge. After we measure them I order the piston kits then hand him the parts and specs from the piston maker.

  7. #7

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    One more (stupid) question for you guys after talking to another resource.

    If the piston is connected and the cylinder is on, at the top of the stroke should the top edge of the piston come to the very top of the cylinder with no space what so ever?

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    Quote Originally Posted by Radmond View Post
    One more (stupid) question for you guys after talking to another resource.

    If the piston is connected and the cylinder is on, at the top of the stroke should the top edge of the piston come to the very top of the cylinder with no space what so ever?
    Not on a shovel.

    Jim

  9. #9

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    Quote Originally Posted by JBinNC View Post
    Not on a shovel.

    Jim
    Meaning there IS a small gap?

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    Quote Originally Posted by Radmond View Post
    Meaning there IS a small gap?
    On a 74 or 80 the edge of the piston, where it's flat, not the dome, will be about 1/8" down from the top of the fire ring on the cylinder at TDC. It does not come up flush like a car motor or an evo or twinkie.
    IF you have 80 pistons in a 74 motor, they will be 1/8 further down than that (half the difference in stroke) resulting in the very low compression figure.

    Jim

  11. #11

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    Gotcha! Thanks jim, I appreciate the insight!

  12. #12

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    Just to end this thread on a round note here, took the jugs to the local shop and they are in fact 80” and the way to tell if the stroke is right is if the pistons reach the top of the jug without more than a 1/8th gap.

    Had the jugs bored to +.020 as they were already +.010
    Putting it all back together tonight.

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    Cool they had a set of .020" OS pistons in stock. That's unusual nowadays.

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    If you have a 74 crank as you say, you will need +.080 74 pistons to fit 80 cylinders bored +.020. If you use +.020 80 pistons on a 74 crank, you will have about 6 1/2 : 1 compression as I noted earlier. Unless some enterprising fellow shortened the cylinders on the last rebuild.
    Of course the motor will run, but why bother to build a 6 1/2 : 1 motor?

    Jim

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