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Thread: TDC when Engine Rotated

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  1. #1
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    TDC when Engine Rotated

    I've finally made time this weekend to do my first engine rotation to replace my rocker box gaskets (entire assembly), but after reviewing several videos of sportster rocker box gasket replacements, realized they find TDC by rotating the rear wheel in gear. Since we have to rotate the engine, what is the best way to do this? Also, are there any other major differences in the process from a sportster? I'm using the service manual and buellxb post for the rotation, and a great video by lo brow customs for the gasket replacement on a sportster, but want to make sure I'm not leaving any gaps.

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    FYI, was able to just get traction with the belt loose in top gear to roll it over.

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    Senior Member Cooter's Avatar
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    IMO a bit more accurate to turn the crank with a wrench, or even the sprocket bolt (in gear) but the rear tire will do. IMHO reference the Buell service manual only, and heed the caution to tighten the rocker box bolts slowly so the lifters bleed down. If you removed the pushrods for some reason note that they are slightly different lengths (I/E).

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    Quote Originally Posted by Cooter View Post
    IMO a bit more accurate to turn the crank with a wrench, or even the sprocket bolt (in gear) but the rear tire will do. IMHO reference the Buell service manual only, and heed the caution to tighten the rocker box bolts slowly so the lifters bleed down. If you removed the pushrods for some reason note that they are slightly different lengths (I/E).
    Thanks, yeah I'm primarily referencing the service manual using videos/threads to supplemental tips/tricks. It's slow, but can't be too careful the first time. I used a combination of turning the wheel and checking the piston with a straw through the spark plug hole to confirm. I will most definitely re-tighten slowly. The interesting part thing to me so far is the gaskets weren't terrible, even looked like they may have been the james gaskets.

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    Senior Member 34nineteen's Avatar
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    Quote Originally Posted by Cooter View Post
    IMO a bit more accurate to turn the crank with a wrench, or even the sprocket bolt (in gear) but the rear tire will do. IMHO reference the Buell service manual only, and heed the caution to tighten the rocker box bolts slowly so the lifters bleed down. If you removed the pushrods for some reason note that they are slightly different lengths (I/E).
    If you go slowly, you could theoretically do it even at overlap TDC. Just tighten them slowly, maybe a half turn, wait a few minutes, another half turn, wait a few minutes, and so on. Since you're running stock cams you *should* be OK, you just dont want to jam it on with one valve fully open.

    Also, and probably more importantly, do not rotate the engine to do the other side until you give the motor PLENTY of time to let the lifters bleed down. How much time is "PLENTY"? I usually give it a time unit I call a "COOTER" (aka the time it takes to enjoy a good frosty beer).

    If you rush it, and just rotate the motor after torquing down the rocker box, the pumped up lifters can hold a valve open and it can/will get hit by a piston possibly bending a valve(s) which will lead to it breaking and destroying the piston/cylinder/motor.

    The straw is used to determine where the piston is, and you really only have two choices, overlap TDC or compression TDC. Getting it to the top of the stroke is mandatory, even if it isn't exact.
    Last edited by 34nineteen; 10-04-2021 at 07:15 PM.

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    Just re-read my answer and realized I didn't say I was using a combination of turning the wheel while watching the rocker arms to make sure valves were closed, then checking piston height with a straw. I will probably be bolting it all back up tonight, but not rolling the motor any more until at least the next day.

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