Prop shaft alignment or a problem with the crank? Knock at low RPM's

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  • Laker
    At least your have this going for you : your engine and shaft access are the best I've ever seen.

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  • hanleyclifford
    I'm wondering if you need to re-align the shaft. With a new cutless bearing the tolerance for deviation is small. I suggest you disconnect the shaft and begin by grasping the shaft and rotating it full circle, exaggerated at the limit the new cutless will allow. This will give you the idea of exactly where lies the "sweet spot" where the cutless is carrying the shaft dead center. You must utterly disregard the location of the engine while doing this. When you have found the "spot", support the shaft and it's coupling (I like to use pieces of cedar shingles). Only now should you begin to move the engine to the shaft. In this matter the "tail" wags the "dog". The position of the engine will finally come to be very different from the location you found it. The tolerance for the A4 is .003".

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  • Prop shaft alignment or a problem with the crank? Knock at low RPM's

    I pulled the A-4 on Lola my Tartan 27 to do a valve job as well as a bunch of other work but stopped short of pulling the crank and accessory drive gears.
    I put the engine back in after replacing the cutlass bearing.
    I have spent hours trying to align the prop shaft to what appears no avail.
    The main problem seems to be a pronounced clunking noise at low RPM's in both forward and reverse. In reverse I can feel a fluctuation in the shift lever and it seems the clunking is coming from the area of the crank. I got the coupling tolerance down to 0.04 but still have some shaft wobble... I didn't have the shaft checked but it rolled well on a known flat surface. I have to assume the cutlass bearing is okay since I never want to have to go through that again! I would hate to have to try to pull the shaft with the boat in the water.. It seems like the big nut in the center of the engine coupling gets in the way of the prop coupling making it hard to see what is happening to the macro adjustment of the shaft to engine and this seems to make fine adjustment really hard.
    I tried having the couplings apart and shimming to get them close and then using feeler gauge to fine tune it... The shaft is well supported by the packing gland...

    I got it to the point where I can use the engine without thinking it's going to blow up but when adjusting dwell... (I did a distributor rotation test test at low RPM's) I get a similar clunking noise. I can also feel what is best described as the gears appearing to move eccentrically against the distributor base right when the clunking/knocking starts. I had the coupling d/c'd when I did this test so I know that it's not totally a shaft problem.
    I did notice some shaft wobble last season as well as very hard shifting which resolved with adjusting of the forward and reversing mechanisms.
    Could this be some kind of crank problem? I don't seem to have any movement when I pull on the engine flange but there seems to be a very ( very as in a drop or tow of oil all season) small leak from the rear seal. Could the rear coupling be wagging the tail? Doesn't seem to make any sense..
    The clunking does seem to go away at higher RPM's but I am concerned that I am damaging the crank? On looking at the video it does seem like there is still a lot of shaft wobble... but the knocking sound with distributor movement is making me second guess the whole thing... The knocking sounds a bit like dieseling, could that be all it is? Is the shaft still out of alignment and that is making the engine try to stall out at very low RPM's?
    Here's a link but for some reason the sound is not right... You can see all the wedges from attempts at alignment... I feel I still have some more work to do there. That does seem like a lot of wobble but it spec'd out at 0.04? Is the shaft bent?