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Old 11-22-2019, 12:07 AM   #1
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Transmission Noise

Need some help from any transmission gurus out there. Just purchased this boat as a project. It has a Perkins 4.108 with what I believe to be an older Borg Warner 72C transmission (model plate is missing). When shifting into gear, there's a tapping noise from idle speed up to about 850 rpm, then the noise goes away. Any ideas on this? Thanks. See video.
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Old 11-22-2019, 05:18 AM   #2
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Need some help from any transmission gurus out there. Just purchased this boat as a project. It has a Perkins 4.108 with what I believe to be an older Borg Warner 72C transmission (model plate is missing). When shifting into gear, there's a tapping noise from idle speed up to about 850 rpm, then the noise goes away. Any ideas on this? Thanks. See video.
How old is the damper plate?
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Old 11-22-2019, 06:36 AM   #3
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No propeller shaft yet? Then this is sorta normal. Also sounds like your idle is set low, that will aggravate it.

At low idle the engine turns in "lumps", causing gear rattle, especially in gear without any load. Heard it many times "bench running" engines in the shop.
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Old 11-22-2019, 07:44 AM   #4
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“At low idle the engine turns in "lumps", causing gear rattle, especially in gear without any load. “

Agree. No load on the gear causes gear rattle.
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Old 11-22-2019, 08:37 AM   #5
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Since it is a project boat anyway, why not pull the tranny and check or replace the damper plate. At this point it will only take an hour or so and given the unknown history of the engine and trannny it is probably a good idea. Cheap fix, less than $150

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Old 11-22-2019, 08:44 AM   #6
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Velvet drives are known for getting noisy if the idle is too low and doing it with no load on the output wouldn't surprise me either. If it's noisy with a load on the output and the idle speed is correct, rattle is usually either a bad damper plate or the gears in the reduction planetary (I've seen the teeth on the stationary plate fail on those).
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Old 11-22-2019, 09:02 AM   #7
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No propeller shaft yet? Then this is sorta normal. Also sounds like your idle is set low, that will aggravate it.

At low idle the engine turns in "lumps", causing gear rattle, especially in gear without any load. Heard it many times "bench running" engines in the shop.
Engine was doing this in the water with shaft attached too (at low rpms). Maybe just need to bump up the idle speed or maybe the engine is running a little rough? Going to check damper plate today.
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Old 11-22-2019, 09:14 AM   #8
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Engine sounded fine, just a low idle speed. Buy/borrow/steal a phototach and see what rpm is. Sounded to be about 600, most four bangers are a bit shaky/rattly/obnoxious below about 750. Some are happy lower than that, but not most.

It is a good idea to check the damper plate, as it is a wear item. Might go ahead and replace it if you going through the trouble to pull the gear. They are not that expensive and then you know its clock is at zero.
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Old 11-22-2019, 12:01 PM   #9
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My laptop speakers aren't good enough to pickup subtle sounds but first impression is it sounds like a normal noisy 4 banger. My imagination may have picked up a faint periodic metallic sound. Almost sounded like a bad piston rod.

I would get an inexpensive mechanics stethoscope to determine the source. Damper plate is an inexpensive wear item so always worth replacing if you pull the transmission. You can mcagyver a large chuck and 1/2" drill to drive the input shaft while transmission is out.
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