Trim Tilt Honda BF50A

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Phil M
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Trim Tilt Honda BF50A

Post by Phil M »

After 10 years I suppose I should check the level of the trim tilt fluid. According to the manual, I'm supposed to fully extend the rod ( it is fully extended) and remove the reservoir cap, except that I don't know where the reservoir cap is located. I'm wondering if the reservoir cap is red in the photo below.

After topping up with automatic transmission fluid I'm supposed to bleed the system to remove air. I'm not sure how to bleed the system. The manual does not say that either.

Any advice?

Image
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NiceAft
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Re: Trim Tilt Honda BF50A

Post by NiceAft »

Is this the manual you have?
Last edited by NiceAft on Sat Jul 01, 2017 6:49 pm, edited 1 time in total.
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Phil M
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Re: Trim Tilt Honda BF50A

Post by Phil M »

NiceAft wrote:Is this the manual you have?
Image
That would be the one. :(
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Tomfoolery
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Re: Trim Tilt Honda BF50A

Post by Tomfoolery »

Your tilt/trim unit is different from mine, and a quick search of parts sites confirms that it did change starting with the 2004 model year.

But knowing a couple of things about hydraulic systems, I'd have to guess that the big red plastic plug is the fill/level check port. The older ones like mine have a separate expansion chamber where oil is stored with an air cushion above it, while yours appears to have the actual cylinder concentric with the chamber, like a common bottle jack. The plus is relatively low in the chamber as that's where the oil level when the rod is fully extended. The volume in that chamber will drop as the rod extends, by an amount exactly equal to the volume of the rod that has been pushed out of the cylinder (length*pi*diameter^2/4), so when the rod is fully out, the oil volume in the chamber has dropped by that volume. The oil level (obviously) goes up as the rod is retracted by lowering the OB tilt.

If it were me, I'd tilt it all the way up on level ground, let it sit for a bit in case of foaming, put a pan under the unit, and crack the plug. Check the level with it in that orientation. If it needs filling, top it up until it just runs out, put the plug back in, and cycle the tilt mechanism. Let it sit again, and recheck the level.

If the pump didn't suck air, it shouldn't need 'bleeding', but if it does, about all you can do is fill it, cycle it, and check the level.
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Re: Trim Tilt Honda BF50A

Post by paul I »

Tomfoolery wrote:Your tilt/trim unit is different from mine, and a quick search of parts sites confirms that it did change starting with the 2004 model year.

But knowing a couple of things about hydraulic systems, I'd have to guess that the big red plastic plug is the fill/level check port. The older ones like mine have a separate expansion chamber where oil is stored with an air cushion above it, while yours appears to have the actual cylinder concentric with the chamber, like a common bottle jack. The plus is relatively low in the chamber as that's where the oil level when the rod is fully extended. The volume in that chamber will drop as the rod extends, by an amount exactly equal to the volume of the rod that has been pushed out of the cylinder (length*pi*diameter^2/4), so when the rod is fully out, the oil volume in the chamber has dropped by that volume. The oil level (obviously) goes up as the rod is retracted by lowering the OB tilt.

If it were me, I'd tilt it all the way up on level ground, let it sit for a bit in case of foaming, put a pan under the unit, and crack the plug. Check the level with it in that orientation. If it needs filling, top it up until it just runs out, put the plug back in, and cycle the tilt mechanism. Let it sit again, and recheck the level.

If the pump didn't suck air, it shouldn't need 'bleeding', but if it does, about all you can do is fill it, cycle it, and check the level.
Maybe I don't have a good understanding of this. With the OB in the up position, the hydraulic oil level in the reservoir should be at its lowest because a significant portion of oil is routed to the cylinder. With the OB fully lowered, oil is removed from the cylinder and routed back to the reservoir and the level then would be at its highest. So wouldn't you want to top off the fluid level with the OB in its lowest position?
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Re: Trim Tilt Honda BF50A

Post by Tomfoolery »

Not if the plug is located at the right level for checking with the cylinder fully extended. Like the little rubber plug on the typical bottle jack, only in that case, it's with the rod fully retracted. But there is a certain amount of air volume above the fluid, by design, and overfilling can make a problem.

There is plenty of volume below the level of the plug even with the rod extended. And remember that it's a double-acting cylinder, with fluid below and above the piston inside that bore. The volume change is that of the amount of rod extended.* So the actual volume doesn't change all that much as the rod extends. The air space above needs to be large enough that the pressure increase when retracted is small, assuming it's not vented (which I would assume, as it could ingest sea water if it was). It also has to be able to handle a rapid increase in volume if an object under water was struck and the piston was forced up too rapidly for the anti-cavitation valves to allow transfer from one side of the piston to the other, and some ended up going to the reservoir - more than is normally present. Air space is also needed for thermal expansion of the fluid.

I'm guessing here (sort of), as I don't have that particular unit, but it would almost certainly have been designed for checking the fluid in the full-up position, especially since you can't even get to it with the OB down. Here's a screen shot of the installation manual page, which calls for checking the fluid with the OB in the up position.

Image

*If the rod is 3/4" diameter and with 8" of stroke (a WAG), then the volume change is about 3-1/2 cubic inches, or less than an eighth of a pint. If the rod came out both ends, there would be no volume change at all - fluid would be pumped from one side of the piston to the other. But it's not, so volume change is identical to the volume change of the rod exiting the cylinder.
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Re: Trim Tilt Honda BF50A

Post by paul I »

That answered all my questions. I never thought about having no access with the OB down. Duh...
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Phil M
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Re: Trim Tilt Honda BF50A

Post by Phil M »

It turned out that the Red Cap was the correct Reservoir cap for topping off the fluid. After 10 years you might find it good to know that my Honda did not really need any topping up but I managed to squeeze in a few drops of ATF anyways.
And because you are only topping up the fluid, you don't need to bleed anything.
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Re: Trim Tilt Honda BF50A

Post by Drifter »

Make sure you do the red knob up good and tight, or otherwise you'll have a leak and spend ages trying to source spare parts and wondering where the leak is... :wink:
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