Twin Disc swap-out MG502-1 for MG5050A on Bruno 35 (3208NA cat) advice

Mark Kopyc

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Hey,

I have a 1978 Bruno 35 that is being repowered with a replacement refurbd Cat 3208NA 210hp.. My mechanic took a look at it and says the original Twin Disc MG502-1 (2:1) is slipping and parts are unavailable. He is recommending that I get a new MG5050A. From the spec sheets, it looks like the MG5050A is about 2.5 inches longer than the MG502-1 is. He is talking about shortening the 1.5" prop shaft by a couple of inches to accommodate. Is this the right approach, or possibly could the engine be moved forward a few inches to accommodate?

The prop is 4-blade 21x19 which seems correct for the 2:1 ratio gear at 210 hp, max RPM 2800. I definitely don't want to overprop the boat.

Thanks so much !! Mark ('Panacea', West Bath ME)
 
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plowin

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Do you have the room to simply slide the shaft and prop back towards the rudder? You would get double the benefit if so because the 35 Bruno is typically limited as to the size of the wheel that will fit. That wont so much be your issue with 210hp however but it would be the easiest thing to do. It may make getting the prop on or off difficult or even impossible though. Its easy enough to slide it back and test it.
 

Mark Kopyc

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Thanks Plowin.. Possibly. You're right, that would be easiest. I do plan to add a Piranha disc line/net cutter (there isn't one there now) so the extra clearance would let me add a shaft zinc also. But I worry about having too much prop shaft length aft of the stuffing box/bearing due to reduced support of the prop (maybe that worry is unfounded though). I'll stop by the boat and take a look at the clearance margin. Seems to me that cutting and TIG welding the 316 stainless prop shaft could be very tricky and work-intensive, all over 2.5 inches..

Can you explain more why moving the prop aft would be a benefit due to my prop diameter limit? Thanks.. trying to understand.. Is it because of the 10 degree downslope of the shaft and the gearbox output? That reminds me, the shaft coupling might not line up the same on the new transmission due to this downslope.. in other words, it isn't going straight back, its doing back and down (although at a fairly shallow angle of 10 degrees). This is why I had the idea of moving the engine forward...

BTW I think it's pretty terrible that Twin Disc wouldn't make a 'drop in replacement' for the MG502-1 that has been around for so many decades.
 
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Keelboater

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Shorten the shaft, but not until the new motor and gear are installed and you can verify the measurement. If you move that motor forward, the heat exchanger will be quite close to the back of the steps and performing a simple belt change will become a real treat. If you slide the shaft back instead, you will have more than 1 1/2 x the shaft diameter protruding from the cutlass bearing and you will have trouble with prop to rudder clearance as well. Also be sure that transmission will clear the hull. My MG506 was only about 1/16" away from bottoming out against it when aligned with the shaft. They did not leave a lot of wiggle room with this hull design.
 

Mark Kopyc

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Shorten the shaft, but not until the new motor and gear are installed and you can verify the measurement. If you move that motor forward, the heat exchanger will be quite close to the back of the steps and performing a simple belt change will become a real treat. If you slide the shaft back instead, you will have more than 1 1/2 x the shaft diameter protruding from the cutlass bearing and you will have trouble with prop to rudder clearance as well. Also be sure that transmission will clear the hull. My MG506 was only about 1/16" away from bottoming out against it when aligned with the shaft. They did not leave a lot of wiggle room with this hull design.
THANKS!! excellent advice and insight!
 

Fishbound

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Look at the Zf 220. There foot print is very similar to the 502, and would prob be your easiest swap

I’ve got a Bruno with a 506 to keep the engine down low, since the 506 is Coax
 
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