i don't know if there are diff tooth counts in TR drives. turn input shaft and count prop turns. compare it to your existing drive. they are probably both 1.5 to 1 reduction.

Standard TR drive is 2:1. They did make a TR Commercial drive which was 1.5:1, however their rare and I have never even seen one of those before. The TRS is 1.5:1. As mentioned turn the input-ujoint shaft and see how many times you have to turn it to make the prop shaft turn one revolution. Something else this will tell you is if the input shaft turns the same direction as the propeller shaft it is a left hand rotation drive. If they turn opposite of each other than it is a right hand rotation. Just curious, exactly what size and type of boat is this on? What engine?

it is a 1981 Searay 25 foot Dancer with flybridge. i believe this would be a TRS, not a TR, and Carla is mistakenly leaving the S off. TR is a houseboat drive. i doubt many were put on 25 foot weekenders.

Not many true, but I have seen them on old Searays that weren't even that big and not weekenders. Carla could verify this by posting the propeller diameter or gear ratio. TR's use a 19-20" prop and the TRS can only run about a 15.5" max. Would have thought the seller would have also been identifying it as a TR.

Hello there, I have been very busy a few days with my house that also is a project :-) The boat is a 255 Sedan Bridge from -81 with a 7,4 l Mercruiser. On the original "operation and Mainteance Manual" that was included with the boat says Models 330TR-330TRS. I think I have assumed that the drive is a TR. But maybe it is a TRS ? The prop size is approximately 50 cm or 19,6 Inch on the new one and maybe 1 inch less on the old. I need to turn the outgoing shaft 2 revolutions for the prop to turn one revolution on both = YES ! I guess it´s TR drive I have then. On the new drive the prop rotatets to right and the old one turns to left.
I´m little curious now why the boat will need that big prop ?

// Carla

That's a big heavy boat especially for one engine. Needs the extra blade surface area to move it efficientlly and the lower 2:1 gear ratio drive for the extra power to turn that big prop.