81 Evinrude 25 HP Block Interchangeability

[cchildr84]

I was just wondering what the range of interchangeability for an 81 25 HP Evinrude is. I'm going to take my block to a machine shop here in the next few days as it is at the very least going to need to be honed, but likely bored. As Ive searched for rebuild kits my available options are standard, .020, and .030 over. In the event the shop should tell me that the necessary bore would be more than .030, I've been searching ebay and other sites. In these searches I've seen blocks for 20, 25, & 35 HP motors, but unsure if the block would interchange with mine. For these blocks, would I be able to mount my carburator, made for the 25 HP, as well as the other parts, and go? What are the differences, and what if any changes or parts would I need to convert, say a good 35 HP down or 20 HP up to my 25 HP parts? My boat is only rated for 30 HP so I don't really want to go over that. Your help is appreciated.

 

[Chinewalker]

Re: 81 Evinrude 25 HP Block Interchangeability

According to the parts listings, it is 1981 to 1984. Same block was used on the 25 and 35 of that era, part number 390430. As a unit, I am pretty sure a full earlier 25hp powerhead will work with yours, but the 35s will have a different crankshaft (splines are different for driveshaft). Later 20 to 35 powerheads all had the larger splines and would not work with your driveshaft without swapping cranks - if that's even possible, as they changed the upper bearing in there somewhere. They also used a primer system in place of a choke, so you'd have to swap intake manifolds.

 

[cchildr84]

Re: 81 Evinrude 25 HP Block Interchangeability

That makes sense. I guess I was more interested purely in the block, as my crank is still good. Are the crank & rod bearings the same? I did just drop my block off at the machine shop and they told me .020 over would clean everything up. However, I've considered having an extra power head laying around in the garage for parts.

 

[Chinewalker]

Re: 81 Evinrude 25 HP Block Interchangeability

Your crankshaft will for sure fit the range of years I listed first. Thought of a reason why earlier ones might not work - they changed how the shifter detent works. Yours should have the plastic lock-out for the throttle/shift linkage above the shift handle. Earlier ones had a spring-loaded vertical arm that accomplished the same thing.

 

[cchildr84]

Re: 81 Evinrude 25 HP Block Interchangeability

Fair enough. I suppose I should worry about getting the one I have back together, before worrying about a back up. Haha. Thanks.