[QUOTE=John Reimann;8385]I just came across this thread. I had a similar problem, although I have the newer style heat exchanger. I unscrewed the clamps that attach the HX to the motor, raised the hx up so I had access to the hole for the zinc, and CAREFULLY ran a drill bit up into the opening a bit. Then I took a tap of the same thread size (I forget what size) and tapped the hole out again. Solved the problem.

If Chachere is reading this, I hope he or she lets us know how they resolved the issue.[/QUOTE]

John -
I am reading this, and here's how I resolved it. Actually, Tom Soko had offered the same suggestion on the listserv in response to my query.

So two weeks ago I took the HX off so I could work on it at home, ran a drill into the broken zinc, chased out the threads, and Voila! It looked like we were good to go. But then I had to deal with the problem of getting all the loose bits of old zinc out (I had -- note the past tense -- the old 2" HX that the early M25s came with, where the port-side end was not removable). So with a lot of careful rinsing and jiggling, and some delicate jabs, I was able to shake out all the old zinc fragments. OK, NOW we're ready to go. I file off the end of the new zinc so its not too long, gently screw it into the cleaned out threads ... and oops, the entire fitting splits into 3 pieces. Quite rotten.
Oh, well, I guess the original HX had served its time (it did have a lot of green corrosion on the outside.)

So, on the advice of an earlier thread on this subject (and Universal's service bulletin from the early 90s that suggested upgrading to the newer style 3" HX), I purchased a Seakamp aftermarket 3" new style HX from AAA marine, it was a bit over $300 from the copper-nickel model.
It was a bit of a squeeze getting it in -- I ended up removing the old bracket from the engine and reversing it and bolting it onto the fiberglass crossmember instead. And the hose connections are not all the same size -- although pretty close. Fired up the engine today and it all seems good.
I'll try to post some pictures at some point.

Thanks again, all, for the suggestions, as always. This is a great resource.

[QUOTE=stu jackson c34;8399]The trick to installing a 3 inch HX on an M25 is to bend the bracket down so the new one fits where the old one used to be. Your solution is also one I'd considered.[/QUOTE]

I tried that, but there wasn't enough clearance around the exhaust elbow and it rendered access to the zinc and the transmission filler impossible. As it is, its all a very tight fit. Maybe the layout on the C34 is a little different.
I also wonder how important that fiberglass crossmember is, i.e., whether its structural, because if it wasn't there things would be easier....

[QUOTE=TomSoko;8402]Matthew,
Yes, it is structural. It's part of the aft berth. Without it, the aft berth, below the wooden shelf, would flex quite a bit. You could probably trim it a bit, but I wouldn't cut it away completely.[/QUOTE]
You're right -- but at least my hands wouldn't have gotten so cut up working in that tight area!

[QUOTE=Chachere;8388]
It was a bit of a squeeze getting it in -- I ended up removing the old bracket from the engine and reversing it and bolting it onto the fiberglass crossmember instead. And the hose connections are not all the same size -- although pretty close. Fired up the engine today and it all seems good.
I'll try to post some pictures at some point.[/QUOTE]
Here's a photo of the finished installation (with the reversed bracket). Sorry, its not only a tight area to work in, its tight to photograph.