CAUTION: a zinc that is too long may possibly damage the core of the heat exchanger and will certainly destroy the anode, thereby leaving your HX vulnerable to stray electrical current.

The anode is best replaced (at least your first time) concurrent with removal of the HX end cap. Be careful to note the relationship between the o-ring and the gasket when you remove the end cap, as the cap should be reinstalled with the same relationship. Why remove the end cap? Because you can readily see what's going on within the HX. You can use your finger to pull out remnants of old crumbled anodes, and you will have a better appreciation of how the anode fits into the picture.

I don't know the proper length, but it is easy to determine what it should be.
When you have a replacement anode in hand, remove the old one. Use a small screwdriver or other probe to 'measure' the maximum depth limit that the new anode will have to comply with. Then hacksaw the new anode to the proper length and install. Do not use any insulating silicone or similar lubricant on the anode threads because it is essential that the anode threads maintain a solid electrical contact with the HX housing.

Just happened to have a new one here as I bought a bunch of spare engine parts. The Universal part# is 301068. The overall length of the whole thing is 2", the threaded part is about 3/4" long and the anode part itself is 1 3/16".

Berndt,

If you haven't already contacted them, you can buy the Universal Zinc Part Number 11885 ( and any other Universal engine spares ) direct from Marine Energy Systems Rivergate Marina Murrarie Brisbane Qld (07 3890 1300). Ask for Prince - he is very helpful. They will ship by overnight mail and they cost about $10.00.

For the reasons Larry has mentioned, I cut off about 3/8 inch to ensure it doesnt touch the heat exchanger core when you screw it in. The actual zinc screws into the plug and is made by Martyr and you can buy the replacement zinc element only (through Whitworths here in Australia) if you really want spend the time to save a couple of dollars!

Note: the brass plug has a tapered thread so you dont have to try to screw it all the way in - only a light touch is required to seal it. For what its worth I don't cut my zinc off and it doesn't touch but better to be sure.

Also for what its worth as I do, you can get all engine spares by mail order from the US at about half the price if you can be bothered.

11885 is a small zinc -3/8" nut I believe, the anode is thinner and about 3/4" longer than the 301068. I have one of the 11885 ones here from our other boat that had an M3-20 engine. It is for the 2" heat exchanger. The 3" heat exchanger on our M35A takes the one I mentioned above 301068. I bought 4 of them and just installed one so I know it fits. But perhaps they put the 2" heat exchanger on M35A's?

Bud, as far as I know, everything after the M25, syarting with the M25XP, came with 3 inch HXs. I put a 3 inch HX on my M25.

I have the M35 in my 1991 hull. I've been using the replacement pencil zincs off the shelf at West Marine, which are 2 inches, with no problem.

Nelson

My 2000, Catalina 36, hull 1897 with M35 diesel:  magnesium anode, 2” long.  A bit of patience is needed...  remove:
- aft cabin engine console (two rubber snubbers need to be popped off)
- aft cabin “L” shaped cushion nearest engine.
- use ratchet wrench or ratchet socket, other wise lots of patience to remove anode
- after removal of old anode (in most cases just a threaded stub of shinny brass or copper threaded plug), use a 4-5” thin rod to test the depth / length of the anode.  There should be a slight dripping of raw water, (fresh or salty depending on where you sail).  You might also have a few piece of old anode or maybe rubber impeller drop out).  
My overall length of anode, including threaded portion, end to end was 2 inches.  It took patience to get the threads started to screw replacement anode in.  
Run engine BEFORE replacing aft cabinet & cushion to make sure there are no leaks.  
***Also a good time to check:
——transmission fluid level
—- your raw water intake filter ( I had a few zebra muscle shells in there)
——fuel filter

Good luck, it wasn’t too hard.