Over the years there has been a lot written about the correct size of the Heat Exchanger zinc for the 3” HEX on the M35 series motor.
There have been concerns about "punching" a hole in the HEX and like others I’ve been guided by advice in the forum to cut a piece off the standard zinc to be on the safe side.
Because of the tapered thread on the brass nut it’s difficult to feel with a spanner if the zinc is located correctly and that its not too long.

In the various parts list the zinc is listed as Part Number 11885.
This is generally a Martyr CME-0 zinc which is 1 ¾” long and 3/8” diameter which has a threaded screw base which in turn screws into a 5/16” brass nut with a ¼” NPT thread.
I have attached a photo of the standard zinc screwed into the HEX and its can be seen that there IS clearance. The blue mark is about 1/4" long. One point to note is that the NPT thread is tapered and it only needs to be lightly screwed into the HEX – definitely not all the way in as I’m sure many of us might try to do. There is little chance of “punching” a hole in the HEX as it is a sturdy casting as can be seen.

For comparison I’ve attached a photo of shortened zinc showing plenty of clearance as would be expected.

The other photo shows the two zincs side by side with ¼ “cut off one.

Needless to say I’ve been using uncut zinc for ages with no problem.

Hope this helps