There is usually a cap at each end of the heat exchanger so you can
open it up and clean it out. The only problem with zinc crud in the HE
is that it will block off some of the passages. It needs to be cleaned
occasionally, mabe every 3-4 years, but just one pencil dropping in
there shouldn't be a problem. That is, unless the PO left a bunch in
there as well! The pencil in your case corroded off and got stuck in
the hole. Probably should have been checked / replaced earlier. If you
haven't done so since you've owned it, probably a good idea to remove
the HE and take it to a radiator shop to have it hot tanked and
pressure tested. Otherwise, remove any end caps and blast out with a
water hose.

Paul C wrote:
I have a Westerbeke 4-107 in my new (to me) boat. Upon checking the
pencil zinc on the heat exchanger, I found that it was not screwed into
the cap, although I could see zinc sitting in the orifice. Either it
was coroded away in the cap, or was not screwed into the cap properly.
Not sure which, although the cap was clean with no zinc residue in it.
I tried to grab the zinc with pliers, but it would not come out.
Finally, it fell into the exchanger. The heat exchanger is a horizontal
cylinder about 3" in diameter and is about 14" long, with the zinc
fitting at the 9 o'clock position. My questions a Is having zinc
scraps sitting at the bottom of the exchanger a problem? The opposite
end of the exchanger is fitted with a bolt in the center. Is the
purpose of this bolt to an access to clean out the exchanger? If so,
how is cleaning accomplished. Thanks in advance for any responses.