My 2004 Sierra catalog lists Sierra #18-2952 as the head gasket for 1957-58  7-1/2 HP.  It might be the same for 1955 , as these motors didn’t change much back then (maybe someone here knows?).  Says  it replaces 203034.  See if it looks like this one:  https://www.marineengine.com/newparts/part_search.php?part_num=203034

If you call marineengine, they will probably know for sure.

Dave

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Rkymtn57

this may help

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Rkymtn57

Regarding the exhaust cover gasket, from the factory there was only one gasket on the outside of the exhaust baffle. On the inside of the baffle some kind of sealant was used. Some of us prefer to use two exhaust cover gaskets. One on the outside of the baffle and one on the inside of the baffle. It may cost a little more, but it makes for a very dependable repair/service.

crosbyman wrote:

check the exhaust cover plate for pin holes … I recall getting a stainless one  from one member on here (sorry) .

Thats a good point about checking the exhaust baffle for pinholes. While you have it apart, clean it up well and give it a check with a bright light behind it to see if any light comes through. If so, you will need a new exhaust baffle.

Was it Wedgie that used to make SS replacement baffles? The factory ones were made from aluminum, and that’s why some of them failed. Especially ones used in saltwater.

The plates were cut with some kind of plasma cutter or CNC metal cutter … super work  perfect fit .