Mercury MK20 restoration

Hey everyone...I'm new to the forum and I'm looking for info from some experts. (but I'll settle for a good shot in the dark and a touch of experience) My brother and I recently obtained our grandfathers old 1958 14' Fleet aluminum fishing boat and 2 1954 Mercury MK-20 motors.

I'm in charge of the motors and I've run into a few problems. When disassembling the motor for a preliminary look, I snapped 7 of the 8 bolts holding the water jacket cover on (at the spark plug end.) I've tried using a bolt extractor, but the bolts were very soft and simply expanded when the extractor bit down. I'm down to drilling the bolt out and retapping the holes. Has anyone every done this and do you have any advice?

Second, The gasket between the motor and the water jacket cover was in sad shape after I separated the two. Should I track down a factory replacement or would a homemade gasket out of gasket material be sufficient.

I'm very handy with all things mechanical and other types of internal combustion engines, but this is my first outboard motor so any help would be appreciated.

Mark

Re: Mercury MK20 restoration

First i am jealous, not one but two Mk 20"s!!! Parts are still available as are gasket sets, parts lists are also available. With an engine that old you will have to assume that the bolts will be corroded and will break. Sometimes you can tighten the bolt first then loosen it a bit and let it soak in wd40 to try and break the corrosion, heat often helps but the bottom line is you will break bolts. Drilling them out carefully and retapping is usually the only recourse. Water jacket bolts are the worst. I am sure you know to replace the impeller in the lower unit before you try and start them up. The carbs are simple and should be removed and cleaned with carb cleaner after they are dissassembled, you can not clean a carb with it installed. When you take the carb off you will find the reed valve behind it, make sure that it is closing properly and that it is not broken. You will be able to inspect the piston through the port covers on each side of the block. If the pistons are not scored and the rings are free I would suggest you not dissassemble the block.

Re: Mercury MK20 restoration

Dave,

Thanks for the info. I am a newbie at this outboard stuff, so can you explain why I need to replace the impeller. I just recently starting making contacts with some folks who restore older outboards, so I'm no expert by any means.

Mark

Re: Mercury MK20 restoration

Impellers have a finite life and not all of it need be actual service time. They can dry rot or take a set just from sitting for a period of time. Besides, for as little as you spend on an impeller, having one go bad on you while running the motor is a good way to ruin the motor, and possibly ruin a weekend in the process. Cheap insurance....
The Mercury 1965 & Prior Service Manual is a must-have for these old Mercs. Available at www.oldmercs.com, among other places. They turn up on eBay from time to time, too....
- Scott
- Scott

Re: Mercury MK20 restoration

Scott, Chinewalker, did a good job of describing what happens to the impeller. Whenever I come across someone with a mark 20 I remind them because when I sold my mark 20H and hydroplane in the late 80's I told the purchaser, who was a fellow co worker, that he MUST change the impeller before he ran it, I knew it was old and had to be changed. He decided that since there was water coming out it would be just fine. He enjoyed the motor for exactly 5 minutes in a straight line at wide open throttle and then enjoyed the 5 mile paddle back.
Post lots of questions as you go with this and you will get answers, this is a great forum and you will find knowledgeable people.
Post pics if you can when you have a problem, it will help folks understand what you are looking at.
I assume yours is the green and gold model with the gold faceplate, that was the most popular in the day. The paint is available too from old mercs I believe as are the decals for the gas tank (ebay) Recently I saw a fuel connector on ebay for that engine too. Take your time and you will have a wonderful motor(s).

Re: Mercury MK20 restoration

Dave,

Actually, I have the green and silver model. The 16hp version. I have two identical motors. Lucky, I know. One of them has the fuel connector and one doesn't. I ran across someone the other day who was selling those fuel connectors. Can't remember where though. I'm hoping that between the two motors that I can get one good one running and then use the other for spare parts. I'll post pics when I can. One final question regarding my project. These motors are going on my grandfather's old 1958 14' fleet fishing boat. A few of the boat rivets are loose and need to be replaced or snugged up. Was there a common rivet size for these aluminum boats? I know they are solid rivets, but they come in various sizes and without drilling one out I won't know. (And Drilling could quite possibly ruin the rivet so I couldn't tell it's original size anyway.) Thanks for everyone's advice.

Mark

P.S. On the manual that was mentioned earlier. We found several of them on Ebay for around $45. I also found a 1953 MK20 owner's manual on ebay for $9.99. I purchased it because the cover said that it has maintenance and repair in the book. Has anyone ever seen one of these books and am I better off just throwing down for the bigger, more expensive book?

Re: Mercury MK20 restoration

FYI, the green and gold only came on the Mark 20H, which was the racing model seen in my avatar.

The owner's manual will tell you how to change the gear lube and maybe adjust the carb, but is pretty much useless for actual repairs. Get the service manual - it'll pay for itself in saved frustration and broken parts...
- Scott

Re: Mercury MK20 restoration

Chinewalker said:

FYI, the green and gold only came on the Mark 20H, which was the racing model seen in my avatar.

The owner's manual will tell you how to change the gear lube and maybe adjust the carb, but is pretty much useless for actual repairs. Get the service manual - it'll pay for itself in saved frustration and broken parts...
- Scott

Click to expand...

Agreed Scott, mine was the 20H hurricane, man I wish I had not sold that motor, it would still be running today. The other thing that I found useful when I owned the motor was the parts manual. The diagrams often showed how things went together better than the service manual did. At least you could see what order the washers went on the various shafts

Re: Mercury MK20 restoration

Hi Dave,
Did you race or was yours on a flea? I've got two, both built from parts over the course of a couple years. One is the original green & gold, and the other is a Cornpopper conversion...
- Scott