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Thread: Finish on 20H Conversion LU

  1. #41
    Administrator Ron Hill's Avatar
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    Default 4000 Mark 20-H's WOW!

    Quote Originally Posted by Mark75H View Post
    There were 4,000. As original equipment, approximately 1,000 had the long skeg.
    No wonder we had boat racers in the old days. Now days we try to build a class with 10 motors from Japan....and less Sidewinders.

    How does a 20-H housing differ from the Mark 30-H??? Does the 30-H have a larger needles bearing in the driveshaft??? Seems me dad changed a 20-H housing to fit it on a 30-H...

  2. #42
    BoatRacingFacts VIP Aeroliner's Avatar
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    Default 20-H housing

    If I remember correctly the deep skeg housing were the same basic design. The big differences was in the completed assembly. The 30-H had a stronger drive shaft which was later adapted to the 20-H. Early 20-H shafts when run on a conversion tended to fail in the upper ball bearing area. Broke a bunch years gone by. Note that the production run had about 4,000 engines but there were additional engines built for development.
    Here is some data from the OldMercs website:

    •This model was painted "Mercury Green" and "Gold."
    •Model & serial numbers were on the instruction plate riveted to the swivel bracket above and between the clamp screws.
    •Serial numbers range from 786613 thru 790612.
    •This model was sold in 1954-55-56. Low numbers in this serial group can be assumed to be 1954, high numbers 1956, and mid-range numbers 1955 area.
    •Many of these models were "converted" in about 1959 to make them faster and more powerful. The Conversion Kits changed the exhaust system, fuel system, and lower unit gear ratio. Conversion engines have a remarkably different appearance. (See photo.)
    •Original, unconverted engines used a special pressurized 2-line fuel system.

    Although there might be some 20-H engines produced with the deep skeg I have never seen one that is unmolested with the unit installed. I just bought a never run 20-H S/N 789158 which is in the 1956 range and it has a short skeg. The short skeg was still being produced when Mercury introduced the 22 cubic inch SS engines, of which I have 2. These units have the same water pickup as the deep skeg units.

    Alan

  3. #43
    Team Member A/B Speedliner's Avatar
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    Default

    Early gear conversions that used the original gear case required some minor scraping of the inside for clearance. When I put Alan's conversion together for him in 1959 the instructions that came with the kit showed how to do the relief.

    The only parts that are different when you use the later gear case with either 1:1 or 16:21 gears is the driveshaft pilot, driveshaft seal and the upper ball bearing. The thin S7K bearing used a thick pilot which was changed to the thin pilot when the thick bearing 16MM ID bearing is used. The OD of the ball bearing may be a little different also. The thicker bearing also required the use of the heavy duty driveshaft which required a different seal.

    David
    Old Race Boats Still Flip You Out

  4. #44
    Sam Cullis Mark75H's Avatar
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    Can you give me the part numbers of the 2 different driveshafts?

    Although there might be some 20-H engines produced with the deep skeg I have never seen one that is unmolested with the unit installed. I just bought a never run 20-H S/N 789158 which is in the 1956 range and it has a short skeg.
    Unmolested 20H's of all variations are rare; easily accounts for any particular person not seeing any particular variation. Your serial number is 1,454 from the end, not sure that is truly in the '56 range; again a serial number well within the range where I expect to find a small skeg.

    With the help of several friends I surveyed hundreds of 20H's ... The survey started giving a consistent result at about 100 motors and continued no matter how many we checked, so we stopped after several hundred. About 25% have large skegs, well over 95% of these have serial numbers in the last 1,000. I attribute the remaining 5% to parts swapping over the years. The <5% number also held well for early numbers with large skegs. I welcome you to do the same research for comparison.
    Last edited by Mark75H; 05-25-2014 at 07:23 PM.
    Since 1925, about 150 different racing outboards have been made.


  5. #45
    BoatRacingFacts VIP Aeroliner's Avatar
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    Default 20-H lower units

    Hi Sam,
    Thanks for the info on the 20-H. I have all of the old Mercury racing parts manuals and looked up this data so far. Starting with the KG-4/7 the drive shaft is listed as M50-1360 which became the 45-20909 in the short skeg lower unit. These shafts had the thin lower section and also used a thin ball bearing and thick pilot. The original lower unit part number was 1604-135A1 and I believe the empty housing was 1604-135. The deep skeg housing assembly was 1604-1162A1 and the open housing was 1604-1162 I believe. From the 30-H info and my 1968 Mercury parts status report the open deep housing became the 1604-1116A4 for the 20-H as well as being used on the 30-H. Some time after May 1968 Mercury introduced the metric shaft as a replacement for the normal 20-H drive shafts as the P/N is not listed in the report. I believe they made these shafts with the run of 22 cubic inch black 25SS engines, which had the short skeg lower unit. The early deep skeg lower units came with the standard 45-20909 drive shaft from my parts manual which would indicate that they came installed on an engine. I am still looking for the SB that introduced the metric drive shaft to the 20-H.
    If anyone would like to have a copy of the 20-H, 30-H and 55-H status report I will take the time to scan it and make it available by Email.

    Alan

    Note: The 25SS engine was a 1973 offering.

  6. #46
    Sam Cullis Mark75H's Avatar
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    Great info! Thanks!
    Since 1925, about 150 different racing outboards have been made.


  7. #47
    Team Member MTECHMARINE's Avatar
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    This 20h has a very small "1956" steel stamped between the model and serial number on the id plate.

    Serial number 790039

    pretty sure it has the original gearcase.
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  8. #48
    BoatRacingFacts VIP Aeroliner's Avatar
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    Default 20-h

    Great info on the 20-H. At least we now have one S/N for the 1956 run. Maybe other folks will share their S/N info.

    Alan

  9. #49
    Team Member A/B Speedliner's Avatar
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    Default More numbers

    Here are a few more numbers to digest. The H length driveshaft for the KG4 & 7 H and the original 20H is 45-20909, the 30H part number is 45-25726 the 25SS part number is 45-65844 which uses a narrow bearing and is the same length as the H 2 cylinder engines and the same spines for the crank shaft. It is thickened above the pinion gear but not like done on the 20H metric shaft or the 30H. There were two shafts for the 25XS engines a short one 45-14544 and a longer one 45-12639. The 2 cylinder H engines used the gear case 1604-135A1 which is the small skeg. The empty gear case for the large skeg 30H, 25XS is 1604-1116A2. The empty case for the 20H is 1604-1162A2 large skeg. The 7.5J full case is 1604-1116A8 I do not know the driveshaft used but they did have an adapter to fit the smaller splines in the crank shaft. The 25XS short shaft full case was 1604-1116A17 and the long shaft 25XS full case is 1604-1116A14.
    Old Race Boats Still Flip You Out

  10. #50
    Sam Cullis Mark75H's Avatar
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    I have never seen the year stamped between the model and serial on any others. If someone else has one like this, please share a picture.

    The last 20H number is supposed to be 790612 ... so this long skeg 20H, approximately 600 from the end of production, is again what I would expect.
    Since 1925, about 150 different racing outboards have been made.


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