Name this lower unit

**Mark75H** · 02-10-2008, 09:54 AM

No, it is a Merc. If you read through the whole thread you will learn about it and its close relatives.

The visual similarity/familiarity is probably the smooth prop shaft on this Merc unit and the 75/105/120/135 Chrysler unit (that most people call the 135 Chrysler unit, 'cause that was the last Chrysler to use it). Big difference between the shafts though ... this Merc is 3/4" , the Chrysler is 7/8", so props will not interchange.

Both the Mac and Chrysler are completely different from the Mercs and each other.

The Mac and Chrysler were both white, did not use exactly 1:1 gears, used fat, reduced diameter gears, had a thrust boss cast in, the exhaust exited above the cavitation plate, were slightly larger than a Speedmaster, may be co-winners of the butt-ugly racing lower unit award and were designed by the same guys; but after that they are completely different.

I guess I should be doing a separate thread on the Mac and a separate thread on the Chryslers to go thru all the variations of each of them, too.

**smokin'joe** · 03-30-2008, 12:05 PM

...were first built to bolt onto production Merc 800 downhousings in 1960. The fat, short direct reversing, low geared Mark 75, 78 and Merc 800 gearcases gave the John-Rude 75 a good chance in NOA Pleasure Craft Racing, in fact until we (and also Paul Allison) got ahold of a Sportsmaster unit (geared between the club foot and 1/1, and some what thinner than an OMC 3 cyl. looper gearcase of later years), Johnson held the unlimnited record on a 14' Allison at about 57 mph. I held the 70-80 class record with the standard gearcase Merc 800 on a heavier 14' Allison at 51 mph, and upped it to nearly 57 mph with the Sportsmaster unit (and ran over 60 mph with the same motor on my 13' Allison in unlimited class, driving on my knees from the transom--Paul ran a similar rig at over 61 mph and set that record). My Dad had complained until he turned blue in the face about Kiekhaefer 6 cyl. gearcases, the new gearcases were the result of such feedback (the Speedmaster was disallowed in pleasure craft racing).

Originally Posted by Mark75H

Who can name this lower unit EXACTLY? From this image tell us the year this was made, gear ratio, rotation and the motor it fits on.

When the guessing is finished I (or DaveS) will explain how you can tell all the details.

DaveS knows, so he can wait for the guesses before he tells us. Raceman may know, but might be short one detail.

**Mark75H** · 03-30-2008, 12:51 PM

Joe, you are correct, 1960 is the first year the Speedmaster was legal. The first spec sheet I have showing them is dated Jan 1, 1960. I had the spec sheet and never looked at it. You made me go back and do thorough complete research to learn that date.

At the same time the left and right 800 Speedmasters came out, so did the one in this thread ... not an 800 but a 700. There were subtle differences between the two and that is what this thread is about. 80hp Speedmasters used 3/4" smooth propshaft with 5/16 shear pins, this one has a 1/4 hole, so it was for a 70hp.

Info on the Sportmaster lower unit is in this post: http://forums.screamandfly.com/forum...1&postcount=24

**smokin'joe** · 03-30-2008, 01:18 PM

Sam,

There were also both left and righthand rotation Mark 78 standard gearcases in 1958, we sold at least one.

Best,
Joe

Originally Posted by Mark75H

Joe, you are correct, 1960 is the first year the Speedmaster was legal. The first spec sheet I have showing them is dated Jan 1, 1960. I had the spec sheet and never looked at it. You made me go back and do thorough complete research to learn that date.

At the same time the left and right 800 Speedmasters came out, so did the one in this thread ... not an 800 but a 700. There were subtle differences between the two and that is what this thread is about. 80hp Speedmasters used 3/4" smooth propshaft with 5/16 shear pins, this one has a 1/4 hole, so it was for a 70hp.

Info on the Sportmaster lower unit is in this post: http://forums.screamandfly.com/forum...1&postcount=24

Link to the next Speedmaster version

**Mark75H** · 03-30-2008, 01:44 PM

True, but I don't know how that relates to this???

**smokin'joe** · 04-01-2008, 02:54 AM

Jim Hunt was my co-driver in 1979. Financed by Fred Schoellkopf (who became a fine driver in his own right in 1983) who co0drove with George Laycock. I set up both boats, my motor came from Ireland and Jost, Kenny Shaw (who won with similar boat) and bill muncie got the other two factory motors. In 1981 I drove the 4 hours alone, really loved it although the boat (Louis Collins' winning 1980 Laser) was no longer competitive because they changed the rules at the last minute under pressure from the Cal. crowd to permit beneath gerarcase water pickups. that allowed the tunnels, which previously had zero advantage, to run upwards of 100mph. We were stick at about 85mph.

**jerry chichetti** · 04-24-2008, 12:21 PM

Hi 75H,.
That was a cool video. I was 13 years old in 1957 and my family had a Merc dealership in Seattle Wa. I ran lots of Merc 75's 78,s and all the other D R', AS THEY CAME ALONG. I had a pair of them on a 14' Powercat. It is fun to look back on the old days and realize just how much advance has been made in engines. I love my 20' ally w/the 280. Jerry

**RBoso** · 04-07-2009, 03:39 PM

was it used on a marc 75 78 made in 1957 or so????????

**Mark75H** · 04-07-2009, 04:13 PM

No, it does not fit towers made before 1960

**AirRide** · 04-07-2009, 05:17 PM

First SpeedMaster made. Single shaft. Single pinion. Weak unit, thats why there's few left, and the reason the twin shaft, twin pinion was developed soon after. The driven gear(pinion) snaps right off the shaft. Its so weak, I would hesitate to even use it on a historical piece(motor).

**Mark75H** · 04-07-2009, 05:55 PM

Originally Posted by AirRide

First SpeedMaster made. Single shaft. Single pinion. Weak unit, thats why there's few left, and the reason the twin shaft, twin pinion was developed soon after. The driven gear(pinion) snaps right off the shaft. Its so weak, I would hesitate to even use it on a historical piece(motor).

I have seen quite a few of them still in use and almost as many apart, and I have never seen one with a broken pinion gear as described. These work quite well as long as you don't run them with more than 100 horsepower. About 500 were made and a surprising number of them are still around.

The main failure is gear tooth wear

Link to the next Speedmaster version

.
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**AirRide** · 04-09-2009, 09:31 AM

Originally Posted by Mark75H

I have seen quite a few of them still in use and almost as many apart, and I have never seen one with a broken pinion gear as described. These work quite well as long as you don't run them with more than 100 horsepower. About 500 were made and a surprising number of them are still around.

The main failure is gear tooth wear

Link to the next Speedmaster version

.
.

Broke plenty of them years ago. The driveshafts sheared right at the top of the pinion, the gearcases weak point. You know, in retrospect, your right...the horsepower was the straw that broke the camels back. lol

**Mark75H** · 11-15-2009, 09:38 PM

If anyone who has a single pinion Speedmaster is interested in new gears ... there is an on going project that could yield gears as a side product

In order to produce the driveshaft gear, the guys involved would need to invest an additional $700 in tooling to produce the driveshaft gear. If we don't hear anything, these will not be produced.

These will be the real deal, but they will not be inexpensive.