33 VS 35 hp OMC.

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Adam1961
Posts: 392
Joined: Sat Mar 14, 2015 7:46 pm
Location: Montreal, QC Canada

33 VS 35 hp OMC.

Postby Adam1961 » Wed Jun 13, 2018 7:01 pm

Hi all,
I am doing some comparing of the 1960's 33hp and the late 50's 35hp motors. Does anyone have two heads to compare. I am wondering if there is a visible difference in the chamber design. I would also be interested in the port height on a 33 if anyone has a block apart

Thank you,
Adam

Dan in TN
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Joined: Mon Mar 09, 2015 3:58 pm
Location: Smyrna, TN

Re: 33 VS 35 hp OMC.

Postby Dan in TN » Wed Jun 13, 2018 7:50 pm

This may not be what you had in mind, but while you are waiting for an answer I use eBay pictures to help. I put in the 33hp head #311170 and a few came up. Pictures are pretty good. One looks like they actually stamped 170 near the spark plug holes in the combustion chamber? I have a 1965 model. It's missing the motor cover, starter, etc. I know the pistons are different (part number) & of course the head doesn't have compression relief valves. Never got a chance to measure port height or anything?

https://www.ebay.com/itm/NOS-OMC-EVINRU ... 1438.l2649

https://www.ebay.com/itm/P45-New-OMC-Jo ... 1438.l2649



Dan in TN




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VinTin
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Joined: Tue Mar 10, 2015 11:13 pm
Location: N/E Georgia

Re: 33 VS 35 hp OMC.

Postby VinTin » Wed Jun 13, 2018 9:38 pm

I believe they are the same bore and stroke.

reivertom
Posts: 550
Joined: Tue Mar 10, 2015 6:49 pm
Location: Eastern Kentucky

Re: 33 VS 35 hp OMC.

Postby reivertom » Thu Jun 14, 2018 12:38 am

I never understood the 33, or the 28. I always thought they might just be marketing schemes from some suit instead of really different.

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Phil B
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Re: 33 VS 35 hp OMC.

Postby Phil B » Thu Jun 14, 2018 12:55 am

I'm sure it's come up here before, but found this:

https://forums.iboats.com/forum/engine- ... p-big-twin
[the Aerocraft & OMC boat guy; formerly "Silver Bay"]

FrankR
Posts: 3594
Joined: Tue Mar 10, 2015 6:32 pm
Location: Florida

Re: 33 VS 35 hp OMC.

Postby FrankR » Thu Jun 14, 2018 7:25 am

reivertom wrote:I never understood the 33, or the 28. I always thought they might just be marketing schemes from some suit instead of really different.


Low priced motors to fill the gap between the 18/20 and 40hp motors. Made simpler to keep costs down. But while holding onto the legendary OMC quality.

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Chinewalker
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Location: 1000 Islands

Re: 33 VS 35 hp OMC.

Postby Chinewalker » Thu Jun 14, 2018 11:19 am

I've used 33hp pistons to rebuild an older 35 with no issues. As far as I recall, the head shape was more or less the same, but the 33 head lacks the expansion chambers for the compression release. The 33 also has the beefier redesigned 40hp crankshaft.

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twostroke
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Joined: Fri Sep 15, 2017 12:34 pm
Location: Central Lower MI

Re: 33 VS 35 hp OMC.

Postby twostroke » Thu Jun 14, 2018 12:38 pm

Low priced motors to fill the gap between the 18/20 and 40hp motors. Made simpler to keep costs down. But while holding onto the legendary OMC quality.[/quote]

Amen, Frank. I've got a late '80's 28SPL on my pontoon boat that just runs....and runs. Feed it mixed gas and a water pump impeller every few years. It just might be as good a motor as one of my beloved 5 1/2 CD's...

Jim
I say "Pardon Me" a lot. I had a 20H with a toilet bowl, then raced open mod sleds. :D

amuller
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Re: 33 VS 35 hp OMC.

Postby amuller » Thu Jun 14, 2018 4:59 pm

No thermostat on the 33.

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Garry in Tampa
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Location: Florida

Re: 33 VS 35 hp OMC.

Postby Garry in Tampa » Fri Jun 15, 2018 12:45 am

A little history - - - when the Big Twin came out in 1951, Evinrude had been working on a 25 horse power apposed four. The Johnson twin was lighter and much cheaper to build. It also put out 26 prop shaft horse power where the Evinrude 4 tested 26 horses at the powerhead, The prop shaft power was 24. After fine tuning the prototype the production Big Twin averaged 27 horse power at 4000 RPM on the dynamometer. It hit 25 HP at 3750 and dropped below 25 at 4450 RPM. At the supreme test of being put in the hands of the public some failures were noted. As more aluminum was added to various places in the outboard, you will note an increase in weight over the years.

In1955 big changes were made to quiet the noise the Big Twin produced and to isolate the noise from the boat. This resulted in an increase in weight and loss of power. The tight fitting sound dampening motor cover and carburetor intake muffler did not allow enough air into the motor. Service bulletins were issued to open the quarter inch hole in the front lower motor cover to two inches and to remove the rubber elbow and the acoustical liner from the intake air silencer. These changes brought up the horse power up to 25. They pushed it to 30 the following year by higher compression pistons and heads, needle bearings on the wrist pins, and revised port timing.

Meanwhile they were working on a bigger model. The 35.7 cubic inch Big twin was bord out 3/16" (from 2-7/8" to 3-1/16") to 40.5 cubic inches. The prototype was judged to be hard to pull over by hand and compression relief valves were added to the production model. So if you have a weak battery that is having trouble cranking your 35, you can reduce cranking effort 30% by pulling the starter rope out about six inches. A well tuned motor should start in just a few revolutions. This Big Twin was bored out another 1/8" in 1960 to 3-3/16" which brought the piston displacement to 43.9 cubic inches. It had a long life as a 40 horse power outboard. This left a big gap between it and the the 18 horse model. A slightly modified 30 horse became the 28 horse Ski Twin. Then as the 18 horse was "souped up" to 20 and 25 HP, a slightly lower compression 35 took it's place at 33 horse power. I hope this explanation helps clear the air . . .
8-)
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Last edited by Garry in Tampa on Wed Jun 20, 2018 4:27 pm, edited 1 time in total.
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