Evolution of the early 1950's 25 hp OMC's

[Motor Boater Bill]

It seems like the early 1950's 25hp Johnson RD-10 through RD-17 series motors (and the Evinrude equivalent), evolved into 35 hp motors around 1957 (RD-19 through RD-21), then became 40 hp motors in the 1960 or so (RD-22 through RD-30). I think I read that the RD series were around 38 cu. in. displacement, although I don't know if that is correct. That seems large for a 25 hp, about right for a 35 to 40, perhaps.I have three questions:1. Am I right about the evolution of this motor?2. How did they do it? Did the displacement increase by changing bore and stroke on the same block, or did they change other things like carbs, ports and exhaust--or all of the above?3. How much parts interchange is there among the various generations of these motors?I'm picking up a 1954 RD-16 this weekend, so I'm trying to education myself about them.Thanks!

[Chinewalker]

Hi Bill, Here goes. The first "Big Twin", as they are commonly referred to, came out in 1951 as a 25hp. They were approximately 35.7 cui. This model ran through the 1954 model year, more or less unchanged.1955 saw a fairly substantial change toward quieting the motors, as they sealed the powerhead and made changes to the exhaust and lower unit sections. Horsepower remained 25 and displacement remained 35.7. They also changed the throttle arrangement from cables to gears where it entered the powerhead area.1956 saw a change in the cylinder head and piston dome to go from a 5:1 compression ratio to 6:1. Displacement remained 35.7, but hp bumped to 30.1957 they bumped the displacement up to 40.5. Same stroke, larger bore. Horsepower was bumped to 35. 1958 marked the introduction of the "Super Quiet" motors with the double wall mid-sections and better sealing for sound. Evinrude's was the Lark, and Johnson's was the Super Seahorse. You could get either the Super Quiet version or the standard Big Twin model in 1958 (similar to the 1957s without the quiet package).In 1959 the quiet models were the only 35s available.1960 saw a bump up in displacement once more to 43.9 cui and horsepower went to 40. Unfortunately, they were still using essentially the same crankshaft that they started with in 1951 and the 40s began breaking cranks. 1961 saw a beefier crankshaft introduced, retaining the same stroke as 1960 and prior.Hope that helps!- Scott

[OBJ]

You did that from memory....didn't ya' Scott.....

[Chinewalker]

Mostly... I had to double check on the displacement for the 40 horse.. Forgot to handle point three of Bill's query...Lots of interchangeability between years. lower units from 1951 to 1954 interchange. Units from '55 to '57 interchange. '58 units get a little interesting as they had the two versions offered. Silent units interchange up 'til they went to electric shift in the 1960s. Base (non silent package) model units interchange with earlier models without the silent pkg. Units generally interchange as an assembly between years, within a style, as there was sometimes variations in the innards themselves between years. Magneto parts are fairly standardized throughout the run, but flywheels changed periodically. Confused yet?It's interesting that the Big Twin design remained in the OMC lineup well into the 1980s as the WorkTwin 40. Made for a 30+ year run using the same basic design....- Scott

[Motor Boater Bill]

Wow--thanks Scott. I sort of figured you might have the answers on this one!I will pick up the motor on Sunday, and already got my hands on a manual tonight. I feel like a bit of a traitor to my Mercury roots but it went for a song on eBay ($29) and is about five minutes off of a route I drive pretty regularly--how could I pass it up? Of course, old outboards are like dogs. You can get them pretty cheap, it's the care and feeding that costs ya! At any rate, since all my fishing buddies have OMC's, it will help me to learn a bit more about them. I'll let you know how it goes...Motor Boater Bill (aka Peter H on John's Old Merc board)

[OBJ]

Interesting reading Scott....and a nice bit of history. Thanks!Jim

[Paul Moir]

I have to agree with OBJ, thanks a lot!I just learned the other day about the original Evinrude 4-stroke "Big Twin". Once bitten?

[Jho928]

Hi,Were do the Speeditwin (62-64 28hp) and the Skitwin (65-70 33 hp) figure in this mix? It seems to me that the Speeditwin was made from left over 1956 BT30hp parts and the Skitwin from 1957-BT35hp parts,is this so ?The similarities between these motors is very close.

[JOHNSON110]

Scott, i see you mentioned compression ratio on these motor"s no one ever says what that is, do you know what the compression ratio is on a 1986 110 v4 cross flow omc , i am curious, Thanks Cliff

[Paul Moir]

Jho928, The Speeditwin is in many ways like the 30hp, and in other ways like the 40hp. Really it's an enhanced '56 30hp. It possesses the same midsection as the 30hp & '57 35hp, but with the lower cowl pan of a 40hp. The 28hp cowl and the 40hp cowl are interchangeable.I think the internals of the lower units on some of the 28hps are different for the equivalent year 40hp. I don't think the 40hp ever used the Timken bearing on the pinion gear, but the 28hp did. Externally they're identical.The '62 and '63 28hp are identical as far as I can tell, and the '64 is only different in the fuel pump and the transom clamp arrangement.As for the engine, the 28hp is only slightly different from the 30hp. The crank and bearing arrangement is identical to the 40hp, as is the con-rods. So it's got roller bearings throughout unlike the 30hp. The reeds, intake, and bypass covers are identical to the 40hp (which are slightly different from the 30hp).The Skitwin is the same thing, only with the '57 35hp block. The chief differences between the 28hp and the 33hp are the larger pistons and different port timing. The heads are different as a result. The 33hp does use the 40hp three bearing arrangement on the pinion gear, unlike the 28hp.

[joaklay]

Interesting tidbits here indeed as I have several of those variations, some being restored, some almost perfect. I have a '54 Johnson electric start 25 HP, '55 Johnson 25, '56 30 hp Javelin (actually two, one mint, hardly used and another repainted "beater" in comparison), a '57 35 hp Javelin (almost totally restored).

[Jho928]

Thats amazing that the same design evolved into several different models over a 20 year span.Whats more amazing is that these motors are still running today!As long as I can keep mine running I'll never buy a new OB,for some reason I don't think a new one will live for 56+ years.

[Jho928]

Does anyone know if the short shafts are the same length on these motors. I need a SS for a 28hp and I am not sure if one I have from a mid 50's BT will fit.The 28hp had the adapter pan,does this require a slightly longer drive shaft?Thanks

[Paul Moir]

I don't know. But if you can get a couple tape measures going, it shouldn't be hard to find out. The short shaft will be exactly 5" shorter than your 28hp. The splines and everything should be the same. You can pull your driveshaft without disassembling the lower unit. Just remove the water pump and pull it out. It'll go right back in.

[Texasmark]

Hey Chinewalker, Thanks for the info. I was just getting into boats about the times you discussed and I was a big OMC fan at the time. Good to know about these guys. Couldn't believe the power those old 25 put out but you may have explained it; the 35 cu in power head made for some awesome torque.I remember some guys had 16' wooden boats with wide beams and lots of stuff, external bait wells hung on the transom (for live shrimp) and all,with 2 or 3 fishermen (women). Punch out and that baby would run em right down the bay.....no 50 mph, mind you , but at a right smart clip (probably 25ish).But on the who done it for how long, don't forget the paltry Mercury Mark 55. That baby, was born around '50 (give or take...forgot) and I have it listed in a '97 Merc sales brochure (latest I have), still 4 cyl, and still cross flow, and still in the lineup after all these years. Only big change was it got CD electronic ignition and the belt driven magneto disappeared. Don't know how long it stayed there or if it's there today. No recent info.But in the timeframe you mentioned, Mercs seemed to be in the shop all the time. We OMC fans used to laugh about Merc owners. "Go the shop, pick up your Merc (and boat). Go to the bay (lake); come home early; drop off your rig at the shop cause it broke".....Ha! Ha! Seemed to be true at the time......no mo tho.Mark