Differences between ’50’s era 30, 35 and 40 hp big twins?

[instigator]

I’m collecting goodies, including motors for a twin project.

Hull is a tin, ’56 14′ Crestliner and I want to do matching era power. Very rare model that came w/wide motor board and splash well for twins.

I know the motors very well but am unsure in the differences between HP models and with it being difficult to find similar motors, let alone matching, especially semi-local to me, I am expecting to convert one to match the other.

Can anyone tell me yrs and models where a carb or head change made a 30 a 35 or a 35 a 40? I know on the V-6 OMC’s that I hot rod, only difference between a 200 and 225 is carbs.

How about powerhead mounting? Anyone know if I buy a 1960 40 hp (had one), will that powerhead bolt to a 1956 30 hp (had one) mid section?

Should be a fun project once I get it rolling. Already have the twin control set and am collecting propellers.

Thanks guys!

Gary

[r-c]

Apples, oranges and pears! The 1956 30hp is a 36 cubic inch engine, a hopped up version of a 25 from 1951 to 1955 of the same size. The 1957 to 59, 35 is a 41 cubic inch model, much differnet than the 25/30, but same bolt pattern. The 1960 to ? 40 has a totally different bolt pattern until you get the the later 1970’s and beyond models that used the narrow tower housing, like the 1950’s model.
One problem you will have with dual engines while can’t having one engine counter rotating at the prop, is it will turn well one way and not the other. That was not available back in the 1950’s.
If you found a left handed prop, maybe a Scott-Atwater, and ran one engine in reverse; it would wear out the rear bearing which is a brass bushing, not a ball bearing like in the front.
Just run one engine.

[1946zephyr]

I happen to own two 25’s, a 30 and two 35’s. Each 35 is different builds. R.C. pretty much got it.

The bolt patterns were the same from 1951 to 1954 on the lowers, which was a 6 bolt pattern. The 7 bolt started on the 1955 models". The 25 and 30, if I’m not mistaken, had the smaller pinion gear and I understand that the other two are smaller too. The single wall motors went into 1958 and the Super Quiet model started in 1958, so, both style motors were only built in that year. 1959 on was Super Quiet only, until the introduction of the 28hp, in 1962. This was just a later style 25/30 with a fiberglass hood and two ring pistons. In 1965, the 33 replaced the 28 and that was basically a later style ’57 35 with a fiberglass hood and two ring pistons. The single wall mid section looked a lot like the ’50’s versions, but the cosmetic appearance is slightly different too. Everything inside is pretty much the same though.

Here’s the kicker though. The Super Quiet and the single wall style motors have entirely different mounting surfaces on the lower units and powerheads plus entirely different cooling systems too, so be careful, if you buy a lower unit, or powerhead for your motor.

I hope this helps

[instigator]

Great info guys. MUCH appreciated!

This is a long term project and I’m not in a hurry. My thinking is spend the next yr or so collecting donor motors to be able to combine the pile into two complete, as in identical as possible motors.
In process using biggest carbs, tightest heads, best pistons etc.
Think "restofication" and not restoration. I’ve owned the hull for a yr and been watching for motors since.
Problem has been location of donor motors. I’m by Cincy Ohio so of course I find one motor in Michigan and another in Cleveland!
Annndddd, one will be a 30 and one will be 35 or 40.

If I could find what I wanted, I’d probably buy two 40’s w/the later era plastic cowlings, double wall mids and bolt the powerheads to a pair of 30/35 mids w/the clam shell cowlings.

I raced kneel downs for 9 yrs (’76 – ’84), built my own boats/motors and have continued w/the go fast gene to this day w/hot rodding V-6 OMC’s on light/fast (100 mph+) boats so this will need to run too.
W/the frustration of motor shopping I even considered buying twin recent era 40/50 hp OMC’s and grafting on ’50’s era cowlings. That would give me power trim, better props etc.
Wouldn’t be the same though.

So, pull start, 30 -40’s and probably own it forever.

W/a 200 lb hull weight, it should run good.

As to both being RH rotation, that’s the way they did it back then and I’m OK w/that. One of my dads first pleasure boats was a plywood 18′ w/twin, 1953/1954 25’s.
Also, I bought a 24′ twin outboard Sonic yrs ago and before I repowered/restored it, both motors were RH rotation and I couldn’t even tell.

[1946zephyr]

Well, be careful there, friend, the powerheads from the Super Quiet 40’s wount bolt up to a clamshell style Big Twin. The rear half of the engin mounting is completely different.

[instigator]

quote 1946Zephyr:

Well, be careful there, friend, the powerheads from the Super Quiet 40’s wount bolt up to a clamshell style Big Twin. The rear half of the engin mounting is completely different.

I got that from your previous quote and is much appreciated.

I’d like to compare the two.

I may be limited at 35 to keep the cowlings I want.

[chris-p]

You CAN bolt a 40hp powerhead, onto a 35hp Super Quiet from ’58/’59 and have a sleeper. As stated, you cannot bolt a 40hp powerhead onto a 25/30/35hp single wall exhaust motor though.

[olcah]

Hopefully not moving too far off subject – What were the hop up mods done to go to 30 HP for 1956?

[Anonymous]

Hi OLCAH, the 1956 30hp motors had higher compression and were very sensitive to engine damage if over propped.

[Casey Lynn]

quote Chris_P:

You CAN bolt a 40hp powerhead, onto a 35hp Super Quiet from ’58/’59 and have a sleeper. As stated, you cannot bolt a 40hp powerhead onto a 25/30/35hp single wall exhaust motor though.

Wanna’ bet?
I bolted up a ’74 40hp single wall exhaust to a 1957 35hp. single wall exhaust engine. Have to cut an eyebrow at the forward end of the lower bearing housing on the crank and match it on the mid section and use the later model gearcase due to the differences in driveshafts and cranks……… Been there done that!
Getting the clamshells to fit was the fun part (Ignition is different) plus a couple other smaller issues. If you cut the late model ventilation plate to match the 1957 profile and paint it nobody is the wiser!

[Author]

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