Re: [omc-boats] CONVERSION NOTES; was BAD NEWS

From: lib1@...
Date: Mon, 19 Dec 2005 12:26:42 -0700

Hi Ryan:
> What kind of speed you get with the V8?

I didn't do the conversion for speed. I did it for reliability ( a smooth
475 RPM idle means electric shift spring tabs will last forever) and ease of
maintenance (Chevy parts are everywhere),
and economy (yes the V8 typically uses less gas than the V6). Holeshots
"feel" more powerful.

But since everyone wants to know about the speed; you can view the
GPS-measured results of my V8-powered 66 16-ft Sportsman at:
(Note: that the test contion altitude above sea level is 6095 ft., also
note: 21:16 gears and 13-inch pitch prop)

Believe it or not, for every 1000 ft you effectively loose 4 per cent power
to changes in aire density) Effectively that reduces a V8 to a V6, or a V6
to an inline-4.
Us high-country boaters are always looking for bigger breathing engines!

The deeper vee design of your 69 -70 Sportsman/Reveler/SeasSport hulls are
roughly 2 - 3 mph faster than the older 64 -68 Sportsman/Reveler/SeaSports.
You can play with the speed calculator at My guess is your boat will gain
about a 10-12 percent increase in top speed if you swap the 3.7L Buick V6
for a 5.0L Chevy V8 (210 to 220 hp). That would put you at about 44 -46 mph
at sea level. A stock 5.7L pushing 260 hp might get you to 50 mph at sea
level. This all assumes a clean boat hull and smooth water.

> About changing the upper gear...what about changing the prop instead?

The OMC electric stringer props go from 10-inch pitch to 22-inch, but the
most commonly available sizes are 12, 14, 16, 18, and 20. You will need to
change the upper gear, my Buick V6 had the 25:25, not sure about yours. The
V6 25:25 gears will cause the V8 to over rev regardless of prop selection.
You will basically shorten the useful life of the sterndrive if you bolt up
the V8 to the original V6 25:25 upper gears. Fortunately there are gearing
ratio alternatives: The seventies OMC Chevy I-6 and the OMC Ford V8's used
21:19, 21:18, 21:17 ratio's, while the Chevy V8's used 21:16. see: For sure, count on changing the gear
ratio, and possibly the prop pitch as well. If you look for a used electric
stringer V8 outdrive, make sure you get the "Long-leg" version and not the
more popular "short-leg."

> You say the exhaust manifold fits the V8?
Just to clarify: There are numerous sources for Small Block V8 Chevy NEW
water-jacketed Marine manifolds; OMC, Barr, OSC, Volvo, MerCruiser to name
but a few. Used OMC Chevy V8 manifolds are easy to find on eBay or Marine
salvage yards. Stay away from manifolds that have been exposed to saltwater
use. The Buick and Chevy manifolds DO NOT interchange and same goes for V6
and V8 manifolds.

You can read more about the conversion project I did on my boat at:

Lee Shuster
Salt Lake City

----- Original Message -----
From: "Ryan" <kmoore12@...>
To: <omc-boats@...>
Sent: Monday, December 19, 2005 5:41 AM
Subject: Re: [omc-boats] BAD NEWS

> What kind of speed you get with the V8? I might be able to do some engine
> swapping and maybe afford to do this. This boat has allways been a little
> slow coming out of the hole. About changing the upper gear...what about
> changing the prop instead? You say the exhaust manifold fits the V8?
> Ryan
> ----- Original Message -----
> From: <lib1@...>
> To: <omc-boats@...>
> Sent: Sunday, December 11, 2005 1:36 PM
> Subject: Re: [omc-boats] BAD NEWS
>> Hi Ryan,
>> I'd say that before you go much further you probably need to pull the 225
>> and determine the health of the short block, ie the crankshaft, bearings,
>> pistons and rings. Most likely they are getting pretty tired and you
>> won't know how much damage has been caused with water breaking down the
>> oil that you say "looks like mud." This is best done with the engine out
>> of the boat so you can pull the oil pan and inspect the crankshaft and
>> bearings.
>> If it were mine, I would not go to the trouble of "swapping" or
>> re-installing the even-fire 231 into the boat; without first thouroughly
>> checking out it's running condition as well. While it's on the stand, you
>> could pull the pan and check out it's crankshaft and also also check out
>> it's compression.
>> BTW, it's getting hard to fine an engine rebuilder who will still do the
>> 225 Buick/Jeep V6, but Jasper Engines completely redo long-blocks if you
>> send them the core. Not sure what they charge these days, but I'm guess
>> the charges will fall in the $2000 to $2500 range, which is quite a bit
>> more than a small block Chevy V8, simply because they don't get much call
>> for the Buick V6 anymore. But you do get a fresh engine, including heads,
>> that has a warranty.
>> Just a thought: ->>> I don't know about your 69 Sportsman, but I suspect
>> you could get the small block Chevy V8 to fit in your engine bay, as I
>> did on my 66 Sportsman. See:
>> If you consider the SBC V8 swap you'll need the exhaust manifolds from
>> the 210, 215, 225, 235, or 245 hp OMC. You should also change out the
>> sterndrive upper gear ratio from a similar V8-powered OMC electric
>> stringer sterndrive, as the V8 would be revving too high with the stock
>> V6 gearing. SBC Chevy parts (even OMC-specific) are plentiful and cheap
>> on eBay. The 210 hp (307 cid - 5.0L) was used in the 68-70 19-ft Evinrude
>> and Johnson boats. It makes the16-ft Sportsman run much smoother and it
>> gets better mileage than the V6, because most of the time it's not
>> working as hard.
>> Just curious if you have checked the sterndrive yet? You can tell a lot
>> be draining the vital fluids and looking for water or metal.
>> Lee Shuster
>> ----- Original Message -----
>> From: "Ryan" <kmoore12@...>
>> To: <omc-boats@...>
>> Sent: Saturday, December 10, 2005 1:13 PM
>> Subject: Re: [omc-boats] BAD NEWS
>>>I check compression on all 6 cylinders 3 times each. I found that I have
>>>one bad cylinder that is squirting water out of it when I crank it over
>>>and the compression is low. First test was 100 then 90, then 70. :( The
>>>other cylinders were 125, 125, 130, 130, 145 average. Oh! The oil looks
>>>like a light colored brown mud.
>>> Should I change head gasket and possibly the head or just swap motors
>>> with that 231 that come out of a 83,84 Buick Regal?
>>> I'm still shooting for spring!
>>> Ryan
>>> ----- Original Message -----
>>> From: "Ethan Brodsky" <brodskye@...>
>>> To: <omc-boats@...>
>>> Sent: Saturday, December 10, 2005 12:08 AM
>>> Subject: Re: [omc-boats] new member
>>>> On Fri, 9 Dec 2005, Ryan wrote:
>>>>> Ok..I got a compression gage. The guy who loaned it to me said I
>>>>> should
>>>>> check not only the compression but also the tolerance between
>>>>> cylinders??? I assume that means make sure one cylinder isn't way
>>>>> more
>>>>> or less the than others. Any idea what the tolerance is + or - ? Is
>>>>> this necessary?
>>>> I recently picked up the factory service manual for the 1970 155 HP OMC
>>>> V-6. It specifies the following procedure for compression testing:
>>>> (a) Remove all spark plugs
>>>> (b) Put throttle and choke in wide open position
>>>> (c) Remove wire from coil to distributor cap and ground securely to
>>>> engine
>>>> (d) Attach remote starter switch to starter assist solenoid
>>>> (e) Insert compression gauge in spark plug port. Crank engine through
>>>> at least 4 compression strokes to obtain the highest possible
>>>> reading.
>>>> (f) Check compression of each chlinder. Repeat a second time and
>>>> record
>>>> the highest reading for each cylinder
>>>> (g) The recorded compression pressures are to be considered normal if
>>>> the
>>>> lowest cylinder pressure reading is more than 75% of the highest
>>>> (h) Example: 129,135,140,121,120,100 - 75% of 140 is 105. Thus,
>>>> cylinder 6
>>>> is less than 75% of number 3. This condition, accompanied by low
>>>> speed
>>>> missing, indicates an improperly seated valve or worn or broken
>>>> piston
>>>> ring.
>>>> (i) If one or more cylinders reads low, inject about a tablespoon of
>>>> engine
>>>> oil on top of the piston in low reading cylinder through the spark
>>>> plug
>>>> port. Repeat compression check on these cylinders
>>>> (1) If compression improves considerably, rings are worn
>>>> (2) If compression does not improve, valves are sticking or seating
>>>> poorly
>>>> (3) If two adjacent cylinders indicate low compression and injecting
>>>> oil does not increase compression, the cause may be a head
>>>> gasket
>>>> leak between the two cylinders. Engine coolant and/or oil in
>>>> cylinders could result from this defect.
>>>> Also, for Mark (the guy with the boat that won't go), the shift coil
>>>> resistance should be between 4.5 and 6.5 ohms, not the 2.xx ohms I
>>>> quoted earlier. The current draw is specified as 2.2 +- 0.2 A.
>>>> Ethan
>>>> -----
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Received on Monday, 19 December 2005

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