Mercruiser engine swap (2.5 for 3.0) for 1968 Sea Ray SRV170


New Member
Jun 5, 2023
Boat Info
1968 Sea Ray SRV 170
Mercruiser 120 HP
Hello, would anyone happen to know if I can replace a Mercruiser 120 HP (chevy 153 c.u.) engine with a 1993 Mercruiser 3.0 (181 c.u.) in my 1968 Sea Ray SRV170? I'm not sure if the engine mounts are the same or if the engine would line up properly with the shaft to the outdrive. Thank you for any suggestions!
Anything is possible. But doing a quick Google search turned up a lot of the GM 153 CI motors. Looks like those are very plentiful coming out of IH Scouts. Might try finding a good rebuilder block and replacing with the original. Summit had a few similar engines for under $2500 in the long block.
Thanks for the suggestion but I already have a 1993 Mercruiser 3.0 that I would like to use. It has almost no hours on it. I'm wondering if it can be used without many modifications. It believe I have to cut away a little fiberglass to make room for the U-Riser. But I'm concerned if the outdrive would line up with the motor.
As long as the mounts at the transom can be adjusted to line up. The front should be able to be adjusted and modified to allow the adjustment, through bracketing the front motor mounts at the block. Seen it done, going from old Italian diesels to LS gassers.

Make sure your drive gearing will handle the power. That might be the harder part of the equation
Okay, so, the 1993 MerCruiser 3.0 IS the successor to the 120/140 R-drive 153ci that your '68 SRV came with. The difference between the 120hp and 140hp versions, was simply a matter of wether the 153ci GM engine, or it's 181CI bigger brother were used.

These engines are in NO WAY related to the GM "Iron Duke" 151, nor are they related to the International Harvester 152 ('slant').

They are BOTH in the same development family as the GEN2 inline sixes... the 153 is a 230ci six less two cylinders. The 230ci inline six's bigger brother is the 250, and they all have the 3.875" bore, but the 181 (and the inline six spinning the cleaver prop in my avatar pic) uses a 4" bore. These engines were ORIGINALLY conceived as automotive and light truck replacement for the "Stovebolt" family, but were such a good product, that they were given very slight improvements, and then spun over onto the 'Industrial' line, and from there, sold to marinizers in very significant quantities, which is why you'll see the same casting numbers in a MCM 120R drive system, as in a Hyster H50H forklift. The industrial flavors usually had high nickel blocks, large diameter flywheels, timing gear oiler nozzles, bronze distributor gears, very little valve overlap... and if they were intended for multi-fuel, sometimes fancier valves and valve seats.

The 153, 181, 230, 250, 292... all share the SBC flywheel housing bolt pattern. Inlines do NOT use the same Mercruiser bellhousing/flywheel cover as V-engines, on account that the exhaust riser and down pipe wrap around the housing differently AND... they use a longer coupler shaft on the drive. There were SOME runs of 153, 230, and 250ci marine engines that came with forged cranks. My (formerly) 250 is a forged crank... but the rest of the industrials were usually a nodular (cast steel) unit.

IF you were to put your 181 in, you'd use the existing flywheel cover and front mount (bolt on, no change) and the same exhaust manifold and riser setup. You'd use the same flywheel coupler, but the balancing INSIDE (the crank) is different, and I believe they MAY have something specifically different about the marine 181's flywheel in comparison to the 153, I don't know off top of my head.

Additional height of the riser is not a problem, you can trim the aft bulkhead, probably won't need more than 3/4", but you can get away with less clearance by leaving the exhaust manifold off 'till you get the engine IN, and then install the manifold and riser as a SET, place the elbow into the bellows boot before swinging it into position.

RE hull/powerplant performance... Your original 153 was running a gear ratio of about 1.98 or 2.04:1, with a 17p prop, probably making 33mph at 4800rpm. With a 181, and same gears, you'll be able to get a 19p to do same at about 4300, and possibly make 38mph at 4800rpm. The '69 drive will have no issues handling the power (Aside from gearing changes between fours, inline sixes, and V8's, it's same drive) just keep it below 5000rpm, as they'll melt the upper gearset quick if you don't. For what it's worth, sour SRV-170 was available with that same drive behind a Ford 302. This setup was pretty darned heavy, it would manage 50mph with 1.5:1 gears and a 23p prop, but the Ford engine's marine rating only to 4400rpm during this period was a significant limitation, so the inline 6 has potential of being significantly faster, and climbs out of the hole quicker, too. Going to a 181, particularly a fresh one, will certainly pop you up quicker.

Aside note- If you study the '69 Pachanga hull, you'll find that there's no doubt in the lineage, very small differences in the SRV170, with just a little length. It's a great hull. It looks a little clumsy at low speed but once up no plane, the higher you push it in planing, the faster it'll go, and (until wind drag starts taking over) the less fuel it'll use.
The forklift guys agree.... and the 181 is just as tough, but with more displacement... IMO, they could have made the 181 a cross-flow 4-valve (Honda CX500-style, in iron) and had one ghastly strong mill.

The forklift antithesis to the GM 153/181, was Ford's 134/172/192, which was a gutless pig... My H50H was unfortunately fitted with that motor, not the 181...

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