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Thread: MERC 470 questions

  1. #1
    I own a Silverline Nantucket with at Mercruiser 470. When I bought it I was told that with a mounting adapter and engine mounts I could install a Chevy smallblock with up to 300HP. Is there any truth to this? I don't want to go through the install if the sterndrive is going to shell out. Any help is appreciated.

  2. #2
    The 470 was built from 1976 through 1984. The drive is an early design Alpha One. This drive was sold as a pakage from Mercruiser with a 260 HP 350 Chev. from 1978 through 1986. It will handle 300 HP IF it is in perfect condition. If it is a 1976 with original gears and shafts and 28 years of use You can expect a failure in the near future.
    The Chev. bellhousing will bolt up to the transom plate with no adapters. You will need the correct flywheel, engine coupler, and front mounts.
    Hope this helps.

  3. #3
    You might want to consider drive ratio in your plans. Most V8 used 1.5 ratio. Just my 2cents

  4. #4
    good point big

  5. #5
    spectras only
    My friend just bought a 1981 19' sunrunner with 470 and will upgrade it to a 350 chev .Talked to one guy with same model boat at a marine shop, who did a swap years ago .He's still on the original Alpha drive and loves the hole shot. Just take it easy on acceleration

  6. #6
    Actually you're talking about a Mod.1 drive , or "R" drive, or Pre-Alpha drive......
    Not much different from the Alpha1s, But Some.....
    The lower unit vert. shaft bearing is loaded by a spring + pin(Mod.1), rather than shimmed under the waterpump(Alpha)......
    The upper units have Much More in Common... The Big difference here is the added Metal in the Casting.... Less Flex.....
    Your 470 Should have 1.84/1 gears in it now...Possibly 1.65/1s....
    If you built a Healthy 4.3l... You could run the ratio you have now.......
    If you go to the V8, you'll need to Regear, in order to get into a usable Prop Pitch Range.....
    IMO, a Freshly built Mod.1 will take 275hp, Reliably, All Day Long......

  7. #7
    Bondo is right this is an "R" with a preload pin and was built until aprox 1985. The tapered bearing under the water pump is "upside down" compared to the Alpha One. The Alpha uses a retaining nut and shims to set the rolling torque. The replacement drive shafts have eliminated the o-ring grove at the top of the shaft which was a weak link.

  8. #8
    spectras only
    I've had a SB V-8 [approx 200hp] with the "R" drive ,and it lasted 800+ hours before the shaft broke. The reason why the shaft broke ? The same friend who just bought the sunrunner grounded my boat on a sandbar two weeks prior to the mishap. The "O" ring grove was too weak to handle the sudden stop .He knows now what will happen if he's not careful with his own boat BTW the weak link wasn't just the "O" ring groove itself , the hole drilled in the shaft for the pin was too deep and lessened the wall thickness at the groove .:frown: I think it was a make work project for mercury ,because almost all these units gave up prematurely if you hammered on the throttle all the time.

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