Mercruiser 470 end of life. Need advice.

[Oshawapilot]

Hey all. So, about 6 years ago here I came seeking advice on a 30' twin cruiser my sister and BIL were looking to buy. Twin Mercruiser 470's. Received the expected "run away!" messages, but they bought it anyways as it was all healthy and passed all tests at the time with flying colours.. 6 years later and around 600 hours later it was still going great with the usual mishmash of issues (B.O.A.T, after all) of any 35 year old boat, but no show stoppers.

Anyhow, that was the story up up until nearly the end of this season. Port engine started backfiring and then struggled to get any significant power. Checked all the usual issues (went over the ignition system, cleaned and adjusted points, etc etc etc) and it still ran like a sack of crap. Compression test came back 150/150/120/115. Leakdown tests came in around 30/50%. One white plug, 3 black. Marina is suggesting "full rebuild". I'm leaning towards the timing perhaps having jumped a tooth - the engine always had a weird rattle that the starboard engine never had and who knows, could have been a chain/tensioner issue. Grasping at straws right now.

Needless to say sinking much money into a 470 isn't probably the best decision versus finding a healthy donor and doing a pull and replace instead.

Found an awesome prospect but it's a 488. From what I gather here...it's effectively the same engine for all intents and purposes?

Anyhow, thoughts on the issues based on symptoms at hand, and thoughts on the 488 donor?

 

[GA_Boater]

Jumped timing chain won't cause low compression on 2 cylinders. A blown head gasket or warped head will. A clean plug tells you coolant is leaking into the cylinder. Checking timing will tell you if it's timing.

Is the motor losing antifreeze? Has the motor ever overheated? That leads to eventual 470 death.

Can't say if a 470 and 488 are 100% interchangeable.

 

[Scott Danforth]

the 488 is a 470 with a different intake and carb.

if the motor spacing is 35" or greater you can put in a few V6's

if you dont want to do a head job on your current head, buy a set of edelbrock aluminum FE heads (427, 460) and drill the hole that Mercruiser drilled for the elbow. this will prevent the head gasket from blowing as much.

 

[Oshawapilot]

Never overheated, very careful there. Boat spends 95% of it's time puttering along not even on plane. And yes I realize the low(ish) compression isn't a timing issue, but I doubt the compression thing is new, the refusing to make any power thing is new.

Re the 488, is it a drop in replacement, intake and carb aside, and will a 470 carb bolt up to it? The carbs on the 470s were completely rebuilt just a few seasons ago and perform amazing so it would be a shame to start over again there.

 

[Scott Danforth]

keep in mind with the 470, if a fish swims past the intake or you run past seaweed, or anything to raise the temperature as much as 10 degrees (yes, 10 degrees), it was over heated and the motor will spit the head gasket. the problem is the open deck aluminum block and the iron head. absolute worst combination of alloys and motor construction you could ever have come up with. do a leak down test, that will tell you everything you need to know about why there is low compression in 3 of the cylinders.

 

[achris]

the 488 is a 470 with a different intake and carb.

And exhaust manifold.

 

[Oshawapilot]

So....not a drop in for a 470 then I'm gathering. Still wondering if anyone can confirm parts interchange however...IE, is the block and head basically a 470 but the carb/intake/exhaust is different? Because if the 488 I'm eyeballing is otherwise a low hour creampuff, swapping the intake and exhaust from the old engine isn't a biggie.

 

[Scott Danforth]

drop in the 488, upgrade the 470 to 488 go boating. longblock is identical. ford 460 pistons, head and rods on a custom mercruiser crank in a mercruiser block with a GM flywheel and GM damper

 

[Oshawapilot]

Thanks. Just confirming, as being a twin just swapping out just one engine for something that was a different HP or whatever would be, of course, potentially somewhat problematic. It'll probably get freshened up anyways beforehand so swapping over intake and exhaust, no biggie, but if there were any other massive differences, well, that'd be more of an issue.

I will still be on the lookout for an actual 470 instead, but the 488 I've come across so far looks far better than some 470 prospects I've also come across.