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Mercrusier 502 advice

humvkev

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Looking for advice. I just received the results of an engine inspection; the boat has a 502 with just under 800 hours. The leak down test results are as follows from cylinder 1-8: 14%, 15%, 20% 74%, 57%, 38%, 20%, 25%. I understand that 10-15 is good and normal and 74% is less than ideal (bad). I guess my question to you all is this something that could get a top end refresh for a few more years of use or is this really the end of life and in need or a rebuild. The engine starts and idles well, seems to get up from say 2,000-4,000 rpms, and the boat seemed to get to 4,600 rpms on the water without hesitation.

Thanks!
 

LargeOrangeFont

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If the leak is from the top end, a top end refresh will help, but you are still on borrowed time. If the leak is coming from the bottom end, rebuild the whole thing. You have 4 cylinders that are down badly and at the point you are that deep into the top end refresh, it is not a heck of a lot more to go all the way, or simply repower.
 

OldSchoolBoats

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I wouldn't count on a "few" years of use left in it. At 800 hours, I feel like you are on borrowed time and I would be getting a full refresh as soon as possible.

Would suck to be in a position every time you are out, where you don't know if your trip will end in disaster or not.
 

ltbaney1

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my .02, a good 3 angle valve job may help, may not. it maybe time for rings. if your looking to buy this boat i would factor in a full rebuild. if its already yours i would probably have the heads done. what does the shop that did the inspection have to say about options?
 

humvkev

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my .02, a good 3 angle valve job may help, may not. it maybe time for rings. if your looking to buy this boat i would factor in a full rebuild. if its already yours i would probably have the heads done. what does the shop that did the inspection have to say about options?
The shop says what you are all saying, a top end refresh will help but not fix it long term. I would be buying this boat from them so I am trying to price the motor rebuild with my offer. I don't typically put lots of hours on boat, do a lot of driving to a spot and playing in water with kids on shore or floating. is getting 100-200 more hours. I am really trying to not get attached to this boat, I like it but is it worth it.
 

humvkev

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I wouldn't count on a "few" years of use left in it. At 800 hours, I feel like you are on borrowed time and I would be getting a full refresh as soon as possible.

Would suck to be in a position every time you are out, where you don't know if your trip will end in disaster or not.
What you just said is what worries me. I want to use it this summer/two summers without putting in a new build. The boat other than this is in good condition.
 

SBMech

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Leak down tests should show and isolate where the fault lies, that's the point of the test. It's not guess work, if they cannot tell you if it's rings or valve/seats, or both, you should pick another shop IMO.

I would be looking to do a full rebuild at 800 hours IMO.
 

Andy B.

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Don't wait mine had 800 hrs detonation two cylinders and it was running great up until..... Take the time do it right rebuild and you'll get another 800 hrs .
 

rivermobster

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Leak down tests should show and isolate where the fault lies, that's the point of the test. It's not guess work, if they cannot tell you if it's rings or valve/seats, or both, you should pick another shop IMO.

I would be looking to do a full rebuild at 800 hours IMO.

Beat me to it...
 

renodaytona

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Full rebuild time, if you don't it's going to me much more costly when that motor eats itself. Ask me how I know.
 

BUSTI

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IMO - Full rebuild. I would not go out and run it long knowing its not in top shape. Worse thing would be to ruin a vacation by having it go down on the lake. I've done that and its a bummer. There are a ton of cost effective parts that are better than stock. SRP makes replacement pistons and rings that are easy to get. Edelbrock marine aluminum heads come ready for valve springs and will make the engine some decent power without breaking the bank.
 

nowski

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Simply buy a used boat with less hours. It might cost more money up front but it will always have less hours, always be worth more money while giving you and the family hours of fun.

If the said boat is approaching 800 hours the same applies for the rest of the boat, how many hours are on that drive???

Said boat made it to 800 hours look for a used boat with 400 hours. You'll still get a good deal and if maintained another 400 hours of serviceable life...
 

Fenderbender

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How much stuff would you have to swap over from the marine 502 to the one from summit?


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checkrd past

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I'm no engine guy, but didn't those motors have a high oil consumption with just regular use? I used to hear plenty of stories of a quart of oil a day.
 

humvkev

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How much stuff would you have to swap over from the marine 502 to the one from summit?


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This is my question too. I just looked online, they are priced well, but what needs to happen to put it in the boat?
 

lbhsbz

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Lots of guessing in here. The answer is...you have to test it. Based on the leakdown test results...we know one of 4 things:

1: The leakdown test was performed improperly
2: The cylinder heads are compromised
3: The ring seal is compromised
4: Both 2 and 3

Assuming the test was performed correctly, that leaves us with 3. Regardless of which it is, the heads need to come off at this point. While the heads are off, perform another leakdown test using an adapter that bolts to the deck using the head bolt holes...AST used to make a nice unit but it is long since discontinued. Not very hard to fabricated with a plate of aluminum and a sheet of rubber. This will test ONLY your ring seal....if the results are good, do the heads and put it back together. If the results are not good, then you need to tear it all the way down.
 

farmo83

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Motors run great until they don't run at all/blow up.

I would call a reputable shop in your area and get a quote on a full rebuild, plenum to pan assuming everything needs to be rebuilt or replaced(which it could), give them all the info you have. Back that off of whatever you think the boat is worth and offer that. You'll meet in the middle somewhere and at that point decided what you want to do.

I know a lot of people that have limped things along and been fine
I also know a lot of people that have tried to limp things along and had their stuff go south midway through the season and been done for the year.

It's just gambling.
 

mash on it

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This is my question too. I just looked online, they are priced well, but what needs to happen to put it in the boat?

Everything would need to be swapped.
Intake manifold.
Distributor and ignition system.
Exhaust manifolds.
Oil cooler and power steering cooler.
Front accessory drive and components, water pump, alternator, power steering pump and sea water pump.
Motor mounts.
Flywheel, drive hub, and starter.
If it's just a long block, possibly oil pan, timing cover and valve covers.
I probably missed something in there, like spark plugs, oil filter...

A weekend job.

Dan'l
 

buck35

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Aren't all the engine tolerances significantly different on marine motor?
 

Fenderbender

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Last summer I was quoted fro a very reputable shop in Havasu 8k for a straight rebuild and up to about 14k for a solid 600hp build. Thats probably a good place to start. I am getting ready for that in the future as I am at about 700hrs now and will probably put another 100 on it this summer.
 

Bobby V

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I'm no engine guy, but didn't those motors have a high oil consumption with just regular use? I used to hear plenty of stories of a quart of oil a day.
I have a 98' Eliminator 502 with about 600 hrs and never had any issues with using excess oil. ;)
 

LargeOrangeFont

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Aren't all the engine tolerances significantly different on marine motor?

No not necessarily, in the high end stuff, sure. You have to run the engine with the correct oil at the correct oil temp. that is the big hurdle. All the LS based marine engines are right off the car assembly line, and call for the same oil you'd run in a car.

You can't go throw a car 502 that is setup to have 10W30 run through it at 200+F in a boat with 20W50 oil temps at 120F... It will eat itself in short order.

That that instance, you would want to run different tolerances.
 

lbhsbz

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No not necessarily, in the high end stuff, sure. You have to run the engine with the correct oil at the correct oil temp. that is the big hurdle. All the LS based marine engines are right off the car assembly line, and call for the same oil you'd run in a car.

You can't go throw a car 502 that is setup to have 10W30 run through it at 200+F in a boat with 20W50 oil temps at 120F... It will eat itself in short order.

That that instance, you would want to run different tolerances.
same tolerances, different specs.
 

guest hs

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I'm no engine guy, but didn't those motors have a high oil consumption with just regular use? I used to hear plenty of stories of a quart of oil a day.
They are Siamese blocks no water runs between the cylinder bores this is why they use oil
 

Ricks raft

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I'm no engine guy, but didn't those motors have a high oil consumption with just regular use? I used to hear plenty of stories of a quart of oil a day.

My current rebuilt 502 uses about 1 qt a season appx 40hrs.
I've heard those rumors about big block Chevys, this is my 3rd. My 1st 454 used about a qt in 15- 20 hrs switched to full synthetic and went to 30hrs. 2nd was a hp500, similar consumption. I do run conservitavly, mostly 2500-3500rpm with short wot bursts.
 

pixrthis

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I did the Summit marine long block swap a couple years ago primarily because of the time our local shops needed for a rebuild, I ordered it and it was at the shop three days later. They were busy so I did the bulk of the work and they helped when they had time, less than six hours later I was towing home to pack for a week at the Delta. Ultimately it took much less time, not sure if cost was more or less, and it’s run great since.
This isn’t a alternative to a high performance engine build but it was all the family ski boat needed.
 

gqchris

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I did the Summit marine long block swap a couple years ago primarily because of the time our local shops needed for a rebuild, I ordered it and it was at the shop three days later. They were busy so I did the bulk of the work and they helped when they had time, less than six hours later I was towing home to pack for a week at the Delta. Ultimately it took much less time, not sure if cost was more or less, and it’s run great since.
This isn’t a alternative to a high performance engine build but it was all the family ski boat needed.

Love it. Got the job done and still running strong!
 

monkeyswrench

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If this boat is at a shop, something you should think about is their time frame and yours. We are rapidly approaching the busy season. It would suck for you to pay for the boat, and have them "start the rebuild", only for it to take them all season. I don't know the shop, and I'm not saying it would happen...but I'm pretty untrusting when it comes to spending "family" money.
I also don't want you to have the hatch up with the family. Been there, done that.
 

FreeBird236

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If you're afraid of someone buying this out from underneath you that is understandable, but maybe we could be more helpful with the full details and price of said boat.;)
 

Icky

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If you're afraid of someone buying this out from underneath you that is understandable, but maybe we could be more helpful with the full details and price of said boat.;)
I was under the impression it was his boat and he had his engine inspected
 

humvkev

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The boat is a Hallett 240, price was negotiated down from asking to 35k, I water tested it last Sunday, felt good and started up quickly, then had a shop in Havasu look it over this week. The results from the inspection above are from a marine shop in Havasu (this shop is highly trusted within this forum) and they believe that as long as I was not WOT I should be ok for a summer or two. It is also a private party sale, not a sale from a shop, it was incorrectly stated in post #5. I am also looking to learn about this engine and what to expect which is also what was gained from this post. because of this post I have also reached out to others on this site about engine rebuilds (which is in the near future) and what it costs. This has been an extremely helpful post and I am appreciate all responses!
 

counterpart7

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I did the Summit marine long block swap a couple years ago primarily because of the time our local shops needed for a rebuild, I ordered it and it was at the shop three days later. They were busy so I did the bulk of the work and they helped when they had time, less than six hours later I was towing home to pack for a week at the Delta. Ultimately it took much less time, not sure if cost was more or less, and it’s run great since.
This isn’t a alternative to a high performance engine build but it was all the family ski boat needed.
Was this one of the "ATK" marine engines from them?
 

rrrr

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The boat is a Hallett 240, price was negotiated down from asking to 35k, I water tested it last Sunday, felt good and started up quickly, then had a shop in Havasu look it over this week. The results from the inspection above are from a marine shop in Havasu (this shop is highly trusted within this forum) and they believe that as long as I was not WOT I should be ok for a summer or two. It is also a private party sale, not a sale from a shop, it was incorrectly stated in post #5. I am also looking to learn about this engine and what to expect which is also what was gained from this post. because of this post I have also reached out to others on this site about engine rebuilds (which is in the near future) and what it costs. This has been an extremely helpful post and I am appreciate all responses!



You didn't say which cylinders had the low compression. If it's the center pair of 3-5 or 4-6 it could be the beginning of a head gasket failure.

Like others have mentioned, you need the shop to specifically tell you the leak down test results.

The engine isn't going to roll over and die. I agree with the assessment by the shop that said run it.
 

2FORCEFULL

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Looking for advice. I just received the results of an engine inspection; the boat has a 502 with just under 800 hours. The leak down test results are as follows from cylinder 1-8: 14%, 15%, 20% 74%, 57%, 38%, 20%, 25%. I understand that 10-15 is good and normal and 74% is less than ideal (bad). I guess my question to you all is this something that could get a top end refresh for a few more years of use or is this really the end of life and in need or a rebuild. The engine starts and idles well, seems to get up from say 2,000-4,000 rpms, and the boat seemed to get to 4,600 rpms on the water without hesitation.

Thanks!
your leak down seems off for the norm...most show neighbor cylinders low compression...for me,... I would pull the motor and go through it.. 502 is a si mease block, first round they like to get out of round... nothing better than seasoned 502 block to build from,... with the motor out there would be no way I would not pull the pan and look at the crank and bearings... also,... where did the leak down show the compression was going???? most comon is between 5 & 7,.. between 4&5 seems strange, but if it's right that would probley be valves... most start burning through 5&7 at the head gasket..... those motors run lean in the rear 5&7 corner... if it's the older 502 mpi, they had a weird fuel system also...
 

2FORCEFULL

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What you just said is what worries me. I want to use it this summer/two summers without putting in a new build. The boat other than this is in good condition.
also . something to think about... merc has a great reman program with a great warranty , and this is what I would do if it was a boat I would keep,..
shop rebuild seem to alway be the start of problems for me,.. always some reason why they leak oil and blow up,... you ran it too hard and such....
with a merc motor you can run it on pin all the way to china,... if it blows, they give you another one...
 

2FORCEFULL

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One tough marine engine. A remanufactured MerCruiser®.
Much tougher. And much better. They have to be after what we put them through. It starts here at the Mercury factory. It's the only place we remanufacture engines. Back where they were first designed and built. First thing we do is full dis-assembly. The powerhead is completely taken apart and goes through a four-step, high-pressure "hot" cleaning process. We inspect everything. We machine, bore, and hone critical components to original tolerances. We replace cracked or corroded components (no weld repairs like you'll find with the aftermarket guys). We replace the oil pump, the valve springs, and all fasteners. We apply new paint. And finish up with a process nobody in the aftermarket can or will do. Hot testing. We fire up the engine and run it for at least 15 minutes under pressure. See if the oil and coolant are doing their job. See if the engine performs to the same high standards as a new MerCruiser engine. And to make it official, we back it the same way. With the same MerCruiser one-year limited factory warranty.
 

2FORCEFULL

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sounds like it's this boat...
1995 Hallett 240 Open Bow - boats - by owner - marine sale (craigslist.org)

I think I looked at this boat when it was for sale about 10 yrs ago, the guy was a friend of a member here


We purchased this boat a little over three years ago and just don’t use it . It has been garage stored very nice overall condition everything works runs great. It has a stock 502 with a Bravo3 has captains choice exhaust motor has just under 800 hours meter shows more the former owner left the key on I had it checked at the dealer can show you the paper work. The trailer has four new Goodyear tires and break away tongue . The interior has a bed insert for the bow and a Bimini top . I don’t need help selling it if you have questions please call or text no e mails if its up its for sale.
 

Runs2rch

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No not necessarily, in the high end stuff, sure. You have to run the engine with the correct oil at the correct oil temp. that is the big hurdle. All the LS based marine engines are right off the car assembly line, and call for the same oil you'd run in a car.

You can't go throw a car 502 that is setup to have 10W30 run through it at 200+F in a boat with 20W50 oil temps at 120F... It will eat itself in short order.

That that instance, you would want to run different tolerances.
You mean 220 degree oil temp? Have seen quite a few ZZ502's live long healthy lives even with the stock cam. I think a cam change and they would be solid performers in a boat for the money. Some ZZ572's also. Agree with you though.
 
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