Best head gaskets for jet boats? MLS or Graphite?
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Best head gaskets for jet boats? MLS or Graphite?

  1. #1
    Senior Member Rivernut's Avatar
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    Default Best head gaskets for jet boats? MLS or Graphite?

    My Vortec 350 has slipping compression and has sprung a couple drippy coolant leaks around the head gaskets of the rear cylinders. Three years ago when the engine was fresh the compression was about 200 PSI per cylinder. Now it is 180-200 with one at only 150 PSI. I've had two previous engines blow out between the middle cylinders. The current gaskets are Felpro MLS. The first two engines that failed and were damaged beyond repair had graphite gaskets. The engine is a stock GM Vortec (Marine Power) with the stock "Ramjet" cam, it turns an "A" impeller at about 4,450 RPM's, about 325 hp. It is also raw water cooled and hooked up to an an American Turbone SD-309. I have never overheated this engine and always allow it to warm up before putting down the hammer. I run about 32 degrees total timing. The spark plugs look normal. There is no water pressure regulator. Is the unregulated pressure likely causing gasket blow-outs? I use this boat all year and winter in Massachusetts. This boat also sees extended full throttle runs of 45 minutes sometimes running up and down the Connecticut River. I'm getting tired of replacing / rebuilding engines every 3-5 years. I understand that MLS gaskets need more precise surfacing. I'd rather not pull the block out for special resurfacing if it looks level. Can MLS gaskets seal better with the old copper spray? Recommendations? Thanks.
    Last edited by Rivernut; 07-26-2016 at 09:24 PM.

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  3. #2
    Member Gopher711x's Avatar
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    I would use a MLS with Copper Coat Sealant sprayed between each layer . I used this on my old blown 454 and it worked for 10 years this way
    I was using Cometics
    How far out of whack are the deck and head surfaces ???
    Last edited by Gopher711x; 07-27-2016 at 08:20 AM.

  4. #3
    Senior Member Rivernut's Avatar
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    I haven't taken the heads off yet to survey the damage. I'm leaning towards using the Felpro MLS with copper spray. I'm thinking of calling the tech guys at Felpro.

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    Senior Member wagspe208's Avatar
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    MLS= awesome
    Fel pro mls=ok
    Cometic mls= awesome.
    Even God can't hold a head gasket in without everything being right.
    You have several potential problems IMO.
    1) unregulated water pressure.... regulate to under 15... I'd do 7 to 10
    2) Install head studs... HUGE clamping advantage
    3) Cometic.
    Run, and be happy.
    Wags

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    Senior Member Rivernut's Avatar
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    I took the passenger side head off today and found the two center cylinders slightly scuffed and their pistons with slightly melted & crumbly edges. The exhaust valves were whitish, burned I guess, from either too much ignition timing or running too lean, but chambers did not look lean. The head gasket was intact between the cylinders, and no visible sanded-in areas of the block. I am guessing the block can be machined for larger pistons. I was hoping I had caught it in time to not have to pull the whole engine. Do scuffed cylinders always need oversized boring or can they be saved with honing?

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  8. #6
    Some guy obnoxious001's Avatar
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    Quote Originally Posted by Rivernut View Post
    I was hoping I had caught it in time to not have to pull the whole engine. Do scuffed cylinders always need oversized boring or can they be saved with honing?
    That's going to be a question for the machine shop, after you pull the engine. No possible way to hone it while it's together. In the old days guys used to hone an inline blocks in the car, but they could get under them. Likely if you think the scuffing is bad, it will need to be bored and new pistons installed.

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    Detroits MLS gasket is Graphite/Steel/Graphite. Will seal to any surface finish. Can use with Aluminum or Cast iron heads. Bore is 4.520". Will hold 20# boost or 15 to 1 compression. A pair cost less then 1 MLS Cometic. Limited to 4.500" bores though. BBC.
    Chris Straub
    Straub Technologies

    3HP is an A$$ Whooping!!! JW

  10. #8
    Senior Member Rivernut's Avatar
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    Upon closer inspection and cleaning, the suspect pistons just had crumbly carbon built up on one side. The light residue is not aluminum but the deposits from gasoline. I'm a little color blind...my world can be confusing at times! The scuffing on two of the cylinders is not feelable with my finger. Here is a pic of the head. Cylinder #5 (3rd from left with lightest colored exhaust valve) had the most suspect looking edge on its piston, but when I looked over the compression numbers, it had the highest @202 PSI. Cylinder #7 on the right had the lowest compression and was the reason for the tear-down @ 135 PSI. It also had a water leak near it and possibly an oil leak too. I'm confident that I will be back in business with my spare heads, MLS Fel-Pro gaskets and careful block deck prep.

    I'm trying to clean the thick carbon off the pistons as I wait for my spare heads to get milled.
    Last edited by Rivernut; 08-03-2016 at 11:16 AM.

  11. #9
    Senior Member stix818's Avatar
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    We use Flat Out Gaskets. They'll make you any thickness you need..
    http://www.performanceboats.com//signaturepics/sigpic3592_1.gif
    Special Thanks to:
    Nelson Speed & Marine
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  12. #10
    Senior Member Rivernut's Avatar
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    I have a few hours of run time with the Felpro MLS gaskets. I readjusted the valves and did a compression check. All cylinders are strong so far with 205-210 psi and no liquid leaks. The bad news is the boat has a vibration that is getting worse. The driveshaft has play in it, so...out comes the engine, again. Just got the Spicer U-joints. If it's not one thing, it's another with jet boats!

  13. #11
    steelcomp was here
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    Unless you have clean, FLAT and smooth surfaces, I would steer clear of MLS gaskets. A good performance composite gasket should work just fine. The Detroit gasket that Straub has was the standard of the racing crowd for a LONG time. A composite gasket is going to conform to the irregularities of the deck and head a lot better than an MLS gasket.
    If God is your co-pilot, change seats!
    Acts 2:38, the perfect answer to the perfect question.

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