Zero Compression on 2 Cylinders...

JR6424

Seaman Apprentice
Joined
Oct 20, 2018
Messages
41
5.0GL-C
Model #3869131
Serial #4012085620 (? Label is really faded)
Aftermarket freshwater cooling
Aftermarket coolant fed heater core/heater system
150 hours since alleged rebuild/reman swap (previous owner reports and provided receipts)

After finally getting to the bottom of an overheating problem (long story short, my boat has an aftermarket heater and guess what? Heater core blown...) I took the boat out on a local lake for a shakedown run. She started up fine and purred like a kitten, ran around a few times doing some WOT runs and everything seemed great until I heard what sounded like detonation, pulled back to idle, ran rough and then died and wouldn’t restart. I limped back on my kicker and dragged her home feeling very defeated. After reading through some old threads on here I decided to do a compression and check and found good results (150-170) on all cylinders except #2 and #5 which both read ZERO. 2 and 5 are on opposite sides and there is no indication of water in the oil so either I blew 2 pistons or I have some stuck valves. This boat had sat idle since the overheating problem started last fall, it was the end of the fishing season for me anyway so I drained the manifolds, winterized the kicker and called it good. Then 2020 happened and I never got a chance to try to resurrect her until mid-August. The overheating problem really had me thrown for a loop until I realized my heater system wasn’t electric but coolant fed... who knew? Anyway, I throw myself at the mercy of the court, what should I do next? I ordered a cylinder leak down tester kit that will be here tomorrow. The exhaust manifolds and risers are off and are going to be replaced with new assuming this engine isn’t DOA. I have all the plugs out and am ready to troubleshoot. If it’s just a few stuck valves can I pull the heads and have them cleaned up at a machine shop? Will any machine shop suffice or are there specific marine machine shops? As far as pulling the heads are concerned what should I know before starting? I’ve never gotten this deep into a disassembly before, it seems pretty straightforward but also daunting. I know the distributor needs to be marked before I pull it, but how/where? Should I pull the valve covers before doing any of this? Is there anything to be learned by looking in there? Sorry for the long post, thanks in advance for the help.

-JR
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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first, use a few breaks in your typing. it makes it easier to read

second, full system or half?

Third, are you in salt water? if so, and you have a half system, your exhaust may have failed and you have two rusty exhaust ports and valves.

4th, all marine heaters are just like a car's and run off hot water from the boat.

yes, any competent automotive machine shop will work. its a GM 305 SBC not a thing special about the long block other than marine head gaskets, brass core plus and a marine cam.

you will need to find the cause of your overheat as that is probably what ate your motor. did you replace the impeller when you got the boat and do all the other maintenance?
 

JR6424

Seaman Apprentice
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Oct 20, 2018
Messages
41
Sorry, was typing on my phone and didn’t notice the run on paragraph there. Thanks for the reply, I’ll answer your questions in order

It’s a half system and ran in the salt at least 50% of the time if not more. I flush it with freshwater after every run in the salt and have been using “Salt Away” for the past 2 seasons. When I pulled the exhaust manifolds and risers there was a fair amount of corrosion but nothing that seemed out of the ordinary for a boat ran in the salt. I will say though that the riser gasket area looked very suspect, and the spark plugs definitely show some fouling that would make one think water intrusion. I’ll try to post pics later.

I’ll take your word for it on marine heater systems but I’ll say that no one out here in the Puget Sound fishing community had ever heard of a system like this. Most guys out here run the little kerosene/diesel heaters under their canvas or have electric systems.

The heater core was blown and was definitely the cause of the overheat, when I finally found the core (up next to the captains chair with the hoses snaked around through the gunwale) there were a good 2+ gallons of coolant just sitting down there. I really like this heater but I hate the idea that there’s a ticking time bomb “out of sight out of mind” up in an area I have nearly zero access to inspect.

Thanks for the info on the machine shop, I assumed so but wanted to confirm. I’ve read on some old posts people talking about stainless valves and special seats and guides and wasn’t sure.

As stated above I’m fairly certain I’ve got the overheat problems fixed now that the heater core has been bypassed. This is my 3rd year with the boat and I’ve been fairly diligent with maintenance. First year I did just about everything, plugs, wires, cap and rotor, all fluids and filters changed, and replaced both bellows. I’ve continued with fluids and filters at winterization time and as part of the overheating troubleshooting I replaced the impeller and thermostat although the impeller that was on there looked to be in good shape.

Is there any benefit in doing the leak down test at this point, or should I just go ahead and start to pull the heads? I have the shop manual but any tips/tricks from folks who have done it before would be greatly appreciated.
 

Lou C

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Nov 10, 2002
Messages
11,802
The leak down test will tell you if its stuck valves or blown pistons or broken rings, so if its valves the heads are coming off which usually you can do in the boat, if its pistons/rings the engine has to come out. might help with planning the repair. If it's just the cyl heads then line up a good automotive machine shop to go through them. If its pistons/rings then starting thinking about a complete rebuild, since its closed cooled yours is worth rebuiding.
Be very careful with those elbow gaskets make sure the sealing surfaces are very good after cleaning them up or if not I'd figure on replacing them to avoid future problems. The problem with that design is salt water gets under the gasket and corrodes and pits the cast iron then it won't seal. When I did mine I coated the gaskets (Barr aftermarket) & sealing surfaces with Permatex Aviation to keep water out of that joint.
 

JR6424

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Oct 20, 2018
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41
Thanks for the reply Lou. My leak down tester is supposed to arrive today, I’ll post back with results when I get the test done.

I have a new set of Barr manifolds and risers sitting in the garage ready to go on, thanks for the tip about the gasket. I found older posts on here with sort of a debate regarding whether or not to go with gasket adhesive or not and was leaning toward going with it like you did. The Barr set came with their one piece gasket and I found a VP service bulletin that specifically calls for Permatex Copper spray on gasket adhesive so I think I’ll definitely go that route. Did you use the Barr gaskets that came with the kit, or did you go OEM?
 

Lou C

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I used the Barr gaskets coated with Permatex Aviation.
 

JR6424

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Oct 20, 2018
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Took me a little longer than I expected to get the test done but I wanted to follow up and seek some more advice. Both #2 and #5 leaked from the exhaust side so I pulled the heads and found what you see in the pics below. #2 has an obvious stuck valve which resulted in a piston strike. #5 also had a stuck valve but nowhere near as dramatic, no obvious damage to the piston.

The heads are out at a machine shop now, they’re going to be cleaned and leak tested before being rebuilt. Assuming the heads are solid the machinist thinks the engine should run OK after he rebuilds them despite the little divot in that piston. What do you guys think about that?

I’m going to turn the engine over by hand and measure the distance from the deck surface of the block to the top of the piston on all cylinders and compare them to make sure the rods aren’t bent, any other “poor mans” way of testing the bottom end without pulling the motor? Any other suggestions on things to check/replace while it’s all torn down? I’m going through all of the rocker arms, pushrods, and lifters now, I’m going to replace the one’s from 2 and 5 just on principle.

What about the head bolts? I noticed the shop manual calls for torque degrees instead of ft lbs which seems to me that they are using torque to failure bolts but the manual doesn’t specifically say to replace them. The machinist says he generally just reuses bolts if they look OK and torque all to 70 ft lbs following the sequence from the manual.

Thanks again for your advice.
 

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Scott Danforth

Grumpy Vintage Moderator still playing with boats
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you have torque to yield head bolts. spend the $35 on another set. these are one-time use bolts regardless of what your machinist says. you may get lucky, however you may also fail a head bolt....

or spend money on a good set of ARP or Manley head bolts

the divot in the piston should be fine. I put 100k miles on a car with a carburetor butterfly screw stuck in the top of a piston.

since you had rusty/stuck valves. buy new manifolds and elbows or suffer this same issue again.
 

Lou C

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Nov 10, 2002
Messages
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I replaced my head bolts with ARP because the OE ones were really too rusted to clean up. Also get a thread chaser and really clean out the cyl head bolt holes in the block to make your torque as accurate as possible. Barr Marine makes decent quality aftermarket VP exhaust...
I also had a couple of stuck valves but not bad enough to cause divots....
 

Lou C

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old head removal....
 

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JR6424

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Thanks for the advice on the head bolts, I’ll spend the money and get a set of ARP bolts when I pick up the head gaskets and other stuff. Any recommendation on thread sealant for the bolts?

I’ve got a brand new set of Barr manifolds, risers, and gaskets waiting to go on as soon as the heads are back from the shop. I saw your above post about the gasket sealant for the riser to manifold connection, but what about manifold to engine do you use the sealant there as well?

Thanks -JR
 

Lou C

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Sealer for head bolts...Merc Perfect Seal, Evinrude gasket sealer or Permatex Aviation
check with ARP for torque specs if you use their bolts they may be slightly different than GM’s torque specs...
make sure to really clean up the block deck surface no rotary tools! Must be flat & level to less than .003”.
And use the thread chaser to clean out the bolt holes and thread each head bolt in by hand before adding sealer to make sure it threads in easily; if not the holes are not clean enough....
 
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Lou C

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PS I used Permatex Aviation on the manifold to elbow gaskets but no sealer is necessary for the manifold to cyl head gasket.
 

Lou C

Supreme Mariner
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PPS might need a lil bit of sealer on the intake manifold water ports sometimes these get some pitting from years of raw water cooling
 
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