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I-6 Engines For GMCs that came with the Inline 6 Engines

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  #1  
Old November 10th, 2013, 03:30 AM
Vern Vern is offline
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Default Blown head gasket vs. cracked block

Ok, folks when I got my '66 (inline 250) there was lots of white smoke coming out of the tailpipe that I chalked up to bad gas and a messy carb. However, after only 20 mi on it, I found the crankcase is down a couple quarts and the rad is dry. Is there a good way to determine if I just need to replace the head gasket or if the engine is toast (cracked block)? The compression is pretty good and reasonably uniform across the cylinders. Ideas? Thanks!
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Old November 10th, 2013, 03:58 AM
BarryGMC BarryGMC is offline
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Default Re: Blown head gasket vs. cracked block

Sounds like a crack in the head somewhere . Good compression would indicate that the gasket is good. I have seen some cracks in the exhaust ports that act just as you describe. The oil use is really extreme and I am surprised that the smoke is not blue. This could be caused by lots of things. Bummer bummer, bummer. When I get back from the woods I will pm you. BC .
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Old March 10th, 2014, 04:49 AM
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Default Re: Blown head gasket vs. cracked block

Howdy, Vern.

Good to see ya.

IME, the weak spot prone to freeze damage on the I-6 is external, just above the freeze plug (ironically) under the manifolds.
I've seen 4 or 5 with the same failure. At least two were 292's....but I do remember a 250/230 or two with the same pattern.

But that's not the only way to crack a block, as we all know.
It doesn't take much compression to puke out all yer coolant.
I'm thinkin if you warmed it up a little, you could pick up an internal crack (or head gasket leak) with a compression guage in the bleed down mode.
You could also pull your thermostat and housing and top it off with coolant (water) and start her up and let it warm up.....and watch for a constant stream of bubbles (or a gusher).

Maybe.

Good luck and keep us in the loop.
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Old March 12th, 2014, 05:32 AM
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Default Re: Blown head gasket vs. cracked block

Rock Driller, thanks for your help! My mechanical skills aren't the best so, bear with me here. I did run a block test and it was negative. The bumper next to the tail pipe is black, so it seems to be burning lots of oil. Given that the engine is really messy and leaking oil all over the place, I was thinking about buying a gasket set and replacing the head gasket. Would that be a potential fix or should I run other tests first? Much appreciated!
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  #5  
Old March 12th, 2014, 08:04 AM
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Default Re: Blown head gasket vs. cracked block

Quote:
Originally Posted by Vern View Post
Ok, folks when I got my '66 (inline 250) there was lots of white smoke coming out of the tailpipe that I chalked up to bad gas and a messy carb. However, after only 20 mi on it, I found the crankcase is down a couple quarts and the rad is dry. Is there a good way to determine if I just need to replace the head gasket or if the engine is toast (cracked block)? The compression is pretty good and reasonably uniform across the cylinders. Ideas? Thanks!
In either circumstance you have to pull the head off for repairs. If you are going to do a head gasket and there is no obvious deterioration of the old head gasket when you take it apart then you will automatically get a mag test and or pressure test done on the head to look for cracks. I think the white smoke does point more to the head than the block.

There is always a risk that a bad gasket could fill up a cylinder with coolant after you shut off the engine and possibly cause severe breakage the next time you turn the engine over and it can't compress the fluid. If the engine still runs and starts okay you can guess that the leak could be going into the exhaust port as suggested already.

A head gasket leak on the compression side could also put a puddle on the ground and pressurize your radiator blowing coolant out the cap, possibly blowing a weak hose and or cause the water pump shaft to start leaking.

Depending on what you know about the engine history you could just have the head gasket/head replacement done but in my experience it makes sense to at least do rings and bearings along with that if not an even more complete rebuild.

Or maybe you have access to another good dropin engine.

Last edited by quest; March 12th, 2014 at 08:32 AM. Reason: missed word
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Old March 13th, 2014, 05:47 AM
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Default Re: Blown head gasket vs. cracked block

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Originally Posted by Vern View Post
Rock Driller, thanks for your help! My mechanical skills aren't the best so, bear with me here. I did run a block test and it was negative. The bumper next to the tail pipe is black, so it seems to be burning lots of oil. Given that the engine is really messy and leaking oil all over the place, I was thinking about buying a gasket set and replacing the head gasket. Would that be a potential fix or should I run other tests first? Much appreciated!
It's always a crap-shoot at this point, Vern.
Pulling an I-6 and doing it on the stand is usually as attractive as leaning over (or jumping in) and doing the job that way.
That being said,....if you do pull it to change the head gskt.... and find out that the head gskt was the problem.....then that gives you something constructive to do with the rest of your gasket set. (and you might find other things that need attention).
But then you have to make a pile of other hard decisions.....like (but not limited to)....do I pressure wash the engine bay....? and while it's so clean and accessible ....do I paint it...? Should I order some grommets...and so on, and on and on...blah blah.
Let me know what you decide...
At least it's not yer D-Driver.
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Old March 13th, 2014, 07:44 PM
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Default Re: Blown head gasket vs. cracked block

Quote:
Originally Posted by Vern View Post
Rock Driller, thanks for your help! My mechanical skills aren't the best so, bear with me here. I did run a block test and it was negative. The bumper next to the tail pipe is black, so it seems to be burning lots of oil. Given that the engine is really messy and leaking oil all over the place, I was thinking about buying a gasket set and replacing the head gasket. Would that be a potential fix or should I run other tests first? Much appreciated!
Based on your earlier description of all the white smoke, leaks, no oil, no coolant(did you see any blue smoke in there or oil burning?) I wouldn't replace the head gasket without getting the head examined at the machine shop. see if your valves and valve guides are still serviceable and get the head magnafluxed for cracks first. It's not worth wasting a gasket set to seal up engine leaks if it needs a rebuild or replacement anyway, if it's worn out... it's worn out! It does kinda sound like it is at the of it's service life.

Sometimes you can get away with a budget rebuild such as saving your crank, cam, lifters, pistons, valve train, even the oil pump and water pump. If these parts are not excessively worn. you could possibly getaway with tank cleaning everything and put it back together with rings, bearings, gaskets and maybe valve guides. If you have not done this before and you like mechanical work just do a little research, ask for a little help here and you will do fine, it's easier than you might think. It will be fun to do!Hope it goes well.
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Old March 14th, 2014, 05:30 AM
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Default Re: Blown head gasket vs. cracked block

YeeeeHaww
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Old March 14th, 2014, 06:11 PM
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Default Re: Blown head gasket vs. cracked block

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Originally Posted by Rockdriller View Post
But then you have to make a pile of other hard decisions.....like (but not limited to)....do I pressure wash the engine bay....? and while it's so clean and accessible ....do I paint it...? Should I order some grommets...and so on, and on and on...blah blah.
OH Yeah, I have been in the "one thing leads to another" position before. Started off by one time to R&R a 350 with a friends rusty 79 Chev 4x4, honestly all we wanted to do was get rid of a worn engine with bad head gaskets and drop a good used 350 in it.....BUT about 100 decisions later and $25,000 + of his cash he had almost completely new 79 Chev 4x4 right down to the carpets and stereo. And it was definitely a "Well we did this so we may as well do that" project as the weeks and months went by!
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Old March 14th, 2014, 07:29 PM
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Default Re: Blown head gasket vs. cracked block

Quote:
Originally Posted by quest View Post
In either circumstance you have to pull the head off for repairs. If you are going to do a head gasket and there is no obvious deterioration of the old head gasket when you take it apart then you will automatically get a mag test and or pressure test done on the head to look for cracks. I think the white smoke does point more to the head than the block.

There is always a risk that a bad gasket could fill up a cylinder with coolant after you shut off the engine and possibly cause severe breakage the next time you turn the engine over and it can't compress the fluid. If the engine still runs and starts okay you can guess that the leak could be going into the exhaust port as suggested already.

A head gasket leak on the compression side could also put a puddle on the ground and pressurize your radiator blowing coolant out the cap, possibly blowing a weak hose and or cause the water pump shaft to start leaking.

Depending on what you know about the engine history you could just have the head gasket/head replacement done but in my experience it makes sense to at least do rings and bearings along with that if not an even more complete rebuild.

Or maybe you have access to another good dropin engine.

Ah, a mechanic family member says I'm crazy and need to drop in a crate 350 with fuel injection. I kinda like the put-put 250 with the manual pull choke... hopefully I can salvage it. Unfortunately, I don't know much about the engine other than it's not the original. The guy I bought it from didn't seem to know too much about it. I think the casting number puts it between '67 and '71.

Is it possible to replace the rings without pulling the engine? It seems like replacing the rings wouldn't be too bad, but could be a nightmare if it's still in the engine bay? How about those rebuilt kits that actually include new pistons and rings?
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