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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old September 30th, 2014, 10:36 PM
705jmguy 705jmguy is offline
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Default hi guys need advice again

hello

have just started my newly rebuilt engine, seems to be over heating

new rad and all seems well otherwise

have plugged the heater hose outlets as I do need heater

does this engine need to have the top and bottom connected by heater hose?

advice is sought and thanks jm
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  #2  
Old October 1st, 2014, 01:21 PM
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sclor sclor is offline
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Default Re: hi guys need advice again

JM:
I just put in a new motor as well and I have both of the heater hose outlets plugged off. The answer to your question is no, you don't need a piece of hose between them.

My only question is are you sure that you have gotten coolant all the way filled inside the motor. I had a problem with overheating until I got all the air our of the block. What I did was to remove the water temp sending unit next to the thermostat housing. That way as I filled the radiator I could see coolant coming out of the sending unit hole to ensure that no air bubble was left inside of the block.

Hope that helps.
Regards,
Steve
New Orleans
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Old October 3rd, 2014, 04:09 PM
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FetchMeAPepsi FetchMeAPepsi is offline
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Default Re: hi guys need advice again

Is the thermostat upside down?
I've heard that can cause overheating.


Oh and did you change the ...cant remember what its called but it senses how hot the engine is and a wire runs from it to the gauge inside. If you changed that out it will show overheating all the time because the newer ones have different resistance levels than the originals. You have to install a resistor to make them show up right.
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Old October 7th, 2014, 03:32 AM
BarryGMC BarryGMC is offline
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Default Re: hi guys need advice again

Hi. I have found 2 reasons for rebuilt engines to overheat. Air in the cooling system. This is rare in a jimmy v 6. And tight clearances in the engine. Piston and rings in particular. I always spend time on the set up and fit each hole to each piston. How hot is it running? Most piston have a one thousands variance and new bored holes can be 1.5 between bores. It all depends on the machining. Lots of old boring bars ang crank grinders can't hold 1.5. Between . So sorry lots of q's to why it's hot. Barry
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Old October 7th, 2014, 05:52 AM
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Default Re: hi guys need advice again

If I'm reading Barry correctly, he is saying a piston can be oversized out of the box by .001 and standard tolerance for a bore allows it to be undersized by .0015, A big piston in a small hole would have a .0025 build up of tolerance. The recommended skirt to bore clearance is .0027-.0033. (305 V6) So that seems like what he is saying can happen to "eat" up all the tolerance if the actual as machined and as purchased mating pair is not measured and matched to create the required skirt to bore clearance. Also wrong ring gap can play a roll in friction heat, I think. IMO, You always buy your pistons first and take them to the machine shop before boring. It is very cool that GMC used aluminum pistons way back, very leading edge for the period.

Last edited by AZKen; October 7th, 2014 at 06:12 AM.
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