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  #1  
Old 09-02-19
disco stu disco stu is online now
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Default Thoughts on an overheated engine?

So I've been on the lookout for a 6g75 to throw in my NL. Been having trouble finding anything with lowish km, though I have found one that is running with about 200 000km on it for $400. I also found one that has a blown head gasket and the guy is asking $100 for it, but its a fair drive away from me (they all are!). When I asked him about how the head gasket blew etc, he mentioned that the top radiator neck cracked enough to restrict flow, and is unsure how hot. All I can guess is that the head gasket blew because it overheated.

My thinking is that I would strip this down and rebuild it, new bearings, rings if needed etc. That way I would know exactly what I'm dealing with, and have a much more reliable engine. But, I wasn't sure if being something that overheated and blew the head gasket, do you guys think this would lead to much warping in the head, possible cracking, block warping, or any other major issue?

I don't mind putting a bit of work into getting an engine that is reliable, I just don't really want to have driven a long way and end up with an engine that I can't use, or have to get a heap of machining done that I could have avoided paying a bit more for the running engine.

Keen to hear any thoughts from you wiser folk, as I don't recall ever having to pull down a car that overheated so much the head gasket blew

Thanks, Stu
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  #2  
Old 09-02-19
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Pajshomoneroguntero Pajshomoneroguntero is online now
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For a $100 Motor if given the skills and tools to do the job myself Iíd be inclined to have a go at changing the head gasket and see how it goes. Iíd be thinking that there is a very good chance that the head has warped or even cracked.

My thoughts on the other hand, could it be possible that the gasket failed because it got hot and dried out? Then there is the question of how long was the engine asked to develop power with no cooling? If it was all discovered relatively quick then there may not be any lasting issues. Itíll come down to how much value you put on the $100, for some itís pocket change and for others can still be a considerable sum to risk.
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  #3  
Old 09-02-19
erad erad is offline
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If the head gasket(s) have blown, it is likely that they have been warped. There is a limit of 0.2 mm distortion and max 0.2 mm grinding allowance. Personally, if I found , say a warp of 0.35 mm, I would get that head shaved until it was clean and flat and still put it in.



Question: What can be wrong with that?

Answer - the compression ratio will be higher with a shaved head. I cannot see any valves hitting pistons etc because they are closed when the pistons are near them.



Anyway, I would be surprised if the distortion was that much - typically they distort very slightly before they blow and then because of lower cranking pressures, the net heat load would be less as well.


If you are ordering piston rings etc from Rockauto, or anywhere for that matter, before you strip the engine down, you are probably OK to order standard sizes. Before you go down that path, check the bores with a fibrescope to verify that there are no vertical gouges (caused by broken piston rings). Obviously, you won't be able to check bore wear until you strip the engine. If you have the time, you can strip the engine and then order exactly what you want, rather than guess how worn the engine actually is.
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Old 09-02-19
disco stu disco stu is online now
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Thanks guys. Appreciate the responses

The $100 doesn't bother me greatly (although I am trying to not throw too much money at this car), to be honest its more the effort and time of driving 3+hrs each way to pick it up. Not been too well lately, and energy is suffering greatly. I don't mind if it all turns out fine, but I'll be mad at myself if I pull it down and it turns out to be junk.

Compression is on my mind a bit. The 6g75 is already a fair bit higher than the pajero motor, and I'm somewhat nervous about upping that too much in a motor without knock sensor. In saying that though I'll be running this mostly on lpg, but I don't want it pinging its head off on petrol as being a 4x4 there will be lots of times I'll run out of gas and resort to jerry can petrol.

I've always been puzzled by the 0.2mm machine limit on the heads. Do you know why that is the limit?

I have the time to pull the engine down before ordering the parts. I have to check sizes on welch plugs before I order, and a few other things, so waiting a little extra is no stress.

Do you guys think a wiser idea might be paying a bit more for the running engine and just use that as is?

Part of me wants to just get this thing running on the cheap and fast, and another part of me wants to get everything spot on and running perfect, so this car can end up being more of the family car...bit torn either way. I'm somewhat tempted to grab both engines
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Old 09-02-19
fourocker fourocker is offline
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The .20mm is especially applicable to overhead cam engines where essentially the cam or cams would be bent when everything is fully torqued, if the head warped say .15mm and then the gasket surface was machined flat again the cam journals etc still have that .15mm error over their length, as you can imagine NO runout is really acceptable for longevity let alone anything over .20mm.
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Old 09-02-19
disco stu disco stu is online now
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That is a good point, thanks
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  #7  
Old 09-02-19
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There are ways to deal with that, just as shaving the head alters the relative position of crankshaft to camshaft, which affects cam timing and the slack to be taken out of the timing chain / belt by the tensioner. The more you shave, the bigger the impacts.

It can all be managed - if you know that it needs to be managed. It's only those who don't know about these issues that have problems with them.
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Old 09-02-19
stumagoo stumagoo is offline
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personally if I was in your position I would go the $400 engine - as long as it has not been overheated... the money you spend in the engine purchase should be saved by not having to deal with warped heads and you have in theory a better starting platform to build your replacement engine from.
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  #9  
Old 10-02-19
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I would go the $400 running one - it’s only $300 more and you will likely spend more than $300 on machining, etc trying to get the cheaper motor up to spec.
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  #10  
Old 10-02-19
erad erad is offline
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Too many yeara ago, I had an engine rebored to the next standard piston size up and rebuilt. The machinist zealously honed the bores such that the pistons were a tight fit - too tight as it turned out. The engine seized when it got really warmed up, miles from home. I let it cool down and drove it back to the workshop. They did something to it (don't know what) and it was fine ever after. The engine did not overheat - it just stopped.


Now, if an engine has been overheated, the pistons are likely to have wiped on the bores. Being alloy pistons and cast iron block, the pistons should be sacrificed first, but there is likely to be metal all through the engine. This is not a total disaster, but potentially a lot of work in cleaning out the engine. Fine if you are doing it yourself, but if you are paying a mechanic to do the work, totally out oif the question.



Even buying the $400 engine can have problems - it too may have been overheated, it may have a blown head gasket - the list goes on. If the engine is still in the car, you can hear it running, then you are in a position to at least assess it better than if it is on the ground. Given the fact that the $100 overheated engine almost certainly will need machining and new parts as wella s gaskets, maybe the $400 one is the better option? If you need parts, when I had my NL enginerebuilt, I bought the parts from RooDogs in Campbellfield Vic. Not sure if they are still in business but they were quite cheap and as far as I could see were genuine parts.
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