Head bolt size tool


New Member
Hello all. I'm bout to do a head gasket and exhaust manifold replacement on 02 TB EXT 4.2l. What size Allen wrench fits the head bolts? I plan to drain coolng system and flush, change oil and filter, replace head gasket and bolts and exhaust manifold and studs and all gaskets associated with. I just replaced all 6 plugs and coil packs, water pump, thermostat, fan blade & fan clutch a month ago or so. Anything else I should replace while I'm at it? Timing chain n cover maybe? Any help would be appreciated thanks.


Lifetime VIP Supporter
Welcome to GMT Nation...

Without the slightest intent of impugning your "Mechanic's Chops"... You are about to embark on the HARDEST Repair you have to date ever engaged in. If you tell us how far along your Repair Plans have progressed AND explain which GMTNATION On Topic Threads you have already researched ... we can save you weeks of agony and steer you clear of the many pitfalls yet to come.

Here are some images of the tool you asked about. The GM Atlas LL8 4.2L Engine sports (14) M11X2.00 Hex Head Cap Screws bearing the "H-10" Tool Size PLUS Three more Smaller Head Fasteners besides. If you look closely at the sacrificial pounding I rendered atop that 1/2" X 1/2" Chrome Adapter plugged into the end of the H-10 Male Hex Socket... You'll begin to understand the very definition of the 'futility' I experienced while trying to 'Beat On & Vibrate' those (14) TTY Bolts loose enough to "just un-scew them...". I broke HALF of the Bolts off in mid-shank after that ...and you will likely do the same, plus or minus a few Head Bolts.

The BEST thing you can do in this situation is let the REST of those Head Bolts just snap off as well and ONLY attempt to extract the Broken Bolt Stubs AFTER you completely remove the Engine Head... THIS is just a Small Example of the Information you will want to know about IF you are going forward with this idea.

You have not explained precisely WHY you think it is necessary to "Replace the Head Gasket" and knowing those reasons will determine what advice best suits your situation. Think long and hard about this and instead, consider just Pulling the Motor in order to Swap in a Decent Used Engine out of a CRASHED (Running at The Time) GMT-360 SUV you might find local to your area. To do otherwise is to Travel a LONG Road... with NO Turns...


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I personally would rather replace the entire engine rather than doing just the head. Head replacement is normally easier but not on this engine. And even if you have a horseshoe up your butt, a 4 leaf clover, a real rabbit's foot, born on 8/8/1988 and win the lottery every week, you will still break head bolts and it's ALWAYS the rear most ones that are the most inaccessible. And you also need a cherry picker to lift it out it's that heavy.

And as @MRRSM asked, what leads you to needing to pull the head? Head failures are rather rare on these engines.


New Member
Well 1st of all truck has 260k miles on it. As I mentioned in original post of the parts I have recently changed. I replaced water pump due to it locked up mid commute omw home one evening and snapped the serpentine belt and left me sitting on side of the road .. so I replaced that then the thermostat stuck open so I went ahead and replaced it well apparently when I bolted the new thermostat housing In place the O-ring fell out the bottom of the O-ring groove and it didn't show it's ugly face until 2 days later my wife and kids were headed home from store and she calls and says "uh I am on side of road the car started running funny and shut off. ". Well it we. The O-ring out and all coolant and she being unaware drove it until it over heated and shut itself off. So I got it home and re installed thermostat and put all back together and ran it up to temp and it ran ok for 20 mins or so and temp began to rise. Well coolant was on vacation and headed south exiting from pass rear frame rail above wheel . I have tried to find the leak and can not locate it. So I summed it up to the Head gasket. Could find any freeze plugs leaking or anywhere else and there's coolant in the crankcase. So that is why I believe it is the head gasket. I'm no seasoned mechanic but do most all my vehicle repairs and have for 20yrs. I kinda like to take on the challenge of fixing things if ya know what I mean. Also a friend of mine is a mechanic and he said the same thing. I would be better off finding a crashed out doner and doing engine swap with as much of a pita it is to do the head.


Lifetime VIP Supporter
If you begin your search on eBay...

...it will give you chance to check out the ones that look promising from the ones you should 'Shine On' and absolutely avoid getting. Here are some additional suggestions:

(1) Avoid anything with over 160,000 Miles. The Atlas Engine CAN run as long as 300,000 Miles... but getting beyond the 100,000 to 160,000 Mileage realm means there won't be but a few more years left on the engine. Anything Older and you are just asking to inherit someone else's problems.

(2) Follow your "Due Diligence" by confirming that the Vehicle was recently towed into the Salvage Yard after an Accident and not just dragged off of somebody's Vacant Lot after sitting there for over a decade. Request additional images and specs from the Invoice if you can get them to email you this data.

(3) LQK Motors sells GM LL8 Engines for a Higher Dollar... but they may include a Limited Warranty.

(4) Keep your Motor choices within the range of your present (2002-2004) PCM if possible. The Later Model Engines will NOT play well in the 2002 First Year of Manufacture SUVs.

(5) If you have a place to work that you can park ANOTHER GMT-360...it might be cheaper to just find another, low mileage SUV that will cost just about as much to get as a Used Engine and allow you to skip having to crawl around to R&R or transport the Motor out of a Bone Yard. Besides THAT... it provides all those additional Parts that may just come in handy.

(6) If you have access to a Pick Up Truck... Your options for doing a nationwide search to find the Very Best Deals becomes possible if the Seller(s) will ship to a Local Freight Yard at a Flat Rate. THIS opens up ALL the doors to you so that you can afford to be *Picky* and find a Motor worthy all of this trouble.

To see what this activity looks like when swapping out things wholesale between one GMT360 and another... look up "The Daninator" Youtube Channel for a whole series of such R&R and Swap Videos.



Not being able to confirm if it's the head or possibly the block that is damaged, a complete engine swap would be better than chancing taking the head out with all its inherent issues and complexities.

Personally, I'd be leery of eBay not knowing how long that engine has been sitting on a rack or outside and being able to look it over personally. Warranty or not, you're still on the hook for yanking it back out if it's borked. Also check out car-part.com where you can search for engines in your area. For easiest compatibility, one from an 02-05 will swap directly. 06-07 will work with minor wiring modifications to move the temp sensor and connecting the newer ignition coils. Stay away from 08-09 engines as they are not compatible at all with the PCM and sensors.


Well-Known Member
I kinda like to take on the challenge of fixing things if ya know what I mean. Also a friend of mine is a mechanic and he said the same thing. I would be better off finding a crashed out doner and doing engine swap with as much of a pita it is to do the head.
In my opinion, you will want to first identify where the coolant is leaking from. If you popped a heater hose, which are located on the rear passenger side, then you might choose to fix that and test the engine operation and performance thoroughly before doing major work. It could also be the engine is pushing coolant out of the coolant recovery tank, also located on the passenger side.

If you find the leak is from the head gasket, or if the severe overheating damaged the engine in any way, then I'd plan on pulling the engine to either repair or replace it. Doing a head gasket while the engine is in the car is an unnecessary trial in frustration and futility.


OK, you have an EXT, which means you have the rear heater. There are a couple of places where it might be leaking.

Well coolant was on vacation and headed south exiting from pass rear frame rail above wheel . I have tried to find the leak and can not locate it. So I summed it up to the Head gasket.
If you mean that it's leaking at the rear passenger side wheel, then the rear heater core is likely the source. I had that problem and had to replace it along with the hoses. It was an exercise in futility as the rear HVAC never worked for me. And replacing the heater core you have to convert it from the original block style connector to the traditional connectors. I'd just block off the rear system unless you really need the heat at the rearmost seats.

It is extremely rare, or even impossible, that a head gasket leak would leak outside the engine. It certainly wouldn't travel all the way to the rear or the side to the frame. If it did leak from the engine, it would just drip straight down and would be directly under the engine. If there is no coolant in the oil, bubbles coming out of the radiator or coolant steam coming out the exhaust, it's likely fine.
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New Member
If you mean that it's leaking at the rear passenger side wheel, then the rear heater core is likely the source
Thank you Guys so much for all your experience and knowledge shared among fellow enthusiasts or weekend warriors/ shade tree mechanic etc. This is exactly why I started a thread on this forum, like I mentioned before, I'm 40yrs old and not an ASE certified Mechanic by any means but I am very capable of doing most of my wrenching myself, for the most part. And I never even thought about a rear heater core, Duh I should've known , but didn't even think that direction due to the rear heat hasn't worked in years so I forgot it even had it. Ha ha. like how the heck would the coolant reach all the way to the pass side rear wheel well. I guess you could say I had a brain fart lol.

I have been all over underneath and in the engine compartment trying to find where exactly this leak is and I just haven't been able to locate it, although I did find a nice size crack on the exhaust manifold. (See below)👇.
I will be checking the rear heater core tom that may be the culprit hopefully haven't created more damage than necessary. I'll let ya know what I find out

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