Oil cooler line gasket - notes on replacing


Lifetime VIP Supporter
(note: I put a separate post in the full-size 'technical discussion' re: Fel-Pro gaskets and orientation, as I noticed the one I used for this was constructed differently than the OEM (which was flat on both sides).

Background: GM includes an engine oil cooler on its 3/4-ton and up platforms (it can be added to other V8-equipped vehicles). Unfortunately, it's the biggest cause of oil leaks. If you see oil dripping from the area of your oil filter - look just above it, and you'll likely find the source where the cooler mounts to the engine block.

Thankfully, it's not incredibly difficult to change the gasket (best to do it during an oil change, as you need to remove the oil from the engine, and remove the oil filter - or else, you're going to have a real mess.) You'll gain room to work with the filter off, as well.

As it is, even after the oil / filter is removed - a few ounces / ml's of oil *will* pour from the cooler / block once the seal is broken on the old gasket, so be prepared.

If you have 4WD, the front driveshaft will be directly in front of and just below the cooler bolts. There's enough room to work around it - a regular 10mm with short extension will be fine. A deep socket with no extension might even work better.
I had no regrets leaving the driveshaft in place.

Although you may not be able to turn the bolts far at first, they're not super tight (see below). After the cooler mount releases and the oil drains from it, you can likely turn the bolts the rest of the way out without the ratchet.

When installing the new gasket, it helps to insert the bolts so that they protrude past the cooler housing by 1/4 in. or so - that lets you 'hang' the gasket from them so that the gasket will be aligned properly. Then push the bolts through and hand-thread them to the block.

(2) Bolts are 10mm. Each are same length / otherwise identical.

Torque is 106 INCH/lb (or 8.83 ft/lb, if you don't have a 1/4" torque wrench. Yes, call it 9...LOL.) Do NOT overtighten; this gasket is said to be certain to leak, if it is tightened past spec.

GM gives a spec of 18 ft/lb - this is just for the one bolt that connects the hoses at the bracket on the bottom of the block. DON'T confuse this for the two attachment bolts with the gasket, or you will DEFINITELY snap them off, if you mistakenly try and tighten them to 18 ft/lb. You have been warned!

GM recommends *no* sealant be used on the gasket (if I have to do this again, I will likely ignore GM's advice).
Last edited:


Lifetime VIP Supporter
So, finally (!) this is done.

Extracted the two broken bolts, cleanly. Dropped the front of the front driveshaft to do it - made a lot of difference. I recommend it now, if you're doing this repair. The exhaust pipe will keep it from dropping too far and overextending at the xfr case end - since there is no u-joint at that end, and it's not easily removed as a result. If you're super-anal, go ahead and mark the driveshaft / bolt retainers, so that you know how to orient them the same way when they go back together.

Even so, it's not an easy thing to lie on a creeper, using a right-angle drill attachment to put a 1/32" pilot hole in that you can then use the extractor in. And you'll need longer drill bits (find those in left-hand; impossible), or a shank extension for the regular RH bit. The LH bits want to walk, even using a center punch beforehand. The RH bits at least let you get the hole started, where you can follow up with the LH bit. For two bolts, this took about 2.5hrs per bolt over two days to get them out (the LH side bolt is harder, due to its location).

I couldn't find any reference to the size / type of those bolts - GM will include them if you buy the replacement lines, but no reference to them anywhere, otherwise. Being cheap, I'm not going to spend $100 or more for essentially two bolts - because I'm sure NOT going to swap out the lines easily, with the 4WD hardware in the way.

Then I ran into someone online mentioning that they removed theirs and bypassed the lines with a blockoff plate.

Found a company that sold 'bypass kits' for LS engines - complete with bolts.
Figuring that I'd probably go for a bypass if it continued to leak after the repair was complete, I ordered it up. $15, and I'd still have the blockoff plate / gasket for potential future use.

Got the kit in the mail. Nice packaging, including a window decal, and a shutoff tag (I don't know what they're called, but they're found in race cars, to help drivers / responders find (& pull) the ignition. I did notice the bolts were shorter, and they didn't have the rounded taper on the end (NBD on the taper; just mentioning for reference)

Tried to put them in, and I didn't have to worry about over torquing - because I couldn't *tighten* them to the specified 106 in/lb. Turns out the bolts were shorter because the blockoff plate wasn't as deep as the cooler plate - *and*, the thread on them is finer than the OEM (when I took them out, they effectively acted as a chasing tap, and brought out some of the thread material in the block. Back to the drawing board. I still have the blockoff plate & gasket, so not a total loss, but I'm no closer to solving my problem. Time to do some bolt research.

As it turns out, these bolts are pretty much a standard 'M6 x 1.0 x 40'. (thickness x thread pitch x length in mm). M6 bolts will have a 10mm head by default, so the next time you're working with a 10mm bolt - it's really an 'M6', most likely. And the 1.0 pitch is the standard for these bolts. Picked up a couple of different ones at the local HW store.

I figure if these bolts don't fit - I'll just install the bypass kit, and hacksaw the cooler lines off / plug the attachment holes in the other end. Do a 'test' fit, and they seem to go in nicely. Now to put them in (along with a 2nd new gasket, since I tightened the first one to 2x spec.)

Get everything tightened up nicely / reinstalled. Fire up the motor and let it pressurize / flow the oil through the system - no more leaks. From *there*, at least. Woohoo! :woohoo:

However, it now looks like there is seepage from either the back cover ('bedplate') or, g*d forbid, the rear main. For now, I have to let it go, because I need to get the Sierra out of the garage, and put my Accord in it.

Seeing this new leak makes me apprehensive about putting anything 'new' inside the motor - I wanted to put a towing cam kit (cam, pushrods, lifters, springs) and the TBSS intake on the engine this winter as a project - along with the goodies you pull off to get to that, like a new WP, timing chain, etc.) But I look at that leak in the back (and whether I can even do it myself in a garage with limited room), assess what it would cost to have it done, and think to myself...maybe I should just cut my losses with this truck, and find another. :bonk:

I will say...after putting on a whole new front suspension, and killing leaks in the front axle, the transfer case, and now the engine oil cooler - along with new brakes / hardware all around, new shocks, etc. - I feel DAMNED satisfied with what I've been able to accomplish on this truck, and I wonder how quickly I could find a buyer on Craigslist for it.

Got a bottle of UV dye when I went to put gas in the truck. Hopefully, I can find out where the leak is, and figure out what I want to do, long-term.

And that's the story of how a repair that should cost $10 and take about an hour to do - wound up costing me about $50 in tools & parts, and took me nearly a week to finish (working intermittently).
Last edited:

Online statistics

Members online
Guests online
Total visitors

Forum statistics

Answered questions
Latest member
robert maynard
Top Bottom