91' gmc 3+3 6.2 rebuild...

[Ben613]

Ok, I'm finally starting the rebuild on this truck. I Have a few ?'s for yall that I'm sure will be answered. First off a little run down about the truck...a few years ago my friend was on a trip to TN, on his way the engine supposedly blew a H-gasket. He made it all the way back to MN with it in this condition. After he got back he kept driving the truck because there was no problem other than the coolant kept leaking out...

So, on a trip in the middle of winter it was -20F outdoors (even colder indoors)...he was pulling a car on a trailer and it shut down. Apparently, most of the coolant leaked out and it overheated so bad that it shut down.

And now we're doing a rebuild. First question is...What size are the H-bolt threads? I am going to chase the threads and can't remember the tap size.

Also, the truck has an aftermarket five core rad, so we will be upgrading to an old 350 gasser five blade fan. 2: I'm wondering what year would fit?

3: What year/code 6.5 diesel felpro gaskets work with the 6.2?
(It doesn't have the weird oval coolant passage)

Any other suggestions welcome.

Thanks for any and all help!;)

Ben

[Ben613]

I also have the three 6.2/6.5 books from TDP. Very nice to have!

[Hat3d1]

Three books? I have found one, mind PMing me links?

[Dvldog8793]

Howdy
What part of MN are you in? I have a 6.5 that I am tearing down and might be willing to part with the heads. I also have some other 6.2-6.5 parts. I am in northern MN by Grand Rapids.

[Ben613]

Hi were in SW MN....around Marshall.


I still need the above ?'s answered;) Don't worry about the delay...I know yall have lives too.


Thank you

[Robyn]

Howdy
IIRC, The head bolts are 12mm.

Use a thread pitch gauge (metric) and check to be sure.

All threaded holes on the 6.2 are metric (except the bell housing, that's 3/8-16

Remember, the head bolts are torque to yield type and once used, are throw away items. Theses are available as a set and felpro makes them (about $50)

The new bolts come with a dry silicone sealer on the threads, use it as is and DO NOT ADD ANYTHING, just install the bolts in clean DRY holes.

Follow the torque sheet that comes with the bolts.
IIRC It's 20 pounds through the pattern, then 55 pounds through the pattern. Once this is done, do the pattern once more and give each bolt 1/4 turn more. This puts the bolt into it's Stretch mode, and allows the max clamping force.

Never reuse the head bolts on these engines, NEVER

The head gaskets for a 92 6.5 will work fine.
Be sure to get the Felpro gaskets, as these are the most reliable ones.

Another item to keep in mind.
The push rods go in one way only. The end that goes to the rocker arm is hardened and should have a copper color to it, butttttttttttttttttttttt, this seems to wear off in time, so mark them as you remove them

Installing wrong will cause rapid wear of the rockers and push rods, resulting in failure.

Be sure to check the main webs on the center 3 for CRACKING near the outer bolt holes and at the register fit area.

Good luck

Missy

[Ben613]

Ok...The heads are off and have the front clip apart...No. 3 cylinder has a crack between the ex valve and the pre-chamber...Have to pull valve and check it out more. ( The cylinder had 350psi) That was the reading we got from most of the cylinders...Lowest we got was 240psi. I'll be bringing the heads to a friend tomorrow...will see what he says. Anyway....For now I'm looking for a good set of heads. Not 6.5 heads...they have to be from a 6.2.

Ben

[Yukon6.2]

Hi Ben
Why are you requesting just the 6.2 heads?
My 6.2 is a 90,when i got rebuilt heads they were the same casting numbers as the 6.5 heads.I used felpro 6.5 gaskets,that was at least 8 yrs ago.The original heads had the same casting #'s.
Thomas

[Ben613]

I want 6.2 heads because the pre-chambers are MUCH different. I just found out that one head is good the other is bad...haha:mad: Now where you think I can find one head??? If anyone has anything I'd be glad if they would PM me.

Ben

[Yukon6.2]

Hi Ben
You can change the pre cups.A lot of the time they have to be changed because of cracks.
Thomas

[Ben613]

OK, one good head...one bad head. What now? I am wondering if 6.2 pre-chambers will fit a 6.5 head? If so, Do 6.5/6.2 heads have the same amount of dome? Because...6.5's have a lower Comp-ratio. I'm wondering if the piston id recessed or if the dome is deeper into the head?

So if the heads have the same dome....I'll be wanting a set of either 6.5 or 6.2 heads. If anyone has any?

[Ben613]

Ok...found a complete hummy 6.2 with a knock in the bottom. I'm wondering what model/year of 6.2 they were based on. Also need to know the pre-cup designation.

Thanks

[Yukon6.2]

Hi
If you look on the block,the rear closest to the driver there will be a casting number.There should be a casting number on the head as well.
Get those and someone will be able to give you some info on it.
Thomas

[Ben613]

Does anyone know which years of the 350 five blade fan work on the 6.2?

[Yukon6.2]

Your best off keeping the 6.2 fan and clutch.They are calibrated for the diesel.If you are looking for something better,i belive Kennedy Diesel used to have a clutch calibrated to their specs.
Thomas

[Robyn]

When replacing heads, it's not so important as to what year or vehicle the heads came from, but that they are a matched set.

Same valve size and precups.

Mixing and matching from side to side is a disaster waiting to happen.

The early 6.2 used very small precups with small ports, and over time the designs changed drastically.

You can swap the cups, just be sure that the register fit in the head and the cup are very clean and free of debris/carbon/dirt ????

The cup must be flush with the head to no more than .005" above the deck.
The cup may NEVER sit below the deck surface.

Cracks in the port window are "said" to be acceptable if they are no more than 3/16 long and or do not cross the fire ring.

Now, this said, why would anyone want to install a broken or cracked part ????


The 6.2 used several different sized valves over the years too. Just make sure the heads match.

The early 6.2 had the coarse threaded injectors, which are ok, but I would shy away if possible.

Missy

[Ben613]

Still looking for heads!!!! The Hummy engine didn't work out.:mad: