Timekeeper explosion in P400

[CursedH1]

Sad to see one return to one component of the "disposable GM diesel engine design" the aftermarket has solved.

Its interesting because literally every single H1 shop/tech I talk to (ModMafia, Rubberduck, Tor and others including the shop that is currently rebuilding the engine) have indicated to me that the gears are not something that they recommend doing as its not necessary. Searching back through the HummerNetworkForum.com (which I run) for "timing" results in little to no results. The maintenance manual from AM General doesn't even mention timing when it comes to scheduled maintenance. I just got off the phone with my guy who had actually just called Tor, who implored him to not install the ARP headstuds because 1) They all leak and 2) He's never seen a problem with the factory headbolts. Now I don't know what to do. For those of you who aren't aware, Tor is one of three remaining AM General factory certified techs in the country still working on these trucks. His reputation amongst our community is absolutely unimpeachable. So I'm getting stuck between two camps of people here. The group that seems to be H1-centric that says "don't do it, not worth it, it'll fail" - the guy doing the rebuild is one of these guys. "I've been building 6.2's and 6.5's for 17 years now and never had a problem with the factory head bolts." Then there are people like Will and Chris who are telling me to do everything from gapless rings, head studs and flash cleaning. In the end I'm probably making all of the wrong decisions all over again and I'll just be back here again in 6 months wondering why I even f*cking bother.

Thank you for nominating me for the Carnage of the Year award - however I should probably occupy a place in that award's Hall of Fame.

In 2019, I blew up my first 6.5 - the #8 cylinder - while driving over the Casco Bay Bridge in Portland Maine. Truck drove about 3 miles puffing white smoke until I finally had it towed away. Had it replaced with a UORVA test mule P400 by ModMafia down in Texas. Never had a problem with that engine.

Until 2021, when a 15 year old girl t-boned me when she pulled out from a stop sign. She hit me right under my drivers' door and I ramped up her hood and did a complete 360' barrel roll. Got it all on dashcam.

Got my current Red 2003 H1 two months later. Managed to get one of the last factory P400's they made in early 2022 - from what I understand it was essentially a "spare parts special" when GEP was cleaning up the factory floor - they found enough components to make 4 more brand new P400's. So I ripped out my perfectly good heavy block 6.5 in my 03 and put the current P400 in with the UORVA turbo and some other things - timekeepers included. Almost universally the Hummer community criticizes me for changing so much on the engine - but my H1 weights 9100lbs, I tow my M1102 (about 2k lbs), daily drive it and also use it for long cross country road trips. It needs to handle the heat generation of going 70mph. Not sure if @Will L. has availed you of the aerodynamic issues that the body of the H1 presents for our cooling stack but needless to say, we overheat when we go fast, whereas we'll actually get too cold when we sit still in traffic.

In August of 2025 I was in the North Maine Woods, about 40 miles from the closest paved road when we pulled off for lunch and immediately everyone in the group smelled coolant. Turns out my fan had intersected with the fan shroud, broke off half of the blades and chucked at least one straight up into the radiator producing quite a leak. We used JB Weld putty as best we could, I filled up a 5 gallon jerry can full of lake water and started driving back towards civilization. Using my two temp gauges and stopping every 5 miles (with the trailer behind me) I managed to get the truck back to our basecamp without breaking 200'F on the temp. I was rather proud of that.

So I had it towed down to my aforementioned mechanic and we had the radiator rebuilt with dimpled cores. Since the fan had lost about 5 of the 9 blades, we decided that it was prudent to replace the waterpump as well. For those wondering if the cover is removed during the waterpump install, per the H1 Shop manual, it is. And this is where the timing gear problem happened I believe.




Timing adjustment is not listed on any of the scheduled maintenance groups throughout the factory manual:

 

[CursedH1]

My guy sent a picture of the bores which he said "look practically brand new".

I respect you posting up as this is some detail I feel I could overlook.

I'm not ashamed of my numerous and repeated "learning experiences" as long as others can learn from my self-inflicted idiocy.

If that makes me look like a moron to people (which I've been told) then so be it. As long as I can at least serve as a warning to others, my place on God's green earth is earned.

 

[Twisted Steel Performance]

The water pump backing plate/adapter cover IS removed because the water pump has 5 bolts on the back that connect the pump to the backing/adapter place., NOT the timing cover or front cover in your pic..... the gears can't be removed or seen very well without removing the timing cover....

As for head bolts or studs, if your boost level doesn't go past 9-10psi, bolts are fine, the higher the boost the more clamping force that's needed...

I would say follow the recommendation of your builder....

 

[CursedH1]

The water pump backing plate/adapter cover IS removed because the water pump has 5 bolts on the back that connect the pump to the backing/adapter place., NOT the timing cover or front cover in your pic..... the gears can't be removed or seen very well without removing the timing cover....

As for head bolts or studs, if your boost level doesn't go past 9-10psi, bolts are fine, the higher the boost the more clamping force that's needed...

I would say follow the recommendation of your builder....

I run 15psi. Ran 16psi on my old P400 with factory head bolts no problem. Of course I'm not doing 150,000+ miles on my H1 though...

 

[Twisted Steel Performance]

Run bolts then, save some money.

 

[MrMarty51]

My guy sent a picture of the bores which he said "look practically brand new".
View attachment 97420

I'm not ashamed of my numerous and repeated "learning experiences" as long as others can learn from my self-inflicted idiocy.

If that makes me look like a moron to people (which I've been told) then so be it. As long as I can at least serve as a warning to others, my place on God's green earth is earned.

Dont look like a moron to me.
Just another 6.5 owner dealing with some problems is all.

 

[CursedH1]

Just to update on the build - the bearings were fine. Some minor wear but nothing that is out of the ordinary from a healthy engine.

Lifters arrive today. Head studs are being installed. Bottom end is all buttoned up (Thank you @Twisted Steel Performance for the torque specs!!!).

Need to order the new IP. Timing chain and gears have been procured. Engine should be going back in the truck early next week I hope.

 

[JayTheCPA]

I had the Internet howl at me (not this community, but others (plural)) not to use the timing gears as they whine at high RPMs (IIRC, 5K and above). These doomsday inputs were from "experts" that raced V8 gassers. When I pressed these "experts" on how to get my 6.5 above 4K RPM without turning it into a grenade, they still howled that the gears would whine. Installed the gears anyway. In fairness, a mechanic did discover that the gears were noisy, but the only way to hear them was to remove the oil filler cover with the engine running. With the oil filler cover securely in place, the engine itself was the loudest thing in the room and could not hear the gears at all.

I had expert mechanics tell me not to use dynamic balancers. But then I explained that the balancer went on the crank after the crank was balanced and the negative inputs stopped.

Oh, and there were shade-tree "6.5 experts" that claimed the only way to get big power out of the 6.5 was to ramp-up the turbo and get above 20-something psi to the intake manifold. Proved that idiocy wrong from dyno numbers with my measly 14psi out of a non-wastegate turbo and *no* intercooler: peak 400'/# torque.

As I only intended to make OE level of psi and left the compression ratio alone, nobody recommended ARP head studs. OE were fine.

Only had the P400 for about ~20K miles as that vehicle could not get past the project stage and sucked too much money away from repair on other vehicles. Engine was good, but the rest of the vehicle did not want to get with the program. I did like the P400 as it was a tad more quiet than the standard 6.5.

By the way, at 20K miles I was still on a 3K mile oil change cycle as analysis did not support longer. Per inputs from a company rep, this was normal.

 

[CursedH1]

This was the only HS roller that had any kind of wear marking whatsoever. It's not flat and spins freely - just like all of the rest of the rollers. These have about 20k on them. Going to go ahead and put these back in.

 

[CursedH1]

By the way, at 20K miles I was still on a 3K mile oil change cycle as analysis did not support longer. Per inputs from a company rep, this was normal.

Oil has been religiously changed at 3k or less. Usually less.

 

[WarWagon]

Of course I'm not doing 150,000+ miles on my H1 though...

Run bolts then, save some money.

First thanks for sharing the good news the glitter didn't get far.

IMO neither of these reasons apply. I had the heads off so often it was cheaper to run the studs than $ order and wait for the TTY single use head bolts. The studs outlived the 6.2 block and are in use in another engine. I say 30K miles was about the time I opened the engine up for one reason or another. And the chains were not something I wanted to ignore looking at them so sloppy with around 30K on them.

@Will L. is there a clearance problem pulling the just heads with studs in a H1?

Yes, the ARP studs need a better sealer and install directions than they included, well a decade or more ago now... Solutions are on here for that as well.

I would say follow the recommendation of your builder....

This is the better advice I agree with for your situation.

As you have spoken to Chris I am sure you have covered other ways to "keep the heat out of the engine in the first place". Turbo, you appear to have gone aftermarket, is step one to not have extreme backpressure keeping the heat in the engine. Chris is an innovator with other ways to keep the heat out of the coolant.

Getting an engine builder to to something different. Maybe they are willing to build it "your way." The risk is real as they can't replace this P400 engine because if something goes wrong with a diesel it's generally hot oily scrap metal.

Almost universally the Hummer community criticizes me for changing so much on the engine

Er, Um... So?! Your money your ride.

Being my own warranty station and one of the few who has used a BD Boost valve on a huge turbo and a Yank high stall triple disc converter behind a 6.2/6.5TD ... I have even warmed up to lower compression builds although the experience I have with it: is from bent valves and or worn/overheated rings. I AM that guy that ruins engine oil and has the UOA's to prove it.

After the ATT (A Team Turbo) thread wars... impossible to get some to think outside the Asthma Attack GM turbo box.

Drop a Yank Torque Converter in and enjoy the heads exploding on the forum. Theirs, not the ones on your engine... Although I maintain my bent valves were from a failed IP governor. Eh, as fun as that may be you are rebuilding an engine for your needs.

Thank you for nominating me for the Carnage of the Year award - however I should probably occupy a place in that award's Hall of Fame.

Submit your entries here... Will will NEVER let me live down the "Warning to others" worm clamp that came off the IP return line and went down an intake port.

6.5 carnage

thought I'd start a thread on just pics of 6.5 carnage. If there is another that I couldn't find mods please merge.

www.thetruckstop.us

Gapless Rings.

3000 miles may be too much on the engine oil. My 1993 called for 2500 mile oil changes under all but "driven by Grandpa to Church on Sunday" conditions. Towing the grades around here, HD use, my UOA's showed 3000 miles was too many miles on the oil. Changing the oil twice a week was buying it by the 5 gal bucket... Esp. since the 6.2/6.5's I had all burned a quart every 500 miles working this hard. No the continuous oil change didn't help the UOA numbers.

My signature has a link to my experience with Gapless rings. I pushed oil changes to nearly 5000 miles and had more to go. This is a dirty soot belching high compression engine. Less blowby with gapless rings keeps the soot out of the engine oil. Like you can read the marks through the oil at 2000 miles on it clean. Less, like near ZERO, blowby as the engine wears is also an advantage of gapless rings. Pic below is 2000 miles on a 6.2/6.5TD hybrid (6.2 block with the ATT turbo) with the wick turned all the way up and you can see the marks still because of the gapless rings I used. Kept my hands somewhat cleaner when wrenching on it as well. Not to brag, but I got 38,000 miles out of the engine before a IP with a bad governor showed me north of 4000 RPM on a Emissions Snap Test.

 

[CursedH1]

In the end, I got some simple, easy to follow instructions from Will and passed them along to my guy who is comfortable doing the studs, and they are now as I type this curing in their loctite bath in the block over the New Year's holiday.

My engine never has had a problem with heat (except when I got a fan clutch that didn't want to engage, fixed that by bending the spring). I've got the 180 Tstats, the BRF, the dimple cored radiator. I tried the electriviscous fan clutch but it didn't work for me. Nearly cooked the engine going up and down the White Mountains in New Hampshire - but the OEM fan and the dmax fan that is currently in the truck have kept my temps nice and cool.

@Will L. is there a clearance problem pulling the just heads with studs in a H1?

It can be done but its a huge pain in the ass. Easier to do if you've got a body lift (which I don't, right now).

Er, Um... So?! Your money your ride.

Yes there's this. But I have been blessed to have so many friends from both the H1 world and the 6.5 world that have a ton of helpful, experienced advice for my situations, and often they contradicts each other. They're all coming from a good place and want the best for me and my situation, so it is very difficult to "pick sides" as it were. It's a great problem to have but a problem it is.

My perspective is that the 6.5 was never built as a performance engine and that the tech and processes that went into its build, even to this day, are based on 60's and 70's methods for the most part. Sure the P400 did a lot to bring it up to modern tech but there's still a lot left in there that can be done to make this engine more "all purpose" than it was originally built for. I'm not in favor of duramax swaps in H1's after seeing the huge bills that come with those. Shops like ModMafia charge $80k+ to put those engines in H1's and I just don't think that the juice is worth the squeeze when the entire platform was designed and built around the 6.5.

 

[JayTheCPA]

About those gap-less rings that WW mentions. 15 years ago my local mechanic was not a fan of them. But... I am in the middle of restoring a Ferd and the mechanic recently asked whether we should go through the engine, and mentioned adoption of gap-less rings among the items to improve. So, there is something to this.

While it appears that it is a little late for gap-less rings in the H1 today, am suspecting another excuse to open the engine will present itself in the not too distant future. Just say-in.

 

[CursedH1]

While it appears that it is a little late for gap-less rings in the H1 today, am suspecting another excuse to open the engine will present itself in the not too distant future. Just say-in.

I hope not. But its nice to see a vote of confidence. Appreciate that!