Do N* heads get machined during HG replacement?

[winterset]

I am just cuirous.

In addition to the required timeserting the block, I don't recall ever hearing anyone requiring the heads being taken to a machine shop & trued for any warpage.

If I am not mistaken the following is required:

Check the block for fractures & check for warpage

New headbolts (Torque To Yield bolts)

Timeserting the block

Machine the heads

this of course only covers that the OEM gasket that is used will last & yield the best results.

Again, not sure if the machining is part of the procedure. I believe truing & cleaning each head is about $125.00 - $150.00

[Cadillac Jim]

Usually aluminum heads can be trued without machining if necessary. Also, an out-of-true aluminum head will usually pull down OK when the head bolts are tightened. I would limit head work to cleean-up and a check for flatness to be within tolerance.

Aluminum blocks aren't as prone to cracking as cast iron blocks, although I would check the sleeves. We have had people here talk about "cracks" including one guy who does engine R&R who had several and came here to stridently warn all and sundry about a big problem with Northstar blocks cracking, but continued dialogue reveals that all of them are apparently damage to the water jacket while the engine was out of the car. Water jackets are fragile on any thin-wall casting and particularly so in aluminum thin-wall castings, which is to say just about any and all new engine designs in the last 20 years or so. Be very careful with your motor while overhauling it, and use a hoist and an engine stand - never roll the block around on the floor.

The factory head bolts of nearly all aluminum engines are torque-and-twist, including the Northstar. I'm not sure that new head bolts are necessary when Timeserting a Northstar. Of course, new head bolts are just about the surest possible way to ensure that the threads are completely clear of debris and thread-lock. and they are not that expensive.

[winterset]

OK.

Reason I am asking. I just performed a HG job on my 3.4 chevy minivan. it's a cast iron block and aluminum heads.

necessary steps:

- machine & true heads

- use the new redesigned OEM gaskets

- new OEM head bolts

findings:

rear head was warped & original rear gasket had a rust pocket between coolant hole & piston opening - causing air to escape from the rear driver cyl & pump air into the coolant. that piston was also clean & white - while the other 5 were covered in a small layer of carbon.

new headgasket seemed to be made of a different material, in addition it had a thin layor of what I believe to be teflon on both sides.

procedure stressed that TTY bolts must NEVER be resused because it's not possible to get the correct torque again after they are stretched.

the shop I took the heads to seemed to be very busy. he said he gets at least five 3.4l heads a week. said he had to remove about the thickness of a human hair to correct the warpage. I didn't ask if he gets N* heads. (I meant to but forgot)

were the N* gaskets also re-designed? I am not sure why it wouldn't be a requirement to true the heads & use new bolts. if I am bolting an aluminum head to a cast Iron block, wouldn't it be equal to bolting an aluminum head to a timecerted N* ?

the heads were covered in bars stop leak clumps, - so I'm sure that stuff was trying to seal up the leak. in GM's defense, I might have been to blame for the failure. I think the waterpump had a small leak, & I just kept refilling. I probably had air in the system causing improper cooling, and leading to the warpage. I don't believe the clumps should have formed on the headgasket unless there was a presence of air.

[Cadillac Jim]

The Northstar head bolts are torque-and-twist, not torque-to-yield, which I have never heard of; I don't think that those would work with aluminum blocks because of the higher rate of expansion would stretch the head bolts when the engine is hot, and the head wouldn't seal at -40 F. Of course, something like torque-to-yield is a one-time-use part.

Improvements are made to parts all the time. I do believe that there have been several versions of the head gasket for Northstars, and that if you get a new set from GM or Rock Auto or FelPro or whatever, it will almost certainly be better than one that is more than three or four years old, regardless of make, model, or engine. Further evidence of this in the Northstar is a change in the torque-and-twist specifications for the Northstar a few years ago, indicating a change in thickness and/or compressibility of the head gasket. Get the part numbers for the had bolts and the tightening procedure and specifications with the head gasket, not from a manual written from date-of-manufacture specifications, to be sure.

I've heard of problems with the heads going bad on the GM 3.4. I had a friend with a 1999 Aztek van with the 3.4 and it had a small head leak. His mechanic simply added stuff to the coolant and didn't tell him. It went bad on a 1,000 mile trip; my wife and I were with him and his wife on a shared vacation. We had to locate a mechanic and yes, the leakage had cut a channel and we needed a new head, and yes, we needed to machine the old head - and the intake manifold and the new head to match everything up. This one had iron heads. My wife's car is a 1999 Grand Am GT with a 3.4 HO with aluminum intake and heads, and the intake manifold gasket went, but we fixed it before it led to other problems. Our mechanic did say that, left alone, this would eventually bring down the engine by letting coolant into the oil through the lifter valley.

I met someone once whose wife drove a VW Squareback with a dead hole. I could hear the whoosh-whoosh-whoosh of a massive compression leak and knew a head was gone. I eventually got them to go to a friend that I knew would fix it without driving up a big bill; that friend told me that my "friends" with the VW Squareback were really weird, because they must have overheated it badly to start the problem and then driven the car with a dead hole for years to do the damage he uncovered; I just said, uh, huh.

[CarlaValentine]

if I am bolting an aluminum head to a cast Iron block, wouldn't it be equal to bolting an aluminum head to a timecerted N* ?

Assuming you are referring to tightening the bolts down and that alone, then it would depend on what type of timeserts are being used. There are different types of timeserts (not all with that name) that are made of different materials ranging from aluminum to stainless steel and carbon steel, and different materials will handle the torquing differently.

If you are referring to the long term, then no. On the northstar the heads and the block are both aluminum and therefore expand and contract differently (more evenly) than an aluminum head on an iron block, be it timeserted or not.

[KHE]

The Northstar heads rarely if ever need to be machined. They do not get out of the flatness specification like other engines do.

New headbolts MUST be used when R&R-ing the heads on a Northstar engine. There are NO exceptions. The bolts are not torque to yield. The only reason new bolts must be used is due to the microencapsulated threadlocker compound that is applied to the threads. There is no way to apply this compound in the field so that is why new head bolts must be used.

[joeb]

when GM builds a new northstar the steel headbolts thread into an aluminum thread. during a repair the block is inspected and new bolts may be used in an original threaded hole if they are ok. yes the consensus is to timesert the block to ensure a more robust repair. are new headbolts with sealer required when using a timesert? a timesert is held in place with locktite, which is a steel/aluminum connection. why not use locktite on the bolt/timesert connection since it is steel/steel?

[Cadillac Jim]

Threadlocker is a must in torque-and-twist head bolts because otherwise the bolts will eventually be subject to working loose over time because of he continual temperature cycling. I didn't realize that the factory threadlocker was mandated.

[KHE]

when GM builds a new northstar the steel headbolts thread into an aluminum thread. during a repair the block is inspected and new bolts may be used in an original threaded hole if they are ok. yes the consensus is to timesert the block to ensure a more robust repair. are new headbolts with sealer required when using a timesert? a timesert is held in place with locktite, which is a steel/aluminum connection. why not use locktite on the bolt/timesert connection since it is steel/steel?

All headbolt holes in the block need to be Timeserted when doing the repair regardless what the threads look like. Not Timeserting the hole is asking the repair not to last long.

New headbolts are required - even with Timeserts. The Timeserts are held in place by a combination of Loctite and a swedging operation with the Timesert installation tool.

New headbolts MUST be used - you cannot apply Loctite to old headbolts as the microencapsulated threadlocker/sealer that is on the new bolts is also under the bolt head and under the washer face. This sealer is required and there is no practical way to re-apply it in the field.

Considering the amount of work it is to pull an engine, remove the heads, install Timeserts, reinstall the engine, there is NO WAY I would risk the expense and labor of doing it again by using old head bolts and Loctite.

[joeb]

torque and twist? lets really go off thread here. if a bolt is not TTY, than it is T&T? what type of bolt is in a 1975 chevy 350 SBC? T&T?

[Cadillac Jim]

I believe that all the old all-iron engines use conventional head bolts. These are simply tightened to torque in sequence and in stages of increasing torque to compress the head gasket. Often is is a good policy to re-torque the heads after the engine has been run a few times to adjust for additional compression of the head gasket.

Torque-and-twist head bolts are a whole different technology. There is a length of head bolt that has no threads and the threaded part is deep within the block near the bottom of the sleeves. The head bolt is first torqued to pull the head against the head bolt, then twisted a specified number of degrees. The amount of twist and the thread pitch determine how much the threaded end is pulled down. This distance is the compression of the head gasket - and the stretch of the long unthreaded part of the bolt, which acts as a spring that maintains a specified clamping force. The major advantage is that when the block expands with heat, the bolt stretches but clamping force increases only as the spring constant of the stretching bolt and the expansion distance, the head gasket is not permanently compressed further, and when the engine cools back to room temperature everything returns to as it was. Thus the clamping force and the head gasket seal do not change over time.

I recall this technology being used for aluminum engines for many years. I rebuilt motorcycle engines in the 1960's and all of them use long studs anchored in the crankcase, and the stretch distance is the height of the cylinder barrel. Usually the head bolt tightening specification was in terms of lb-ft or N-m but I always felt comfortable using my feel with the stretch of the head stud instead, and I never had one come back. I've rebuilt two-strokes in a few minutes without a torque wrench on occasion, including my own once - over a campfire - and at a race when our team's bike seized a piston, and never had a problem. VW engines used a similar long stud arrangement. Those of you with hands-on experience with Corvairs, Porshes, and other aluminum block engines will likely remember observing similar head stud or bolt techniques. Most engines designed since about 1995 use aluminum blocks, and I suspect that all of them also use torque-and-twist.