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Head Gaskets - Achilles Heel of 3.0L Engines

Joined
12 April 2003
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Location
Fort Collins, CO
It is no secret among NSX mechanics, engine builders, racers, and some owners that the OEM head gasket for 3.0L engines is prone to failure. Despite this relatively-common understanding, I have not seen it discussed as much as other much-more-rare problems (harmonic balancer, oil pump gear, etc). I encountered this issue at NSXPO 2012. Despite many years of ownership and involvement in the community, it was the first I (and others I've spoken with) had heard of this being anything but very, very rare.

Mechanics with lots of NSX experience who know how to deal with this are great, but because of location or size of one's disposable income are not a luxury all owners have access to. HG-related experience and knowledge are certainly within our community, but it was not easy for me to find, much less distill, the information down to something useful/pragmatic. Extra-special shout out to Barney Demonbreun for sharing his knowledge/experience.

My goal with this post is to arm people with the information that a competent mechanic [at one's Honda dealer, Acura dealer (that doesn't see many NSXs), independent shop, speed shop, etc] would need to know to do it right the first time. Maybe I'm doing this backwards, but I'm going to start with "what to do" instead of "how to diagnose."

What to do...

Find a competent, trustworthy, meticulous mechanic to...
(All page references are from 1993 service manual)

  • Find the cause to be something else, or confirm bad HG
  • Remove Cylinder Head (instructions start on 6-28)
  • Inspect Head for Warpage (instructions start on 6-41)
  • Replace Head Gasket
  • Reinstall Cylinder Head (instructions start on 6-49)
If the OEM gasket is "prone to failure" what gasket should be used?

The overwhelming consensus answer on this is "Cometic MLS." MLS = Multi-Layer-(stainless)Steel, Cometic is the brand. In 1997 Honda switched to a very-similar MLS gasket for the 3.2L NSX engines. These Cometic gaskets can be purchased from Driving Ambition, Science of Speed, and other NSX specialists in OEM or other thicknesses (i.e. to make up for lost thickness if your heads needed to be shaved) as well as different bores to go along with other major engine modifications. The OEM finish on the heads is sufficiently smooth to make a good seal with these MLS gaskets, the heads don't need to be machined just to get a smoother (more-mirror-like) surface.

Driving Ambition: http://www.drivingambition.us/ProductDetails.asp?ProductCode=DA-132
SOS: https://www.scienceofspeed.com/inde...-multi-layer-steel-cylinder-head-gaskets.html
Technical Info: http://www.cometic.com/technical.aspx

Should I use OEM head bolts or aftermarket head studs?

ARP head stud sets seem to be the most-popular solution but I've also found sufficient support for OEM bolts as a great option (and is what I chose). In either case, torque matters...​

What should the head bolts/studs be torqued to?
(This seems to be the "million dollar question" - key to doing it right.)

The torque procedure, steps, and sequence on page 6-48 should be followed (plus see "pro tips" below). When torqued properly the bolts or studs will stretch a little, which is necessary to keep things together under changing temperatures, etc. Too little torque, the bolts/studs don't stretch, your gasket may fail again. Too much torque you could strip the threads in the block (or hurt the bolts?). How much horsepower they need to handle may be a factor. Here are the guidelines as I've been able to distill them:

  • OEM 3.0L gasket with OEM bolts: 56 lb-ft (spec from service manual, for reference - remember...don't use these gaskets)
  • Cometic MLS or OEM 3.2L with OEM bolts: 71 lb-ft (spec from service manual is 71lbs, others mention success with 71/72lbs)
  • Cometic MLS with ARP studs: 80-85 lb-ft (use the ARP moly lube)
  • Cometic MLS with ARP studs steel thread inserts: 95 lb-ft (use the ARP moly lube)
References:
Sources:

Pro Tip: Be sure to remove liquid (water/oil) from bottom of head bolt bosses. If not, the liquid could effect the torque on the head bolts or you could crack a boss (the block).

Pro Tip: Cometic gaskets will squish once torqued, so will end up not fully torqued, and you'll end up doing the job again if you only torque once per procedure. So... "Torque the heads in three steps over two days. 10lbs less than max torque the first day, wait 24 hours and torque the heads again 5lbs under max torque. Wait 24 hours and torque them to final spec." Use the same sequence as in service manual.​

Should I do both heads or just one?

I can't see much logic to justify doing just one. Sure, you'd save a little parts/labor, but...you might as well have replaced while 80% of the work is already to be performed. It may also be difficult to diagnose exactly where the leak is (and that might overlook smaller leaks). My OEM head gaskets appeared to have a few if not several spots where there were tiny leaks.​

Should my heads be shaved/machined?

Overheating or hot-spots from air pockets in the coolant (caused by combustion gasses getting past head gasket into coolant) can cause warped heads. Only way to know is to have them measured by someone who knows how. Instructions and specs are in the service manual.

If shave is necessary only the bare minimum of material should be removed to avoid increasing compression or changing engine geometry/timing. It seems this is where the different thickness gaskets would come in handy...except it means your engine apart taking up space at mechanic's shop for a few more days if you don't order the gaskets in advance (i.e. if you wait until knowing if/how much will be taken off before ordering gasket of appropriate thickness). When I spoke with SoS about this, they were quick to warn against possible impacts to quench burn from going too thick. They would want someone to tell them how much had been shaved before they gave a recommendation (use in-stock regular thickness or special order thickness which takes 4 weeks).

I have this presumption that a head shop will see any head as in need of shaving, porting, polishing, etc...since that is what they do. So I'm a little skeptical of the likelihood of your mechanic bringing heads into a shop and being told "they're good, don't need any work" even if that might be the case. Some folks reported it is relatively common that the heads are straight so a simple R&R of the heads for gasket replacement will so (matches my experience - heads measured perfectly flat) though at least one experienced mechanic says every one he has done needed head work.​

What else should be done while the mechanic is in there?

  • All seals, o-rings, gaskets, etc removed during this procedure that the service manual says to replace.
  • Timing belt (only expense is part).
  • Upgrade LMAs (see next question for details).
  • Might be a good time to replace coolant hoses (coolant is drained anyway, several hoses already detached, good access to others, tunnel already open, etc).

What is this about LMAs?

An LMA, or "Lost Motion Assembly" keeps the V-Tech rocker arm from flopping around and making noise when not engaged. There are two per cylinder (6 per head, 12 total). The old "piston-type" design is prone to getting sticky/ineffective and you get a noisy valve train. Honda switched to a "spring-type" design with the 2000 NSX. So, if you're in there anyway, you might as well upgrade these. This would be a one-time deal...the spring ones should last forever. SOS carries a set (not sure if source is OEM or not) at a discount to the OEM ones that have replaced the old type in the parts catalog.

SOS: https://www.scienceofspeed.com/inde...lost-motion-assembly-lma-kit-nsx-1991-99.html
OEM 14820-PCB-305: http://www.oemacuraparts.com/parts-catalog/acura/nsx/1991/2dr-nsx/ka5mt/engine/valve-rocker-arm-fr
Reference: http://www.nsxprime.com/forum/showt...ost-motion-assembly-for-1991-1999-NSX-engines

I welcome any/all input. This post may be edited as appropriate to ensure correct/good info is available right here at the beginning of the thread.
 
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Excellent write-up!

Can you touch on the temptation to raise compression ratio by milling the heads "while you're in there" for performance benefit and subsequent side effects?
 
Great writeup Craig! Though, I think the 3.0 gaskets are more of an issue with FI or nitrous systems.

Excellent write-up!

Can you touch on the temptation to raise compression ratio by milling the heads "while you're in there" for performance benefit and subsequent side effects?

This can be done, but the problem is you will likely need new engine management software if you change compression. The factory ECU is programmed for the stock compression ratio. If you change it, you will change timing, air and fuel requirements.
 
Every build I have done I've had the head resurfaced prior to applying a new headgasket. On my 3.8 build I utilized the Cometic headgasket replacing the paper one associated with the 3.0.

A few other motors I've updated to the metal OEM headgasket.
 
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On one hand this seems like a good idea to do, especially if it's time to do a TB/WP job. On the other hand, if those items don't need to be done, why bother? When she blows, she blows. Many of us are going FI and finding that 350-420rwhp can be fun, rewarding and reliable. Most 3.0 NSX's already have 60-120k miles on the original blocks. A rebuild is coming sooner or later, especially at 130hp/L.

I am insanely happy with 400rwhp. But if I had a catastrophic failure, I'd yank the motor and build the correct motor for FI, change injectors go from F/IC to EMS, retune and have a goal of 450-500hp. In the mean time, I set aside a few hundred bucks now and then for my build. Could happen now or 10 years from now. In any case, it seems ringlands are the first to go in failed FI motors.
 
Can you touch on the temptation to raise compression ratio by milling the heads "while you're in there" for performance benefit and subsequent side effects?

This can be done, but the problem is you will likely need new engine management software if you change compression. The factory ECU is programmed for the stock compression ratio. If you change it, you will change timing, air and fuel requirements.

Two more reasons:

If you or future owner would want to go forced-induction (turbo or supercharger) a higher compression ratio would work against such. I would think there would be just as much, if not more, temptation to lower the compression ratio so more boost can be used in a FI setup. The limitation there is that thicker head gaskets (say, over 0.051") may not be as reliable.

As you lower or raise the heads (by shaving or using thicker gasket), you're bringing the camshaft pulleys closer to or farther from the crankshaft. This changes timing of the valves. Large changes here would require use of adjustable camshaft pulleys to compensate for this and get timing back where it should be. I don't, however, know what a "large" change is...how much does the height need to change in order to impact timing enough to actually matter.

I think the 3.0 gaskets are more of an issue with FI or nitrous systems.
Surely FI or nitrus puts more stress on the HG and failure rate in those applications will be higher. Still, from all of the NSXPrime searches and discussions in my research, I think it is fair to say normally-aspirated NSXs (and even Legends) are far from immune.

Every build I have done I've had the head resurfaced prior to applying a new headgasket. On my 3.8 build I utilized the Cometic headgasket replacing the paper one associated with the 3.0.

A few other motors I've updated to the metal OEM headgasket.

Do you only use the 3.2L OEM MLS head gasket when increasing bore (i.e. to 93mm like 3.2L) or do you also use it with stock 3.0L cylinders (90mm bore)? I was tempted to include it in the list of gasket options, but the intended audience here is folks with mostly-unmodified 3.0L blocks. In that instance it seems less-optimal to use a gasket with larger-than-necessary bore. Even for changing bores this option seems a bit redundant to a Cometic gasket with similar specs.

Did you have every head resurfaced because every one was warped or just to clean up (and/or increase smoothness) to ensure a good sealing surface? How often do you have the blocks resurfaced?
 
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Did you have every head resurfaced because every one was warped or just to clean up (and/or increase smoothness) to ensure a good sealing surface? How often do you have the blocks resurfaced?


Piece of mind and Head resurface to clean up and ensure smoothness of a good sealed surface. Same concept as a clutch change. I have the flywheel resurfaced.

I used the Cometic gasket since it was donated to my project. Otherwise I would just have utilized an OEM metal.
 
On one hand this seems like a good idea to do, especially if it's time to do a TB/WP job. On the other hand, if those items don't need to be done, why bother?
I'm not advocating this as a preventative measure to be done with TB/WP. This is a "fix" - what to do once your HG starts leaking.

Many of us are going FI and finding that 350-420rwhp can be fun, rewarding and reliable.
Myself included, until my HG started leaking. I'm at 333rwhp, which is really 270rwhp when you're at 5000+ft altitude like I am and only 60k miles. So it is not like only high-boost, high-horsepower (400+), or high-mileage engines are experiencing this. Relatively-mild FI (like me) and normally-aspirated engines may also exhibit this failure...and those folks may want a simple fix (not a rebuild to achieve higher power).
 
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...........followed by "blue" smoke........:wink:
 
Can the heads be removed without dropping the engine?
 
Can the heads be removed without dropping the engine?
Yes. Dan-o-land illustrates such in this DYI (timing belt DIY gone wrong necessitates R&R of heads for repair). The service manual procedure for head R&R does not include dropping the engine.

However, I've seen a lot of experienced mechanics with shops say they drop the engine to make things easier.
 
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My stock, NA car popped the HG because a misplaced hose clamp following a all-hoses-replacement let some air into the cooling system... I am now running MLS gaskets as well as ARP studs and a brand new Koyo radiator and I am finally (almost) satisfied with the cooling.

Though at $249 that Performance Package coolant tank is looking very tempting indeed.
 
Lets say you have a boosted stock 3.0 could you slightly decrease compression to increase boost reliably?

This was vaguely covered above but not in much detail.

Would this be advised? if so what thickness gasket compared to oem and if you are settled on 400whp being a safe number how much could you increase that to perhaps?
 
Lets say you have a boosted stock 3.0 could you slightly decrease compression to increase boost reliably?

This was vaguely covered above but not in much detail.

Would this be advised? if so what thickness gasket compared to oem and if you are settled on 400whp being a safe number how much could you increase that to perhaps?

The biggest advantage (to me) in decreasing CR for boosted applications comes from the ability to run a more aggressive timing table.

It is possible to lower CR with a thicker gasket, but not necessarily a good idea. Thicker head gaskets are usually weaker, and they kill quench. It's better to use dished pistons to achieve a lower CR.

This is general theory and not NSX-specific info, FWIW. Experienced Primers please feel free to correct me if my previous experience doesn't translate.
 
Lets say you have a boosted stock 3.0 could you slightly decrease compression to increase boost reliably?

This was vaguely covered above but not in much detail.

Would this be advised? if so what thickness gasket compared to oem and if you are settled on 400whp being a safe number how much could you increase that to perhaps?

Yes, you could use a thicker gasket to reduce compression, but this would cause timing problems, as you are moving the centerlines of the cams further away from the crank centerline. You would need to use adjustable cam pulleys to compensate.
 
I'm scheduled to get my timing belt/water pump replaced in a couple weeks, I am also constantly toying with returning to FI. It sounds like replacing the head gaskets is a simple addition to the service I have scheduled, if its just a matter of parts why wouldn't I (or anyone) go ahead and upgrade? Parts cost?
 
I'm scheduled to get my timing belt/water pump replaced in a couple weeks, I am also constantly toying with returning to FI. It sounds like replacing the head gaskets is a simple addition to the service I have scheduled, if its just a matter of parts why wouldn't I (or anyone) go ahead and upgrade? Parts cost?

There is a decent bit of additional labor, namely removing intake manifold, removing headers or pipe after exhaust manifold, disassembling much of both heads (removing camshafts, rockers, etc...but not valves, springs, retainers) - all to make it possible to unbolt the heads from the block. Then putting that all back together.

HGs are not something I would personally have done preventatively. While HG leak is common relatively to other failures (oil pump gear, harmonic balancer, etc) it is still rare in an absolute sense and not cheap (even if 1/4th or 1/3rd of the labor is already planned). There are lots of engines, NA and lightly boosted, for which it may never happen. The majority also seem to be helped along in their failure by another factor like overheating (i.e. after blowing a hose and losing coolant) or FI at higher boost/power levels.
 
Great thread. Though I own an NA2, I just learned about the improvements and changes incorporated on the 3.2 and why my NA1 engine was so noisy in comparison. Thanks for sharing.
 
Quick follow up question. I just got in a set of the OEM head gasket kits for my repair (06110-PR7-020). They include the factory gaskets 12251-PR7-A01, manufactured by Ishino Gasket in Japan. This part number is designated as the replacement gasket for all NA1 engines 91-05. I attempted to research what material the gaskets are made from to no avail. On visual inspection, they appear to have a bronze-colored metal core with graphite coating on both sides.
 
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As mentioned in section on torque in first post of thread, the spec did not change. I thought it had changed to 71 but a reliable source (Maybe Larry B) said it was still 56. However, I have not independently confirmed this. Would be nice to inspect a 1997+ manual to verify.

Since you're talking about these spec numbers, safe to assume you're using (a new set of) OEM bolts?

Care to snap a pic of these gaskets and post?
 
Nope- I confirmed in the 1997 manual. The M/T spec is 71 ft/lbs. Since that is for the metal gasket, I will go with that number and we will torque to 25, 56 and then 71.

Also, a word on the NSX head bolts. After discussing with several NSX experts and Honda engineers, the NSX bolts are not torque-to-yield (TTY) and are designed to be re-used. TTY bolts are designed to stretch and provide spring tension to provide the required clamping force. Many Ford and Chevrolet products use TTY bolts. They are use-it-and-lose it bolts, meaning you must replace them after removal. Standard bolts like the NSX bolts are designed to use friction (along the inclined screw plane for you mech engineer folks) to provide the clamping force and are designed not to stretch. This is why both the 1991 and 1997 service manuals do not require replacement of the bolts.

Still, the experts advise to inspect and measure the bolts before re-use. Check the threads for any obvious signs of stretching or damage. Measure the bolt to make sure it has not elongated. If it is ok, you can re-use it without any problem.

I will try to grab a pic of the gasket.
 
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