emeryz28

I pulled the heads on my '92 Z-28 5.7L (L98) and discovered that the #8 cylinder had several small pits on top. It was the only piston with pits on it. Also, the #8 cylinder wall was the only cylinder wall that had some minor scoring on the opposite side of the piston where the pits appeared. I could feel the scoring by running my finger nail over it. (See attached pics)

It was not burning any oil and there was no antifreeze in the exhaust. The heads were pulled to reseal a leak from one of the head bolts near the #8 cylinder to the outside of the block. I don't believe I used enough sealant on the head bolts when I did the heads back in 2002.

By the looks of the pictures, can anyone give me some thoughts on the degree of damage here and what would cause this kind of pitting/scoring?

Thanks!

emeryz28

I think I may have partially answered my own question. I just looked at all the plugs that came out. The #8 plug had half the electrode missing (picture). I can see this making the pits in the piston, but I still have some concern about how much damage is really there. Maybe it's nothing, but I'm not used to looking at the tops of pistons and I am hoping I could hear from someone with some experience in this area.

Also, since the leaking head bolt was right next to the #8 cylinder, if water did happen to enter the chamber (I sure didn't see any smoke or smell any antifreeze in the exhaust), could it cause some of this scoring to the cylinder wall?

Damon

A piece of debris went through the chamber while the engine was running. Those divots are where that debris got "squeezed" between the top of the piston and the flat "quench" area of the cylinder head above it at TDC.

The scoring on the cylinder walls was probably smaller pieces of related debris that found their way into the top ringland and rode up and down with the rings a few times, cutting those grooves.

88 vert guy

I happen to work on pistons for a living. No doubt the divots you see are a result of the failed plug pieces that became trapped in the squish area. Concerns with the divots are two fold. 1) sharp edges may create hot spots which may negatively affect combustion (pre-ignition or detonation), and 2) they may create a micro crack initiation site, especially if located directly over the pin. I've seen a case where a very small piece of ceramic created a very sharp divot above the pin, and literally cracked the piston in half. By the looks of it, you may be OK, but you may want to CAREFULLY knock down any raised burrs. Avoid using sharp tools and disturbing the piston crown

If you had coolant leakage into the chamber, it would have a tendency to "steam clean" the chamber and remove a lot of the carbon from the crown. What did #8 look like compared to the others? How does the chamber on the head look like compared to the others? Depending on size of leak, over time the continuous compromise of the oil film layer on the bore wall can lead to top ring scratching or scuff, which could have the effect shown. However, also entirely possible that debris (the ceramic) got trapped in the crevice area and scoured the wall as suggested. In my experience, I have also seen "carbon scratching" where if enough carbon builds on the piston in the crevice area (it becomes very hard), it will cause scratches seen. Some of your scratches appear to go above the top ring travel.

I'm not familiar with the head bolt condition on this engine. Are you saying it has a "wet bolt" that goes into a coolant or oil passage and therefore MUST have a thread sealant? Don't recall that being the case, but let us know. What did the head gasket look like in this area? Could you visibly see a leakpath to the combusion chamber?

How many miles on this engine? How long had it been running with this condition?

emeryz28

Thanks for the replies. This is great information and will help me to deal with the problem.

What did #8 look like compared to the others?

It looked the same.

How does the chamber on the head look like compared to the others?

I dropped the heads off at the machine shop before I really took the time to check them over closely.

Are you saying it has a "wet bolt" that goes into a coolant or oil passage and therefore MUST have a thread sealant?

I think one of the bolts back there goes into a water jacket because as I was removing the head bolt, coolant surged out of the hole. I have asked a number of people about the use of silicone sealant on the head bolts and have received incredibly divergent answers. There seems to be no consensus.

What did the head gasket look like in this area? Could you visibly see a leakpath to the combusion chamber?

No leak path to the cylinder was present and the gasket looked to be in pretty good shape.

How many miles on this engine?

This engine has over 200,000 miles on it but I have changed oil every 2,000 miles using Mobil 1 since I drove it off the showroom floor in May 1992.

How long had it been running with this condition?

The leak had been an ongoing issue for three years. I used the CRC head gasket sealant to buy some time and it seemed to work up until a few weeks ago.

88 vert guy

Sorry, didn't see your model year and mileage info below your note until now.

My : With that amount of mileage and the fact that you've changed oil religiously with really good stuff, probably not worth the time and expense to take your engine apart completely to correct the condition. You may get a little bit more blowby past the top ring, but as long as your oil consumption isn't affected too much, who cares. Running Mobil 1 this amount of time, I'm sure your rings and ring grooves (as well as many other critical parts) are probably in decent shape. How did your valve seats look? If it were me, knock off the sharp edges, put it back together and go. Have you checked Service Manuals, or elsewhere in TGO about whether the engine has a "wet" head bolt?

emeryz28

I have to talk to the machinist tomorrow about the condition of the heads. I am inclined to take a chance and slap the old heads back on because I don't think I have much to lose. Your idea about filing down the jagged edge on the piston is very helpful and I will do that.

With regard to the "wet" head bolt question, the factory service manual says:

Coat the threads of the cylinder head bolts with sealing compound (GM part number 1052080 or equivalent).

I presume that all the head bolts should be coated with sealant, although I didn't cross reference the GM part number to find the equivalent sealant to use. Do you happen to know what the equivalent sealant would be?

An interesting note in the shop manual, and I think I may have made a mistake because I listened to the guys at the dealer and didn't use any sealant on the head gasket, is that I used an original factory head gasket which I think is steel. The manual says:

On engines using a steel gasket, coat both sides of the new gasket with a good sealer. Spread the sealer thinly and evenly. A paint roller is a good tool for this job. Too much sealer may hold the gasket away from the head or block. Do not use a sealer on engines using a composition gasket.

I didn't use any sealant on the gasket the last time I replaced the head gasket 9 years ago. Maybe the sealant would have stopped the leaking bolt.

emeryz28

I did some research and found that the GM sealant 1052080 was replaced by GM 12346004. The equivalent Permatex part is 59214.

http://www.permatex.com/products/Aut...ad_Sealant.htm

TTOP350

iTrader: (8)

Clean your threads in the block and the bolts using thread chasers. I Use teflon paste on the bolts. Stock type head gaskets go on dry.

http://www.google.com/products/catal...d=0CDgQ8gIwAQ#

emeryz28

I don't have a thread chaser. Do they make one specifically for head bolt threads?

88 vert guy

emeryZ28, I'll try to address some of your points.
First of all, I cringed when you said "file". No files or anything sharp! Put pistion at TDC, then cover the crevice around the perimeter with tape or something. Then take a small 3/8" or 1/2" dowel and wrap a piece of fine or very fine emery cloth around it and gently knock down the raised material. Then using fingers only, soften any sharp corners. Be gentle, just get rid of any sharp corners, removing only the minimal amound of material necessary. Wipe surface clean of any grit. If you don't you'll get more scratches on your bore wall; Aluminum oxide or whatever material you'll is quite abrasive and you must not leave a trace. If this was my project, I'd vacuum around the crevice using a stiff nylon detailing brush to dry to loosen any crud. I would vacuum all cylinders vs using shop air since shop air has a tendency to blow debris into coolant passage and into adjacent cylinders (which all can't be at TDC).

Regarding "Wet". While anything's possible, I doubt the chevy small block has this. Since I presume you drained coolant, there's one easy way to find out for sure: put some water in the hole! If it drains down, then you got a wet hole (doubtful).

I may be wrong, but I believe the purpose of the "stuff" on the bolt threads is torque (clampload) retention. The other possibilty (to be confirmed by guys who know these engines inside out) is maybe bolts are notorious for getting moisture from cooling passages and will rust and seize.

Regarding head gasket: Long ago in another life, I used to be a powertrain sealing engineer working mostly on exhaust manifold and head gaskets. Around that time the U.S. was beginning the transition over to multi-layer steel head gaskets with rubber coatings. Those gaskets are superior, but have a much lower tolerance for surface imperfections and roughness typical of what was acceptable for fiber gaskets. Did your engine originally come with a composition (fiber) head gasket? I know during this transition period, a steel gasket could have been offered for Service on engines that originally had fiber gaskets. The conditions of some the block and head decks can be quite poor from porosity (when new) or pitting due to cooling system neglect and surface finishes were generally rougher (when new). So, in those instances, manuals or TSBs stated to clean deck surfaces carefully (no die grinders or sharp tools), then apply a coating of sealant. The sealant was a "belt & suspenders" to fill in all the imperfections. Engines new from the factory with steel head gaskets have MUCH better deck flatness and smoothness characteristics. CAUTION: More is not better! Too much coating and material will flow and creep and you'll loose clampload (not good!) If you didn't use a sealer and original gasket was fiber and replacement gasket was steel, you probably should have used sealer. Don't use sealant on composite gasket.

Regarding TTOP350's suggestion - great suggestion. Make sure holes are clean afterward. Blow out. If head bolts are torque-to-yield, you may want to consider changing those out. Don't know what head bolt torque spec is, but anything increasing friction (ie, rust) will take away from clampload for the torque only portion. If you have a torque+angle spec, the angle portion is unaffected by friction.

Damon

When you take out one of the bottom/rear head bolts coolant will spill out even if your drained the rad. That's normal, if that's what you observed. Only way to prevent that happening is to pull the block drain plugs down by the pan rails before removing the head bolts.

SBC head bolts are not torque-to-yeild. You can use them repeatedly.

ALL head bolts on a small block Chevy go into the water jacket. That's one of the reasons you need sealant on the threads. Using the right sealant ensures proper fastener "stretch" when recommended torque is applied since it also acts as a thread lubricant.

88 vert guy

Thanks for setting me straight...now I know if I find myself removing heads. Is torque spec straight torque, or torque + angle?

InfernalVortex

iTrader: (4)

If it was running okay when you tore it down, I wouldnt worry too mucha bout it. I would try the most gentle method you can think of to just round off any sharp points on the piston, and make sure get all the fine particulate debris OUT OF THERE before you re-assemble.

Whenever you rebuild that engine the pistons will be replaced and the cylinder walls overbored, so neither will matter. Any performance it robs and oil consumption it causes has already happened, not much you can do now. The only reason you smooth out that damaged area is to prevent detonation which may or may not even be happening.

I guess waht I'm trying to say is the damage is done, it's still running and it'll likely live and run just fine for a long time to come, so dont fret about it too much.

Why did you tear it down anyway?

emeryz28

sjaluia: I will use Emery cloth to knock the dent down and vacuum up afterwards. The original and first replacement gaskets were metalic with a thin coating of what looked like gray paint, possibly to keep it from rusting? The closest thing I can describe it as would be the coating they put on replacement rotors for European cars like Mercedes to keep the rotors from rusting. In any case, I will use the standard Felpro composite gasket this time. The head bolts are torqued to 92 n-m (68 ft-lb), according the factory manual.

Damon: I figured many of the bolts went into the water jackets but I didn't know that all of them did. I wasn't as careful last time to make sure I applied enough sealant on the head bolts. This time I will pay special attention to this and use the correct sealant.

InternalVortex: I tore it down because of a leaking head bolt in the rear, somewhere around the #8 cylinder. My plan at this point is to see what the machine shop says. If it is going to be more than $250 to redo the existing heads, I'll probably just order the Edelbrock Performance aluminum heads (60879) for $654 and see how they work.

Thanks, guys, for all the great information. This is all very helpful.

emeryz28

FYI: Here's a picture of the old head gasket. I got it from the dealer back in 2002 and it was the GM specified replacement gasket. Sure does look different than the Felpro.

88 vert guy

emeryz28 and TGO audience:
The gasket in the photo is a typical composition-style head gasket. Construction of these is usually a perforated steel core on which a layer of hi-temp dense material or graphite is applied on each side (imagine all three layers going into two rollers to compress together), then the steel "fire rings" are applied around each bore opening. Some gaskets have an elostomer coolant/oil openings. A variation of this is material is bonded to a flat steel core.

Here's what a Multi-Layer Steel head gasket looks like:
http://www.jegs.com/p/Fel-Pro/Fel-Pr...58649/10002/-1 (go to item #375-1142, then click on the image, then click on the smaller cut-away image).
Though the coating is a type of rubber, it's very thin and can be mistaken for paint. Sealing stress is achieved by the embossed beads around each opening and the rubber coating is what creates the "microseal" into the deck surfaces of the joint. When putting these on older engines originally with a composition gasket, an OE may recommend a light coating of roll on or spray on sealant/adhesive. When doing this surface prep is key.

InfernalVortex

iTrader: (4)

MLS is better but VERY picky about surfaces. I'd personalyl stick with a composition gasket if you can get one in the thickness you need.

Measure how far in the hole your pistons are, and subtract that from .040 inches.

emeryz28

Thanks for all the information. I wish I had known about the need to apply sealant to this kind of head gasket 9 years ago. I think I will go with the standard Felpro gasket this time and I won't use any sealant except for the head bolts.

Motogyro

Wow that sucks. I hope you don't have any more trouble after you put the new gasket on.