Robert wrote: <snipped} But replacing all head bolts can be quite costly and un-necessary. Especially 26 bolts X 6 dollars a piece. 48 lbs of torque on the ZX engine isn't that great and if a person follows the book, he should be ok. Make sure you are really clean with all surfaces. The bolts are tempered !! {snipped>

Head bolts are the same hardness all the way through, which actually isn't very hard (Rockwell scale) I am not certain that they are tempered, as head bolts can, do and will stretch if over torqued.

If a head bolt has NOT been subjected to excessive torque, it is most likely reusable a second or even a third time
.
Determining which bolts have been stretched requires no special tools other than sharp eyes
.
After thorough wire brush cleaning of the entire bolt, (this includes the torque surface under the head of the bolt) select any two bolts at random.

Pick up one bolt in each hand, holding each by the "head" end. Superimpose the threaded portions of each bolt over top of each other in such a way (hold them up to a bright light) as to be able to see the zig zag "line" which is formed between the threads themselves. This "line" is the light source illuminating the variation or deviation in thread pitch, width and depth
.
Worn threads (photo 1 ) will show a wider zig zag line. Un stretched, non-rusted or worn threads will create an almost non-existent "line" with each thread seating fully into the other. Superimposing just the first five or six threads (usually an unworn portion of any head bolt) will show you quickly what the pattern should look like. Superimposing ALL the threads will quickly reveal any stretched bolts.

The bolts always stretch just above the last thread which enters the block bolt hole threads.

This stretching causes the last three or four "peaks" of the threads to be wider apart than their un stretched brethren. These wider peaks will not allow the ENTIRE threaded area of each bolt to mesh fully with each other. The bolt which is stretched will allow for the first few threads to mesh into the other bolt, but not the entire bolt's threaded length. With a little practice, anyone can become proficient in determining bolt stretch of ANY TYPE of bolt using this method.

I also reject bolts which have ANY rust pitting anywhere along the length of the bolt. This rust can change torque twist transfer values during the torquing sequence.

Remember that in this situation, LESS IS MORE. Resist the temptation of add a few extra pounds of torque setting above and beyond the FSM recommendations. DO NOT continue to add "just a tich, a tad, a smidgin" of extra twist beyond the point where the torque wrench indicated that full torque has been achieved.

The reason that more is less is that dissimilar metal (cast iron versus aluminum block/head combinations) experience two profoundly different rates of expansion between cold and fully warm.

The head gasket must not only fully seal the combustion chamber explosions, it must also seal all the water passages as well as be subjected to dissimilar stretching forces coming from above and below.

Aluminum expands some three times fast than cast iron, so the tendency is for the head side to want to slowly "rip" the top side of the gasket's "fabric", over and over and over, each time the engine heats up and cools down. The fact remains that super smooth (mirror like) finishes to the head surface allow for a smooth transfer of the expansion without "tearing" the gasket fabric.

It is torque which seals a gasket, not adhesion or the clamping of two rough surfaces together that does the job of sealing. That is why I always re coat composition head gaskets with a spray emulsion of either copper or aluminum beads. The colloidal mix of either of these two metals in a sticky spray matrix, both seals small aberrations in the two clamping surfaces (head and block) as well as providing for heat transfer through the gasket.

This material also provides a "bearing" surface upon which the rapid expansion of the head can occur (repeatedly) without the attendant "tearing" effect that occurs without this coating. In short, excessive torque only speeds up the degradation of the head gasket.

As the all initial torque values and sequences are designed to be applied with the engine DEAD COLD, one should always re torque the head bolts when the motor is dead cold.

Very rapid failure of the head gasket will result if re-torques are done with the engine fully warm.

Note: The total allowable deviation (lack of true plane parallel flatness) of BOTH the head and block combined can not be more than three thousandths of an inch. This is two hair wide from an average persons head. (who still has hair) :)

This means that all cleaning of these surfaces must be impeccable. Clean all surfaces until all dark carbon/gasket buildup is removed. You should be able to see the original factory milling marks across the entire surface when the surface is correctly cleaned.
This usually requires sand paper, which adds abrasive and grit into the picture. Oiling the cylinder bores prior to commencing the cleaning process will help to capture this abrasive dust, allowing you to clean it out just prior to final assembly.

Be SURE not to allow this abrasive to enter or plug the two pressurized oil passages which exit the top of the block. One is near the number one cylinder and one is between three and four cylinder on the passenger side of the block.

Failure to do so will cause an immediate delivery of this abrasive into the cam oil journals where is can and will wipe out the head's cam journals!

Doing a good job of bolt and surface preparation will pay excellent dividends of longevity and reliability down the road.

Kim Blough Idaho Z Car

 

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