Why put screws back to their original holes?

Car mechanics usually mark the position of each individual bolt when removing the cylinder head and then, when reassemblying it, they put each bolt back to it’s original position.

Why not put the bolts back at random? Is it some old wife tale or is there concrete science behind this practice?

IANAMechanic but from a laypersons point of view I can imagine that the last thing you want to do is cross-thread or damage a cylinder head bolt. By putting them back in the original holes you know they should work OK.
Also, by marking them you have a fair indication that if they don’t line up anymore then perhaps there is a glitch in the re-assembly process.
I find that mechanics and hands-on engineers are practical people and these little “quirks” tend to have very good common-sense reasons (even they aren’t the ones that I just mentioned)

I once made the mistake of putting the bolts back random on the head of a Chevy 283 small block. There were 3 different lengths, but I only thought there were two different lengths. I managed to strip out one of the hole when I put one of the medium length bolts where a long bolt should go. I was a dumb 17 YO at the time. I’m older now.

Also head bolts do stretch. In fact there’s a few engines out there where it’s highly recommended to replace all the head bolts each time the head is removed.

Head bolts are supposed to stretch. They are designed that way and should never be re-used. This can also apply to Engine mounts, Flywheel, CV Axle to tranny, Rod bearing cap, Main bearing cap.

Actually, all bolts are stretch bolts. The tension of stretch is what holds the parts in place. The difference is how much you stretch the bolt. If the bolt is starting to yield, the grain structure is starting to change. The bolt hardens into holding the part in place without coming loose. A standard old bolt continues to stretch in use and actually gets a little looser with time.

More info here - http://www.gomog.com/allmorgan/stretchbolts.html

IANAMechanic either.

I just view it as good practice. Even when all the fasteners are theoretically the same, sometimes the real world throws a curve ball. In fact, last summer I replaced the valve cover gaskets on my antique car, and did NOT do my usual practice of keeping the bolts to their original holes. Sure enough, one bolt would only go back in its original hole. Somewhere along the way it may have been crossed or damaged as a set, and I had to work through all the possibilities until I found the one that fit. Fortunately a valve cover is not as critical as a head, but given the situation, even if it all fit together I might not have had a good seal at that location.

And that’s without accounting for the weird stuff you see sometimes, like when someone forces a metric fastener into an SAE hole of close size, essentially re-tapping the hole. Best to play it safe.

ETA: I realize the OP was asking about head bolts specifically, which are often TTY, but nonetheless appropriate I think. Oh, and IANAMechanic.

I spent over 40 years as a mechanic and have never heard of an actual mechanic doing this. As said above replacing the bolts is the preffered method on certain parts of the engine. We don’t always do it. Most mechanics I know just throw everything into a box and sort it out when they start reassembly.

Good way to get repeat customers. :wink:

That’s not true for all head bolt applications. It is true for torque-to-yield bolts. But for some designs the official instructions are to measure the length of the bolts, and replace any that have exceeded the spec, and for some other designs the normal procedure is to reuse the bolts.

I agree. Generally bolts are laid out by groups (head bolts, valve cover bolts, etc.) with note taken of any that need to go into certain locations due to difference in length or bolt head design.

This has not been the case in my experience. Too haphazard, too much chance of getting something significant mixed up, and too much time spent sorting them out.

Cleaning or assembling PC cpu coolers etc., I religiously put back the screws that haven’t disappeared in their home holes since I know that they at least worked once there.

I’ve never bothered with this on head bolts. Just keep track of the different lengths.

Now valve lifters, pushrods and rocker arms I always put them back exactly as they came off. In the old days of flat tappet cams it was absolutely critical that the lifters went back in the same hole as they wear in to their cam lobe (making each lifter unique). These days it’s mostly roller lifters which don’t care that much, but it’s still a good rule to follow.

I won’t comment so much on bolts, but if you look at internal engine parts it’s critical that they are replaced in the same location. One item that I can think of is valve stems. The Valve stems are all the same when put into a brand new engine, over time each stem will develop unique characteristics and you don’t want to mix up the order they were in. These characteristics could be stretching or a minute bend in the stem.

As far as bolts, It is important to note length, but as someone upthread, even if the bolts are identical if they are returned to their original locations you know for fact that they will thread. I’ve had the problem of an identical bolt not wanting to thread into a hole and having to switch the bolts around until I found the one that will thread in.

I am in the process of restoring a car now and i just pulled the engine. While taking it apart I have made sketches of each part and labeled the screw/bolt holes on the sketch as 1, 2, 3 and so on. Then I’ve attached tags to the bolts/screws with corresponding numbers. But then again I can be quite anal about such things.

One particular example I can think of is lug nuts. My older cars had rusty studs that would be finnicky as to the re-installation of the lug nuts. I assume that each nut gets galled in a unique fashion that, after time, will only fit the stud it came off of.

Some time after I began wrenching on my own vehicles, I developed the habit of re-installing all screws, bolts and nuts in their original locations. This habit has carried over to electronics, etc.

Late edit, by valve stems I meant the valve pushrods. These are the rods the cam lobes push up on that enable the valves to open and close. My apologies for the mix up in terminology. Apparently I’ve succumbed to a few brain farts tonight.:smack:

This, as an A&P mechanic who has rebuilt more then 1000 aircraft engines, I concur. It is one of the tricks I teach new mechanics in my shop.

obbn, Pictures help a lot when restoring a vehicle. In the days of digital pictures, when one can store a gazillion of them on one flash drive, take lots of them, and then take some more!! You will never be upset that you took to many pictures, but I guarantee that you will wish that you took some more! AMHIK!

No we don’t.
The only time I have ever done this was on a V engine timing case where there was something like 5 different length bolts. (Idiot design)
I grabbed a piece of cardboard and punched the bolts thru in a pattern resembling their location.

I’ve used the cardboard cut-out trick on a motorcycle crankcase cover before and it helped a lot. That one had three different bolt lengths.

As I’ve gotten older and learn from my mistakes, I tend to put bolts back in their original holes as soon as possible. Too many times, I’ve disassembled something, thinking I was going to put it right back together, and then run out of time or daylight, or find I need to order a part, and then not get back to the project for a week or two. It’s too easy to forget that there were two different sizes when you took it apart and often the difference are subtle and they’re all dumped into one coffee can.

This is news to me. And, we don’t know each bolt will go back into its hole. If it’s possible to cross thread them before, it’s possible to cross thread them during a later assembly. More to the point, preserving the use of a bolt that has been cross threaded by reinserting it in the same hole sounds like a way to lose a chance at preventing stripped threads. Unless the female threads are very short indeed, you probably can’t get the bolt all the way into place at the wrong angle anyway.

Back in the early 50’s, I accidentally swapped the heads (L to R) on one of the 331 cid hemi-head V-8’s, and it worked – I didn’t bother to swap them back, just tied the spark plug wires to the firewall, as the little strap for holding the 4 plug wires for each head was now on the front of the heads . . .