OT: Cylinder Head Replacement

I, too, buy from Rock Auto. Our passenger car is a 1983 MB 300SD with

About the broken cam, that is a strange tale! I have never owned a Toyota, but they seem to run forever. The engine probably had rocker arms, but I still don't see the connection.

Paul

No, it had a SOHC engine. The guy probably didn't set the crank and cam, then sequentially tighten the cam bolts to keep stress off the cam. It likely bent and broke.

-- Knowledge speaks, but wisdom listens. -- Jimi Hendrix

If it is an overhead cam with the valves and springs underneath it and then think about it. If you tighten a camshaft cap at one end and the valve at the opposite end is closed then you are using the length of the cam shaft as a lever to arm to close it. Cams are usually sort of brittle so....

Great, a chance to do some crappy ASCII art!

| equals a bearing cap O equals a cam lobe =3D eqauls cam shaft ^ equals where I start torquing

|=3DOO=3D|=3DOO=3D|=3DOO=3D|=3DOO=3D|=3DOO=3D| ^^^^^^^ ^^^^^ ^^^^ ^^^

If the lobes opposite of where I start torquing have a valve spring acting against them then the middle of the cam shaft is going to have a strong bending moment working on it.

I think you will find the SOHC Toyota engine of that time used rocker arms between the cam and the valves.

Paul

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And their customer support is top notch, too. I am really pleased to buy from them instead of my local dealers.

That's reassuring; it makes me feel less stupid. :)

--Winston

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But wouldn't it have snapped as soon as he started it then? He mentions that the car ran just fine for two short trips and only konked out during his third trip from work to home.

I'm still puzzled why the camshaft worked hunky dory for those first two trips. Confidentially, it sounds as if he cracked the camshaft on removal and the break completed during his third trip.

--Winston

(...)

I'll take your word for it. :)

Ah. So the shaft *could* have cracked during installation and separated during his third trip. It is starting to soak in.

Thanks!

--Winston

OK, Paul. I'll trust you on that. I was wrenching back then, but didn't do overhauls, so I never got into a little Toyonka engines. I do recall seeing things like that, though.

That was a whole 'nother lifetime ago.

-- The unexamined life is not worth living. --Socrates

You might be right. I seem to have entirely missed the part where it was ackshully _running_. Neeever mind.

-- The unexamined life is not worth living. --Socrates

(...)

But wouldn't it have snapped as soon as he started it then? He mentions that the car ran just fine for two short trips and only konked out during his third trip from work to home.

I'm still puzzled why the camshaft worked hunky dory for those first two trips. Confidentially, it sounds as if he cracked the camshaft on removal and the break completed during his third trip.

--Winston

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The materials used in those cams have changed since I played with engines, but stock cams today, and for a couple of decades, are usually ductile iron -- "selectively austempered" or otherwise. They're hard but...

It's good stuff but it has neither the strength nor the elongation of steel. It's sensitive to stresses for which it wasn't designed, and, with high spring pressures, it has a reputation for becoming marginal very quickly. Racers prefer steel for most engines.

So, that doesn't tell much about this situation, except that stressing one of those sticks in the wrong way can break them. A crack that wouldn't result in a break until the engine had been run for a while sounds unusual, but it's within the realm of DI properties. It propagates cracks more readily than steel.

This could all be a distraction from what actually happened, but it's something to keep in mind.

First of all if he had the head rebuilt the cam would have been installed on the head. Tightening the head down might have bent valves if he had the engine and cam in interference position.

But the guy says this

"I matched up the break on the cam and carefully bolted the cam back in place. It was a very rough and noisy ride the rest of the way"

That is someone who is either making things up or isn't isn't competent to accurately report what actually happened

-jim

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I searched a little yesterday regarding the situation. As you all say, these cams are abuse-intolerant, where 'abuse' can look fairly innocuous to the uninitiated. (Very peripheral reference to a temporary bolt installation procedure intended to flex the head enough to allow safe removal of the camshaft. Sheesh!)

This was extremely welcome information as I am looking at replacing valve stem seals on my '00 Camry. 'Sounds like a professionally rebuilt head would be a much faster and less frustrating way to go.

My first search uncovered a rebuilt head for $275. Do I want to disassemble mine to save say, $150? Nup!

Thanks!

--Winston

(...)

I searched a little yesterday regarding the situation. As you all say, these cams are abuse-intolerant, where 'abuse' can look fairly innocuous to the uninitiated. (Very peripheral reference to a temporary bolt installation procedure intended to flex the head enough to allow safe removal of the camshaft. Sheesh!)

This was extremely welcome information as I am looking at replacing valve stem seals on my '00 Camry. 'Sounds like a professionally rebuilt head would be a much faster and less frustrating way to go.

My first search uncovered a rebuilt head for $275. Do I want to disassemble mine to save say, $150? Nup!

Thanks!

--Winston

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Yeah, that sounds wise. You really have to know individual engines today to comment on fixing them -- not that it hasn't always been somewhat true, but it seems to be getting worse. The last engine I worked on at that level was the 3.0 Mitsubishi V6 in my '89 Dodge Caravan. I had to study it for many hours before I was ready to take the heads off.

As Jim says, this story on the whole sounds pretty strange. It's hard to imagine that you could get enough power delivered through the break in the camshaft to actually work the valves on the broken end. And if it held together when he reassembled it that way, how did it separate in the first place?

But if the story helped you avoid big trouble, then it's a good story, I guess.

Ah! You're right! 'Sounds like the rebuilder cracked the cam?

I dunno if the Tercel is an interference engine. Most Toyotas are not.

It's a puzzlement.

--Winston

(...)

Deep puddle. Looks shallow, Kemosabe. Ugh.

Perhaps it is an allegorical tale. 'Morally uplifting but not to be taken literally.

:) :)

--Winston

And the lesson learnt is "don't have your cylinder head rebuilt"??

(...)

Mystery solved. Thanks, Jim!

--Winston

"early '80s" means an "A" series engine. From what I remember the cam was on one side, the valves on the other, and a rocker assembly between them. It's been YEARS since I was last inside one.

As no one ha mentioned it so far, most cam shafts on sohc engines are now cast iron, with induction hardened surfaces of the cam lobes. Wether the cam shaft bears directly on the valves or via rockers, one needs to pull down the bearing caps a little at a time so as not to get the valve springs to load it incorrectly. then as someone else has said, one needs to put the head on the engine block with the pistons at the half way point in the stroke to ensure that the pistons dont foul the valves. Then you set the crank so that No 1 piston is at TDC with the valves for No 1 both closed. Then of course one follows the correct timing procedure via the book and appropriate marks. Much easier on older engines when the camshaft was gear driven. Then the gears had the timing marks on the gears. As I found out to my cost, if you dont change the cam belt on scedule, the cam belt breaks, breaking the camshft and bending the valves. That was on an 84 Peugeot 305 2ltr diesel. Hope this clarifies it a bit further.

Ted Dorset UK.