Never Say Never. Assume You Will Break Shit
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For those of you keeping up on WrongWheelDrive’s adventures in Miata repairs, things went south last week. He was doing a cylinder head job as he’s been dealing with a constant, but minor miss-fire on cylinder two for a while. And now he needs to smog his car. So it’s time to fix it. So a few weeks ago he brought it to me and I did a leak down test and found cylinder 2 was in fact completely borked. But figuring out WHERE the cylinder was leaking was a mystery. It sounded to me like it was coming out of everywhere. But after consulting a friend of mine with WAY more years of experience, he advised WWD to pull the head, which he did.
He then brought the head to my friends shop who found a burnt exhaust valve on cylinder two. This is good news since it’s an easier fix than piston rings. So he took the head to a machine shop and replaced the valves.
Now comes the part where I should have stepped in as I obviously have more experience than WWD when it comes to installing cylinder heads. I’ve done both timing belt and timing chain (SCARY) cylinder head replacements, so I know my way around. But WWD never expressed concern, and he is competent, so I let him do it himself.
So he gets the car all back together but it won’t start and cranking sounds like shit. It’s hard to tell over a cell phone video but I suspect it sounds like no compression. He runs a compression test and cylinder three doesn’t look good. So he re-checks the timing belt and discovers it’s way off. The exhaust cam is off a tooth and the crank is off another tooth. So I offer to come up, re-do the belt installation and try again. But I realize his no-start issue isn’t actually related to the no compression on one cylinder. The engine should have started. It would have run like shit on three cylinders, but it would have run nonetheless.
So I redo the timing belt. Before going up I read the instructions and watched videos. The written instructions made zero sense, but in practice it looked easy. So I went up and I was actually irritated how easy it was. Most timing belts are a pain in the ass. It’s a balancing act of keeping tension on the belt while not moving any of the gears AND keeping the belt on the pulleys while you work on the next one. Obviously I’ve developed tricks over the years (that I learned from others), like using a binder clip on the cam pulleys to keep the belt on. But the Miata engine is SO easy I didn’t need any of that. The belt literally slipped on and I was done.
So we finished putting it back together and despite one major issue (NO MAGNETS ANYWHERE TO BE FOUND) where I dropped a bolt into an abyss and couldn’t grab it, I didn’t think to bring a magnet screwdriver. Everything else went fine until I got to putting the crank pulley back on and I told WWD about a post I read where someone said ALWAYS REPLACE THE PULLEY BOLTS BECAUSE fRAgILE! I laughed. I’ve never broken a single bolt in my years working on cars, and I always use impact and power tools. I assumed (along with everyone else in the post) that the guy was an idiot who overtorqued them. And as I’m saying this the inevitable happened. I broke one of the bolts. So either the guy online was right, OR, more likely, the bolts had been boogered so many times that all it took to snap was one last ugga dugga from me, and I was the smartass lucky enough to be there.
But I soldier on as it’s getting late and I’m exhausted. We run into a few more issues like WWD buggering up a coolant hose clamp, and I didn’t bring replacements (I have all this shit at home…) so that means I can’t vacuum test it and fill it up. So I have him go to start it and it sounds exactly the same.
Fuck.
So we run a quick compression test and cylinder 3 has zero compression. And it also still won’t start, but I know this is a separate issue as I stated before. So I go home and consult with my friend the next day and we go over the issues we found. And this is where the wisdom and experience of my friend comes in.
The Miata is a non-interference engine. Essentially what that means is if the timing belt breaks you won’t bend/break any valves because there’s no overlap between TDC of the piston and full open of any of the valves. WWD knows this, and the Miata forums swear up and down you can’t bend the valves by screwing up a timing belt job. But my friend, who has been working on cars for decades, knows never say never. Just like I shouldn’t have been laughing about that one online dude breaking a bolt. So based on the information I gave my friend he’s almost positive there’s bent valves, even if it’s not clear from the outside (the cam followers have no excessive gap). But we have WWD pull the head off again and bring it to him, and he found the valves on cylinder 3 leaking, and partially on four.
So the lesson here kids is NEVER SAY NEVER. Just because the forums said something can never happen, doesn’t mean it can’t with determination. And don’t listen to a technician who laughs at the idea of breaking bolts just because hasn’t broken one YET.
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I put a cylinder head on my Datsun 240Z back in the early 1990's and we got the timing 90 deg off. So when I turned over the engine I bent 6 exhaust valves. It was not a good day.
And you may ask yourself, "Where does that highway go to?"
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@highlander said in Never Say Never. Assume You Will Break Shit:
I put a cylinder head on my Datsun 240Z back in the early 1990's and we got the timing 90 deg off. So when I turned over the engine I bent 6 exhaust valves. It was not a good day.
I'll bet you never made that mistake again and you checked the timing multiple times before you even thought about putting it back together.
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Just because I used to have one, and know about it: The Miata pulley bolt issue is limited to the early 1.6 motors. For some stupid reason, the nose of the crankshaft only extends about half the width of the lower pulley. The bolt tightens the pulley against a shoulder on the crank. Over-tigten it, and you'll strip or split the crank nose. Under-tighten it, and the key will wiggle in the slot and cause wear and you'll never get it timed right again.
The "fix" was to always replace the bolt when removing the lower pulley, and always Loctite the key in place, as well as using Loctite on the bolt, and use a good accurate torque wrench. It shouldn't be an issue on the car in question, because it's a 1.8, but it's become common to recommend always replacing the crank bolt on all NA/NB Miatas, even if it's not strictly necessary.
Not that this has anything to do with interference/non-interference, but just as an explanation...
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Oof - these are the stories that have kept me from ever taking a head off. I've done a lot of other mechanical work, but this just terrifies me.
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The binder clip trick would have been a god send when I did the 4Runner's timing belt. That shit pissed me off, was constanly off a tooth until I had my wife hold the belt in place.
2014 Mini Paceman S
1995 Ford Probe GT -
@highlander OUCH!
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@rctothefuture I've used zip ties on a prior timing belt change. The engine was out at the time which made it much much easier.
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@his_stigness I replaced timing belts, and or head and or injector pumps on VW diesels a couple of times many years ago. Very tricky. I proceeded with thoughtful caution and at one point even gently rotated the crankshaft without the belt in place to confirm where the interference would be. It's been a long time... Is there any chance the machine shop was borkey?
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@66p1800inpieces good idea too... I was tired and just wanted to get the belt on as the tensioner had kicked my ass. Good tips for the future.
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@his_stigness Ugh. This makes me scared to do mine, despite as you saw, it supposedly being easy. And who cares if you bitch it, its none interference! shudder
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@mark-tucker said in Never Say Never. Assume You Will Break Shit:
Just because I used to have one, and know about it: The Miata pulley bolt issue is limited to the early 1.6 motors. For some stupid reason, the nose of the crankshaft only extends about half the width of the lower pulley. The bolt tightens the pulley against a shoulder on the crank. Over-tigten it, and you'll strip or split the crank nose. Under-tighten it, and the key will wiggle in the slot and cause wear and you'll never get it timed right again.
The "fix" was to always replace the bolt when removing the lower pulley, and always Loctite the key in place, as well as using Loctite on the bolt, and use a good accurate torque wrench. It shouldn't be an issue on the car in question, because it's a 1.8, but it's become common to recommend always replacing the crank bolt on all NA/NB Miatas, even if it's not strictly necessary.
Not that this has anything to do with interference/non-interference, but just as an explanation...
I didn't break the main crank bolt, I broke one of the four(?) 10mm pulley bolts.
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@jminer said in Never Say Never. Assume You Will Break Shit:
Oof - these are the stories that have kept me from ever taking a head off. I've done a lot of other mechanical work, but this just terrifies me.
It's just something that requires A LOT of research beforehand. And asking MANY questions to guys who have done them before.
Timing belt heads are really simple compared to timing chain ones. I've only done one timing chain head and it was my own W202. Chain heads are scary as shit because you're not taking as much apart, and they're infinitely more complicated. BUT, with the right tools and the right amount of research it's still not an issue. I did my cylinder head by myself and had zero issues starting it up, and it took me the book time of 10ish hours, which is impressive in itself because the common saying is usually double the stated book time when attempting a big job on the first go around.
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@rctothefuture said in Never Say Never. Assume You Will Break Shit:
The binder clip trick would have been a god send when I did the 4Runner's timing belt. That shit pissed me off, was constantly off a tooth until I had my wife hold the belt in place.
Yup. V engines are infinitely harder to do without four hands. With the clips they're relatively easy, it's just time consuming to get it right.
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@His_Stigness Why did the valve burn in the first place?
...meh...
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I hate timing belts
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@vincentmalamute said in Never Say Never. Assume You Will Break Shit:
@His_Stigness Why did the valve burn in the first place?
It's really common to burn exhaust valves on higher mileage motors. It's not out of the ordinary at all.
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@dogisbadob said in Never Say Never. Assume You Will Break Shit:
I hate timing belts
If we're talking about head jobs I'd rather deal with a timing belt than a chain. But overall I'd rather deal with a chain. Chain replacement is almost easier than a belt since it doesn't need regular replacement.
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@vincentmalamute said in Never Say Never. Assume You Will Break Shit:
@His_Stigness Why did the valve burn in the first place?
Miata engines are a wear item with a 120k mile service interval.
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@qaaaaa It occurred to me that valves are just a wear item from @His_Stigness 's response but seriously? Replace valves every 120K?
...meh...
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I had much better luck. Years ago I bought a poorly running '94, seriously loping idle and barely would keep going. Thought the engine was toast, but I was buying it for parts anyway.
Towed it home and thought I'd just check the timing to see if that could be a problem. Knowing these were non-interference I figured there wouldn't be any damage if that was the case, and maybe I could get a running engine to sell out of the deal. Lo and behold, timing is off a tooth on both intake and exhaust pulley. slipped the belt off, reset it, ran like a damn top.
ended up selling the car whole after fixing a few other issues it had.
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@vincentmalamute said in Never Say Never. Assume You Will Break Shit:
@qaaaaa It occurred to me that valves are just a wear item from @His_Stigness 's response but seriously? Replace valves every 120K?
No no no, the entire engine. For real though, for NAs and NBs, engine failure around 120-160k is very very normal. Luckily engines are cheap, readily available, and very easy to replace.
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@his_stigness Yeah, learned my lesson the hard way but learned it never the less.
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Whooooo-boy! Big-time learning experience! What a sucky experience, but at least you're learning.
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