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Thread: Timing Chain/Belt/Gear

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    Timing Chain/Belt/Gear

    Anyone here who is familiar or knowledgeable with timing belts or chains on GM cars of the 70's ? [my 76 Chev]. Who knows when these components requires attention or replacing ? What are the symptoms of belt/chain/gears going bad, and how long they will last. I don't have my owners manual anymore.

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    The 350 small-block I had in my 1975 Impala used a timing chain (good) running over a fiber gear (bad). Over time, I expect the chain to wear the gear away, producing fiber shreds that build up inside the housing, until one day it fails entirely. I don't think these old pushrod engines were interference designs, so worst-case is that it strands you with a couple-hundred-dollar repair bill.

    I don't know how long it takes to fail. But you can do like I did as a teenager, and run it up to redline a couple of times a week, and it'll fail pretty quickly.

    Chip H.

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    What is meant by "....run it up to redline..." ?

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    Quote Originally Posted by ChevyMan View Post
    What are the symptoms of belt/chain/gears going bad, and how long they will last. I don't have my owners manual anymore.
    The classic symptom of a bad chain is that the engine runs okay, but takes a while to start, even with a good battery.

    Confirm the diagnosis by cranking the engine back and forth with a wrench on the crank nose while watching the distributor rotor. More than a couple degrees of hysteresis suggests that the chain has stretched.

    The plastic sprockets tend to die suddenly; the engine just stops, or jumps out of time and runs like junk for a few miles before stopping. Best to change them at <120,000 mile intervals just for fun. Same for fiber gears, except they don't jump time; they just stop.

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    Senior Member grouch's Avatar
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    Quote Originally Posted by ChevyMan View Post
    Anyone here who is familiar or knowledgeable with timing belts or chains on GM cars of the 70's ? [my 76 Chev]. Who knows when these components requires attention or replacing ? What are the symptoms of belt/chain/gears going bad, and how long they will last. I don't have my owners manual anymore.


    I worked on many of these back in the day. The factory timing chain and gears had a plastic coating on the crank gear. Over time, the heat of operation causes this to crack and eventually lose parts of plastic which will let it jump time. Sometime after 100,000 miles, you'll need to replace it. This is the absolute cheapest engine to repair. If it breaks, your engine won't start but will spin really fast since there will be no compression.


    When and if you replace the chain and gears, use a doubler roller timing chain. That will probably outlast the rest of the car. Outside of some seals on the timing cover, there's not much else that needs to be done inside. You water pump will come off to do the repair so if there's any play in the shaft, sla a new one on when the job is done. If you're paying a mechanic, the labor will already be done. Personally, I usually drop the oil pan and clean any junk in it out. Since your car may have used leaded fuel in the past, watch for a grey sludge in the pan. That is the remnants of tetraethylead. It's VERY toxic. Being a '75 though, there may not be much in there.
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    Vulture of The Western World Eric's Avatar
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    Quote Originally Posted by ChevyMan View Post
    What is meant by "....run it up to redline..." ?
    All that means is revving the engine to its maximum recommended safe RPM as indicated by the "redline" on the tachometer. Your car probably doesn't have a tachometer though. But don't worry, unless you floor it - and keep it floored through all three gear changes (or manually put the transmission in Low or "'2" and prevent it from upshifting as you increase your speed to 50, 60 or 70 mph...) you're never getting even close to the redline of your Chevy (which is probably about 5,000 RPM if memory serves).

    Under normal part-throttle driving, the transmission will upshift well before you get anywhere near redline. Even 3/4 throttle (in your car) should keep the engine under 4,500 rpm - so nowhere near its redline.

    PS: It's very hard to exceed redline with an automatic transmission (provided it's left in Drive, at least). It will change gears (upshift) as RPM increases, even under full throttle. If you kept it "floored" and ran the car to its top speed, you might approach redline. But usually by now aerodynamic resistance (or not enough engine power) will prevent the car from building up any more speed, so engine RPM will stay where it is - high, but not beyond redline.

    In cars with manual transmissions, you can easily over-rev the engine, possibly causing serious engine damage. The classic case is accidentally downshifting into second when you really wanted fourth...

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    or manually put the transmission in Low or "'2" and prevent it from upshifting as you increase your speed to 50, 60 or 70 mph...
    I'm not saying I did that. But I'm not saying I didn't, either.


    Chip H.

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    Vulture of The Western World Eric's Avatar
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    Quote Originally Posted by chiph View Post
    I'm not saying I did that. But I'm not saying I didn't, either.


    Chip H.
    My Trans-Am has a "full-manual" valve body, which means I can hold it manually in any gear, as long as I like - until something breaks, anyhow!

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    Quote Originally Posted by Eric View Post
    All that means is revving the engine to its maximum recommended safe RPM as indicated by the "redline" on the tachometer. Your car probably doesn't have a tachometer though. But don't worry, unless you floor it - and keep it floored through all three gear changes (or manually put the transmission in Low or "'2" and prevent it from upshifting as you increase your speed to 50, 60 or 70 mph...) you're never getting even close to the redline of your Chevy (which is probably about 5,000 RPM if memory serves).

    Under normal part-throttle driving, the transmission will upshift well before you get anywhere near redline. Even 3/4 throttle (in your car) should keep the engine under 4,500 rpm - so nowhere near its redline.

    PS: It's very hard to exceed redline with an automatic transmission (provided it's left in Drive, at least). It will change gears (upshift) as RPM increases, even under full throttle. If you kept it "floored" and ran the car to its top speed, you might approach redline. But usually by now aerodynamic resistance (or not enough engine power) will prevent the car from building up any more speed, so engine RPM will stay where it is - high, but not beyond redline.

    In cars with manual transmissions, you can easily over-rev the engine, possibly causing serious engine damage. The classic case is accidentally downshifting into second when you really wanted fourth...
    Then all this mean is that I'm safe as long as the RPM doesn't approach near or go over the redline , as normally, I'm a 'slow' driver, that, plus the odo is still under 55K, and timing chains/belts usually lasts well beyond the 100K mileage. That's a relief to know! Larry

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    Vulture of The Western World Eric's Avatar
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    Quote Originally Posted by ChevyMan View Post
    Then all this mean is that I'm safe as long as the RPM doesn't approach near or go over the redline , as normally, I'm a 'slow' driver, that, plus the odo is still under 55K, and timing chains/belts usually lasts well beyond the 100K mileage. That's a relief to know! Larry
    Given just 55k and the relaxed life your car leads, I doubt you'll need to worry about that timing chain for many, many years to come!

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    Quote Originally Posted by Eric View Post
    Given just 55k and the relaxed life your car leads, I doubt you'll need to worry about that timing chain for many, many years to come!

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    That's a relief!!

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