Jumped timing chain

[Wally A.]

have a 350/350 small block and went to relash my valvetrain, on #1 tdc and all was fine, turned the engine over to adjust
#5 exhaust and found that it was not at tdc. rechecked to make certain i was on #1 tdc. so i turned the engine
over again to make certain i was at tdc, still 5 exhaust was not closed,,,,only thing i could think of was the chain has jumped.
safe to say i am correct?

[David M.]

Re: Jumped timing chain

Solid lifter engine?
Verify #1 compression stroke, not exhaust stroke.

Stick your finger in the plug hole and turn it over till it spits your finger out, (remote start switch recommended, cheap), Then line up TDC on the damper.

Verify the vibe damper outer ring hasn't delaminated.

[James G.]

Re: Jumped timing chain

Do you know if the timing set has been changed from the ORIGINAL???
I am assuming you have a 69-70 L46 - which was originally equipped with a Cast Zinc aluminum sprocket with nylon teeth on them.

I thought something was wrong with my SnapOn timing light when I was trying to set the timing on my survivor LT1 with 45k miles BUT no the gear was missing a couple of teeth when I swapped it out.

If the gear and chain have been changed -
Please do not take offense if the followig question seems ridiculous.
Did you ensure that when you rotated to TDC the FIRST TIME that #1 was TDC on the compression stroke?
If on approaching TDC the exhaust valve on #1 was closing then #1 is on the exhaust stroke.

[James G.]

Re: Jumped timing chain

Solid lifter engine?
Verify #1 compression stroke, not exhaust stroke.

Stick your finger in the plug hole and turn it over till it spits your finger out, (remote start switch recommended, cheap), Then line up TDC on the damper.

Verify the vibe damper outer ring hasn't delaminated.

GOOD call on the damper... mine had also spun on the hub which I had forgotten until mentioned.

DAMPER DOCTORS in california is one of the two largest remanufacturers of original dampers.

My TDC was way off the indicator(spun outer ring on balancer) and erratic as all get out (teeth missing on the cam sprocket).

[Bill B.]

Re: Jumped timing chain

To check the damper timing marks, I used a piston stop that screws into the spark plug hole. I approached it carefully so as not to make any impression mark on the face of the piston. I placed the crank at TDC according to the damper mark. Screwed in the piston stop and slowly adjusted the inner device until I could feel it stop. I marked the inner device position, then backed off 5 complete turns. I next rotated the crank 2 degrees in one direction, stopped, and slowly adjusted the inner device again inward and count the number of turns. It should be more than it was at TDC. Back off the device to the original position before the two degree rotation. Now rotate the crank a little to 2 degrees of TDC in the opposite direction. Repeat the measurement. If more than 5 turns, the timing mark is considered accurate as indicated.

If it is offset, you can repeat numerous iterations of various crank settings and determine TDC and re-mark the damper to at least know where you are before replacing/repairing the damper. If you find this is not working at all, your damper outer ring has probably spun significantly, and the finger technique Dave recommended is probably your best option of knowing where TDC is occurring.

[Wally A.]

Re: Jumped timing chain

sure i am on compression stroke, number 5 exhaust down, just to make sure i turned the crank one complete turn to bring up #6
at top center and still #5 exhaust still down.

[Bill B.]

Re: Jumped timing chain

Wally,

As you rotate the crank, does #5 Ex move? Is it ever fully up anywhere in the 720 degrees of crankshaft rotation? If not, perhaps a hung lifter in the bore or severely galled rocker assy or valve stem as cause?

[Joe L.]

Re: Jumped timing chain

Do you know if the timing set has been changed from the ORIGINAL???
I am assuming you have a 69-70 L46 - which was originally equipped with a Cast Zinc aluminum sprocket with nylon teeth on them.

I thought something was wrong with my SnapOn timing light when I was trying to set the timing on my survivor LT1 with 45k miles BUT no the gear was missing a couple of teeth when I swapped it out.

If the gear and chain have been changed -
Please do not take offense if the followig question seems ridiculous.
Did you ensure that when you rotated to TDC the FIRST TIME that #1 was TDC on the compression stroke?
If on approaching TDC the exhaust valve on #1 was closing then #1 is on the exhaust stroke.

James------


All Chevrolet small blocks from 1966 through at least 1982 originally used an aluminum camshaft sprocket with nylon teeth.

All Chevrolet big blocks originally used an aluminum camshaft sprocket with nylon teeth.

[Larry E.]

Re: Jumped timing chain

IF your engine IS original I would check your Distributor/Int. Manifold chisel marks to see
if you are close. Flint and Tonawanda has done the timing for you. I do not use a timing
light anymore on mine. See picture below. JMHO>Larry

[William F.]

Re: Jumped timing chain

You mean lining up those chisel marks produce the factory recommend initial timing advance?

[Mark E.]

Re: Jumped timing chain

have a 350/350 small block and went to relash my valvetrain, on #1 tdc and all was fine, turned the engine over to adjust #5 exhaust and found that it was not at tdc. rechecked to make certain i was on #1 tdc. so i turned the engine
over again to make certain i was at tdc, still 5 exhaust was not closed,,,,only thing i could think of was the chain has jumped.
safe to say i am correct?

Would you clarify what this means? When the engine is turned one revolution from #1 TDC, it will be at #6 TDC- NOT #5 TDC. With that said, with the engine at #6 TDC, #5 exhaust should be closed (according to the CSM valve adjustment procedure).

sure i am on compression stroke, number 5 exhaust down, just to make sure i turned the crank one complete turn to bring up #6 at top center and still #5 exhaust still down.

When you say "on compression stroke" do you mean #1 TDC or #6 TDC (or maybe #5 TDC)?

When you say "exhaust down" do you mean the valve spring is compressed down (valve open)? Or do you mean the valve is down in its seat (valve closed)?

At #1 TDC, #5 exhaust is open; at #6 TDC, #5 exhaust is closed. Depending where it is on the cam lobe when you start, a valve may stay closed after more than one engine revolution. But if it does not move through two engine revolutions, then there's obviously a problem.

How did the engine run before this work?

[Duke W.]

Re: Jumped timing chain

Most, if not all, Chevrolet engine hydraulic lifter cam valve adjustments can be made on 8 cylinders with the crank at TDC #1 and #6, TDC, end of compression stroke.

See the applicable year CSM for details.

Duke

[Patrick H.]

Re: Jumped timing chain

You mean lining up those chisel marks produce the factory recommend initial timing advance?

If the distributor and the intake are both original to each other, then yes.
Replace one or the other and all bets are off.

[Terry M.]

Re: Jumped timing chain

If the distributor and the intake are both original to each other, then yes.
Replace one or the other and all bets are off.

Replace the cam and all bets are off also. I am not sure about the timing set. That might ruin the setting also, but maybe not. I am not sure.

[Duke W.]

Re: Jumped timing chain

If the distributor and the intake are both original to each other, then yes.
Replace one or the other and all bets are off.

Be sure to set the dwell angle to spec first because changes in dwell due to point rubbing block and contact wear changes timing.

The other issue is, especially on emission controlled engines, timing is established for emissions, not performance or fuel economy. Pre-emission spark advance maps were also conservative and that includes initial timing.

Some engines specify a range, not a fixed value. In fact, the whole spark advance map, which includes the initial timing is a tuning parameter.

If you want best performance and fuel economy you can't beat and optimized spark advance map and a dial back timing light.

Duke