Valve Timing Confusion

[01/65789]

Hi Everyone,

 

I have a problem that I'm hoping someone can help me with.

 

I'm currently rebuilding my Herald 13/60 engine, GE76714HE. It was running before I pulled it out of the car. The engine has covered 53,000 miles and, from what I can tell, had never been disassembled before (!)

 

Things had been going well: I am dotty about 13/60's, have lots of experience with this model and this is my third GE engine rebuild since 1978. But now this issue has arisen and it's driving me mad.

 

1. First I found TDC using a piston stop (marking the crank pulley when the piston hit the stop, turning the crankshaft in the opposite direction until the piston again hit the stop, making another mark on the crankshaft pulley at this point then finding the mid-point between the two marks to give true TDC. It coincided with the TDC mark (hole) already on the crankshaft pulley anyway, but I wanted to be sure).
2. I fitted the timing chain after I got the timing mark on the crank gear wheel aligned with those on the camshaft gear wheel and on the front of the engine block, just like they were before I disassembled the engine.
3. Then I thought it would be an interesting exercise to use a dial gauge to confirm that the valves were opening and closing according to the camshaft spec (I'd never done this with the previous engine rebuilds) ... and this is where things got frustrating.

The standard camshaft fitted to these engines is an 18/58 58/18. I understand this to mean that:

The inlet valve opens 18 degrees BTDC (ie the valve just begins to lift off it's seat at this point). 

The inlet valve closes at 58 degrees ABDC (ie the valve just touches back on its seat at this point). 

The exhaust valve opens 58 degrees BBDC

The exhaust valve closes at 18 degrees ATDC

 

So I set up my dial gauge on the second cam lobe from the front of the engine (No 1 inlet valve), fixed a degree wheel to the crank pulley and noted when the inlet valve opened and closed. This is what I found:

 

The inlet valve opens 58 degrees BTDC (as the piston is coming up on the exhaust stroke).

The inlet valve closes at 78 degrees BTDC (as the piston is coming up on the compression stroke).

 

Maximum lift is 0.217" (slightly less than the spec due to wear I guess) and maximum lift occurs at 110 degrees (which is exactly as spec).

 

First I suspected a non-standard cam, but the number stamped on it (212539) indicates a stock cam for this engine in the parts catalogue. I have two other stock cams and I measured and compared them. They are identical.

 

I'm sure I'm making a mistake somewhere, but I've checked everything goodness knows how many times...

 

As I said earlier, the crankshaft is set to the cam exactly as it was before, according to the scribe lines.

 

Any help would be appreciated to help put me out of my misery!

 

Cheers, Chris

[Pete Lewis]

I would have done a equal lift and protractor on the cam to check the marks are correct for exact lobe tdc

 

out all day sunday on an area outing so more thought later on

 

pete

[01/65789]

Hi All, 

I took Pete's advice and decided to set the crankshaft to the camshaft by the method of equal lift, ignoring the scribed lines on the crankshaft and camshaft sprockets and that on the front engine plate and also ignoring then differences between what my degree wheel was telling me about when the inlet and exhaust valves open and close and what the camshaft spec says it should do.

 

This is what I did:

1. Found and marked true TDC. 

2. Made a makeshift pointer (a bit of coat hanger wire bolted to the front of the engine)

3. Put a tappet onto cam lobes 1 & 2 (exhaust and inlet valve of no. 1 cylinder)

4. Placed another tappet upside down on top of the tappets already installed, so that the dial gauge could reach and therefore measure movement of the tappets.

5. I turned the crankshaft while holding a finger on each tappet, feeling for the part of the cycle where, as the piston is coming up the cylinder, the exhaust valve is closing as the inlet valve opens (the overlap area). This told me I was roughly where lobe TDC is.

6. I then put a dial gauge on the inlet valve, zeroed it when the valve was fully closed and then brought the piston back to TDC. All movements were done in the direction of engine rotation to keep the slack off the driven (lower) side of the timing chain. I noted the reading at TDC (62 degrees).

7. I then moved the dial gauge to the exhaust valve, zeroed it when the valve was fully closed and then brought the piston back to TDC. I noted the reading at TDC (22 degrees).

8. Adding 62 + 22 = 84 degrees. Divide by 2 = 42 degrees.

9. With the dial gauge still reading from the exhaust valve, I moved the crankshaft until the dial gauge read 42 degrees.

10. I then unbolted the camshaft from the sprocket and moved the crankshaft back to TDC; the camshaft did not move as I did this as confirmed by the dial gauge. This of course would now put the holes in the camshaft sprocket and the bolt holes in the camshaft itself out of alignment.

11. Then I removed the camshaft sprocket from the chain and moved the chain with respect to the sprocket in order to line up the holes in the sprocket and camshaft. This took a little time. In the end I reversed the camshaft sprocket in order to get the holes to realign.

12. I bolted the camshaft sprocket back to the cam.

13. I now have lobe TDC at 1 degree before TDC. That's about as near as I think I can get it, and certainly much closer than before.

 

This exercise was precise and a joy to do. Thanks Pete!

 

I decided to have another go at finding out when the inlet and exhaust valves open and close and unfortunately they are still nothing like given in the spec:

Spec: Inlet valve opens 18 degrees BTDC: (my reading is 55 BTDC) 

Spec: Inlet valve closes at 58 degrees ABDC (my reading is 85 ABDC) 

Spec: Exhaust valve opens 58 degrees BBDC (my reading is 82 BBDC) 

Spec: Exhaust valve closes at 18 degrees ATDC (my reading is 58 ATDC).

 

So, although I am confident the valve timing is now set, I'm just going to have to 'forget' the discrepancies between the spec and my camshaft unless and until anyone out there can help. My advice to anyone setting the valve timing is simply to do what Pete suggests and avoid trying to compare your camshaft with the specs given! 

[Pete Lewis]

if it runs like a old dog  its nowt to do with me 

 

you can 'check' the timing when the tappets are on its in the manual ,  needs a 40thou gap and strong tea

you follow the setting clues backwards ,

 

get the back 2 rockers  rocking  fully down and set front two at 0.040" then turn to crank TDC   gaps should remain equal  doesnt matter what the gap is so long as they are the same 

 

Pete

[JohnD]

Chris,

Well done if you have never rebuilt an engine before, for an ambitious and competent job!  But!

"Equal lift" refers to the valves on the opposite piston to No.1.  Nos. 4 or 6.    The both rotate together, reaching TDC together, but when No.1 is on the firing stroke, No.4(or 6) is on between cycles.   It is ending the exhaust stroke and about to start the induction.

Its in this position that both valves are open, they overlap, and at TDC they are equally open.   That's "Equal lift on overlap"!

No need to calculate or even know the precise degreeing of the camshaft.   Its so simple!

John

[01/65789]

Thanks for your replies.

After six months of head scratching, I finally got to find out where I was going wrong. Thanks to Del at Kent Cams for providing the Eureka Moment!

To recap, my problem was this: 

1. I set the cam timing by Equal Lift on Overlap.

2. I confirmed that Inlet valve for No.1 piston reached maximum lift 110 degrees ATDC, intake stroke (the spec for my 18/58 - 58/18 camshaft)

3. I couldn't understand why the valves were not opening and closing when they should (The inlet valve should open 18 degrees BTDC (ie the valve just begins to lift off it's seat at this point), the inlet valve should close at 58 degrees ABDC (ie the valve just touches back on its seat at this point) The exhaust valve should open at 58 degrees BBDC and the exhaust valve should close at 18 degrees ATDC). What was happening was that the valves were opening much sooner and closing much later. 

I was taking my measurements directly off the cam: I had a dial gauge mounted on a pushrod which was in a tappet. The specs given include the valve clearance (0.010"). In other words, if you measure direct from the cam, you have to turn the crankshaft through a further 0.010" of movement at the point where the dial gauge shows start of lift. Once I did this, the valve timing confirmed the spec.

Obviously, If you measure from the rocker with the engine assembled (and the valve clearance is set correct) you don't need to account for the clearance.  

Chris

 

 

 

[JohnD]

Chris,

I hope that you don't think that the Equal Lift on Overlap method misset your cam timing.    By your own admisison the error was in how you checked it.

ELoO done straight off the cam will give you the same setting as from a fully assembled valve train , even off the valve.

John

[01/65789]

Hi John,

No, not at all. I know it's my error; I was so glad that I'd finally discovered it I wanted to share it on here!

I knew that if a symmetrical camshaft is set by ELoO - whether it's off the cam or off the valve train  - then the cam is set and that's that. If I'd just left it there and carried on with the rebuild I'd have saved myself six months of head-scratching.

I just couldn't work out why the valves weren't opening and closing according to spec with measuring direct from the cam. 

This rebuild is just for pleasure, so I'm not under any pressure to get it finished by any particular time. Part of the reason for doing it is that I  wanted to transfer all the reading I'd done on valve timing into practice, so that I could really understand it. 

Chris

[JohnD]

Thanks, Chris, glad you're enjoying it!

John