Cam timing & valve-piston clearance
 

This is a mostly stock 3.0 motor, 0.01" were removed from heads. I installed DC15 cams. John recommends 1.8 to 2.0 mm for the cam timing. I set them at 1.82 mm and went for valve-piston clearance check:
Intake#1@1.5 mm: pass
Exhaust#1@2.0 mm: pass
Intake#4@1.5mm: fail, 1mm: pass
Exhaust#4@2.0 mm: pass

Deck height was 0.9 mm if I recall. On the low side...
Question: Am I too close to 1.8 that it causes interference? Should I retime the cams to say 1.9 mm? What value will maximize the valve-piston clearance?
I`d hate to have to tear everything down...

Aurel

If you are close on the intakes and good on the exhausts, retarding the cams slightly would give you more clearance with the intake valves.
It will also close the exhaust clearance by retarding.
I think I would go for 1.9mm. Checking the exhaust carefully.
You might experiment with closer to 2.0mm just to understand what might be the optimum setting.
2.0mm timing could put the exhausts too close.
You don't really know until you check.

Great info Doug, thanks ! I`ll try 1.9 mm tonight.

Aurel

I have DC-15's, and I set them to 2mm. No issues. What gaskets are you using under the cylinders?
Pat

I have one copper gasket, which I measured to be 0.3 mm.

Aurel

If I'm not mistaken, increasing the valve lift @ TDC overlap actually advances the cam timing.
It's my understanding that TDC overlap is the point at which the exhaust valve is closing and the intake valve is beginning to open, hence the "overlap" term. Increasing the lift at this point means your opening the valve sooner through advanced cam timing.

It sounds like your clearance issue would benefit from retarding the cams a little. Maybe say... 1.75mm @ TDC overlap

Your may want to check with your cam guy and see how the engine would respond to this extension of tolerance.

I would think it would move the torque peak higher in the RPM range which may not be the best thing with a mostly stock 3.0, especially if you're concerned about valve float at high RPM

What are your port sizes, what sort of induction/exhaust are you using? That could affect your usuable power band and make cam timing all the more critical

I would put thicker gaskets under the cylinders, giving your self a bigger safety margin, and set the cams where they belong.

Even if it runs fine like it is, unless you've got some stiff valve springs or shimmed them a little tighter with spring height, What happens if you miss a shift, or get a little too silly with a downshift "in the heat of battle"

You could always leave your heads bolted to the cam carriers and not have to clean and reseal them, just pull the heads with the cam carriers. Slip the cylinders one at a time and change/add gaskets...

just my $.02, good luck!SmileWavy

If you had all fresh hardware it's probably still fine

According to the factory workshop manual for the 2 liter 911S engine, .8mm is enough piston to valve clearance. While 1mm is a little tight I would live with it. The exhaust clearance seems to be a little more critical and you have plenty.

-Andy

So, I tried last night. With the right side at 1.82 mm, my intake clearance is OK. But now, my left side is around 1.85 mm. That is measured with carrera tensioners back in. With solid tensioners and clamp, I get basically same difference, but the values are more like 1.8 mm left and 1.77 mm right. Should I only look at values with the clamps?

Also, I am having hard time getting identical left-right values, because I noticed that with the pin in, if I turn the camshaft I can still get about 0.08 mm valve travel without the crankshaft moving! It means that I may be in the correct pin positions for left and right, but I still need to fine tune with the pins in place. Is that common?

Aurel

Okay, I think I`ve got it. I slightly oiled the cam nut and shim to help with fine adjustment. I played with various timings, and found that my right intake valve has less than 1.5 mm clearance, but more than 1.25 mm, so I think I can live with that. I also decided to set them at 2.0 mm, which is 0.078". Two reasons: max advance should be better for a street motor. I don`t need my power at too high rpm. Also, higher chain tension might cause them to get retarded a bit, right?
I first set them with good tension on the chain, then checked again with the carrera tensioner. There was little difference this time :).

http://forums.pelicanparts.com/uploa...1205006126.jpg

http://forums.pelicanparts.com/uploa...1205006154.jpg

Aurel

Browsing through more threads, I saw that John "Camgrinder" recommends clearances of 0.05" for intake, and 0.08" for exhaust. So, my intake is fine with 1.25 mm, which is 0.05".

Aurel

Just one think to mention. Are you turning over the engine to make sure you have these clearances? Some people only test at TDC and that is not the closest distance.

-Andy

Yes, I used the method that Wayne called "shortcurt" which consists in setting the valve to the 1.5 mm clearance, and very carefully turning the crankshaft untill I felt it touching. Then I backed the valve to 1.25 mm, and I was able to turn freely over the entire cycle without interference.

Aurel

So, everything looked good, I installed the chain box covers, the remanining rockers, got some sparks plugs...Last night I decided to do a last check, and both sides came at 2.08 mm !! Is that too much advance? (1.8-2.0 is the recommended range). It went the opposite way I thought. Gotta redo it...I`ll shoot for 1.9 mm this time.

Aurel

How are you people actually measuring piston-to-valve clearance? I am curious

Spec

1 turn on the valve adjustment screw = 1 mm. You count the turns until you touch. You redo it every ~ 5 deg of crank to get the whole picture. Very laborious...
I prefered setting the valve to the clearance I needed, and checking it never touched the piston. I used the dial guage to get the valve down to the exact value. You could also use the dial guage to measure the exact valve-piston distance at any given point rather than counting the adjustment screw turns.

Aurel

Okay here's the Talk to Dave Brown

Jeezus...that is SO Wrong

What does that mean?

What is so wrong?

Aurel

And who is Dave Brown? And whats the Talk To?

Aurel:

The engine looks great!

Just curious what your machinist left as a signature. Did they stamp a -0.1mm on the exhaust port flange area? Or some other area of the head to alert the next rebuilder? Wondering if American shops use the metric or standard measurements for indicators.

SmileWavy