cam timing PTV clearance check
 

Hey everyone -

Rebuilding a 74 2.7 race motor- everything going well until got to PTV clearance check - Right side - AOK - Left side, the exhaust valve was more than enough but the intake valve for about 5 degrees past just past TDC was about 1.4mm (or from what I read in the bible, about .1mm short). I have a few questions:
1. When I started taking it apart, I noticed that the cam nut on the left side was not torqued to 110 - probably more like 80 - chains were very tight (mechanical tensioners in place) - could this cause my problem on the right side - should I just re-time and then retest.
2. is the .1mm that big a deal? if it is right.
3. If I do go back down to the copper shims and add a .25 (assuming I do for all cylinders 1-2-3), should I do all 6 since the others did not need or is it safe to do just the left side?

(Pauter rods, elgin 314/296 cams, Carrillo pistons, 10.0 CR)

Thanks in advance!!

I'm no expert on this by far, but having recently gone through a similar experience, here's my two-cents worth. All opinions voiced below are my own; other more knowledgeable persons may differ.

I don't think the torque difference on the nut is an issue here. The torque clamps the sprocket on the inner indexed flange and it does all the driving of the cam via the woodruff key. The little dowel pin is essentially just for timing purposes. The bottom line is that with the forces in play checking timing, etc., the amount of clamp provided by the lesser torque should have been plenty. Did you check the timing on that cam before you took the clearance readings?

I don't think being shy 0.1mm on the intake side is a problem at all.

In the issues I was having (a lack of clearance with the exhaust valve) adding an additional shim under the barrel made essentially zero difference in the P to V clearance measurement. And then you have to also consider what is it doing to your deck height? So IMHO adding shims under the barrel(s) will not correct a P to V issue.

No, you should not use an additional shim under one barrel only; this upsets a lot of things. But I don't see any reason why you couldn't do that on one whole side (i.e. 1-2-3 or 4-5-6) but it would be interesting to know how/why that situation is occurring.

Finally, what method are you using for this check? The "screw down the adjusting screw" method? I was doing it this way (per the book) and was getting 'way too little clearance on the exhaust side. At the behest of some expert advice I moved on to using clay in the cylinders and measuring the crush. This is a real PITA with the 911 considering all the parts involved, timing the cams, etc. but once I did it this way I found I had sufficient clearance, which save me from removing pistons to machine the valve pockets. (And with the side benefit I could probably assemble a 911 top end blindfolded by now!)

Anyway, my experience and opinions. As I said others may differ; hopefully they will come in on this.

Thanks for the reply - Appreciate it - I only did the screw turn test but measured the last turn. I decided against the use of additional shims. In doing a lot of research, it seems that the exhaust is a much bigger issue if you were going to be off .1mm - and all the exhaust measurements had plenty of clearance. I had to order a couple items from Pelican but once they get here I will complete (ready to seal heads to cylinders) the process and post results.

Whenever using the rocker arm adjuster method to measure the valve clearances to the pistons, always make sure the valve does not touch the valve pocket edges or the corner radius of the pockets. The pockets want to at least 1 mm of clearance in total over the valve head diameter.

On our web site (performancedevelopments.com – Engine Design) I explain how to measure the piston to valve clearances without using clay or the adjust method. This way you can be sure to know exactly what the clearance is without adding any error into the equation.

You need to make sure all other parts of the engine are measured as well. Measuring on one piston does not tell you all are the same. Case deck height, Cylinder heights, Rod CCL's Piston comp heights etc all need to be checked and made the same.

Now much P/V clearance do you need? Driving habits, engine speed and final CR number you want to achieve all play a part here. Common law is 1.0mm of the Intake and 1.5mm on the exhaust, but I think you can use 1.0mm both sides, +/- 0.25mm. We run even closer today with the piston just cleaning off the carbon from the chamber roof and the valves just missing the pistons.

Street engines don't have to go to such extremes as the performance levels are not affected by running bigger clearances as much. As long as the CR is not affected and lowered too much, error on the side of safety.

You don't have to be of much in timing left to right to have slightly different ptv clearance.
I think I'm pretty tight on my 3.2, about 1mm, cant remember if it was the intake, exhaust or both...
It's been fine for the last 10 years up to 6800-7000.

Thanks for the responses everyone - Put it all together w/ seals etc - and think it was more of a timing issue - everything came in within spec on the final - Picked up the stomski cam tool which made the whole timing process much easier - wish I had bought that instead of the crows feet and cam tool. One other user said they feel like they could put the 911 together with their eyes closed - one thing that all these checks and breakdowns does help is with experience. I have timed my cams I think 4 times now - I just want to get done and RACE!