911SC engine rebuild panic - piston-valve clearance check

bigred23 · 09-24-2017, 04:59 PM

I'm nearing the end (perhaps not?) on a full 911SC engine rebuild. I decided to install a 3.2ss Mahle piston-cylinder set along with a cam regrind to the 964 profile (standard stuff). Deck heights on the #1 and #4 cylinders are 1.08mm and 1.05mm, respectively (average of 6 readings each, taken along the wristpin axis). Cam timing on the 1-2-3 bank is 1.26mm @TDC; 4-5-6 bank is 1.29mm @ TDC. So far, so good.

Minor cardiac moment occurred today when I was checking piston-valve clearance using the method described in Wayne's book. Minimum intake valve clearance on #1 was ~1.6mm (~140-150 ATDC), which is above the 1.5mm prescribed by Wayne. Good. Now, minimum intake clearance on #4 was about 1.1mm. D'oh!

Thoughts? Searching the history here, seems there is a range of recommended clearances. Webcam says minimum intake clearance for the 964 profile is 1.27mm (0.05"). I know Wayne is conservative with deck heights, so there's that. Besides, the tight clearance on #4, I'm concerned with the variation between 1 and 4. Perhaps the piston is slightly cocked (wishful thinking?)? I re-checked cam timing and that was good. Could the re-ground profile be off? I will go back tonight and sweep through the profile to check. The thought of having to tear down the top end to open up valve pockets on the pistons is soul crushing...

Best,
Paul

Catorce · 09-24-2017, 06:35 PM

I did the same build as you on my SC only I used JE pistons. Same cams. I don't believe there are any valve clearance issues with a 964 cam - it's too mild.

Looking at your numbers, I'd just run it. Race cars have less clearance than that.

bigred23 · 09-24-2017, 06:46 PM

I like your thinking! I sourced the piston-cylinder set through EBS and have been consulting with Don, and before I did this check, he didn't have concern regarding clearances. So, I'm surprised with the results given that the 964 cam is by no means aggressive.

Catorce · 09-24-2017, 06:57 PM

To be honest I didn't even check mine. Before someone flips out, keep in mind that I've built several dozen 911 engines and the 3.2SS upgrade is so close to the original and the 964 cam is so mild that I would be shocked if there were interference issues.

I currently own motors that have been iffy - very high lift cams and such. Those are nail biters, you are constantly running on the thinnest of margins.

A 3.2SS made from a 3.0SC is not one of them. Screw the valves out, rotate it 720, call it a day.