What deck height?

[boosted79]

With SC cams on 930? I've got 1.09 - 1.2 mm right now with the original .25mm base gaskets. Original jugs, rods were rebuilt to stock spec. So that's the way it came from the factory and did 80k miles no problem. Heads were cut .08 mm, valve job, should I do a valve clearance check? Valve job with original valves should make up for some of mating surface cut. Thanks.

[NICE 69 S]

When you say that "rods were rebuilt to stock specs" you have no way to be sure that they are "exactly as delivered from the factory". Even if the rebuilder has dedicated machinery that only does your type of rod, which he probably doesn't, you still are not assured that they are the same center to center as the ones that came in your motor. And that "original" base gasket is probably not .25mm any more.
Bob B

[boosted79]

I agree, so what deck height should I use? The original base gasket is still right at .25 mm. Should I stick with the 1.1 - 1.2 mm deck and use new .25mm gaskets or go up to .50mm gaskets which puts deck at 1.35- 1.45mm. Or I can get custom Cometic gaskets for something in between.

[KTL]

I don't think you absolutely need to do a valve clearance check. SC cams in a 930 engine configuration is a common modification. Even the same timing spec is used for the SC cams. Check out DRC's specs near the bottom of the list

Dougherty Racing Cams Porsche 911, 930 and 964 camshaft profiles

but why not do it as long as you're already working on the engine? It's just a matter of turning the adjuster screw in & back out at various locations of crank position. The closest exhaust valve proximity to the piston is when you are approaching the TDC overlap point. For intake valves its just after the TDC overlap point.

Before TDC overlap you are seeing the exhaust valve closing from maximum opening, so you effectively have the piston coming up to top dead center and "chasing" the exhaust valve as it closes. After TDC overlap you have the intake valve "chasing" the piston as the valve starts to open while the piston retreats from top dead center.

Originally Posted by NICE 69 S

When you say that "rods were rebuilt to stock specs" you have no way to be sure that they are "exactly as delivered from the factory". Even if the rebuilder has dedicated machinery that only does your type of rod, which he probably doesn't, you still are not assured that they are the same center to center as the ones that came in your motor. And that "original" base gasket is probably not .25mm any more.
Bob B

Bob,

I'm not sure i'm following what you're saying. If he had the rods rebuilt to the factory specified 126.95 to 127.00mm center-to-center dimension, all should be fine. That rod length, along with a used 0.25mm base gasket, is producing the deck heights he's stating. Where's the problem, other than the measurements not all the same?

You're implying he's assuming those deck heights are similar to what they were pre-rebuild? So what, because that assumption doesn't really matter. All that matters are the numbers he's seeing right now and they are fine. 1.0mm of deck is the generally accepted minimum and is perfectly acceptable for a "stock" build or a 930 engine with SC cams.

[boosted79]

Thanks. I will get new .25 mm gaskets. I am going to check valve-piston clearance just to make sure and for my own edumacation.