Piston / valve clearance way too tight. Help!

[Adrock]

I'm sitting in front of my mostly complete long block and I'm checking piston to valve clearance as per Wayne's book.

I have a 1983 911 SC Japan model with 930/17 engine. I don't know what work has been done to the engine previously, but here's what I've done or had done:

Case has not been machined. (By me)
Rods checked, rebushed and I believe resized.
Crank polished not machined.
Using original SC pistons and cylinders with .25 copper gasket installed
Heads rebuilt, machined (unknown amount),. We Springs and guides.
Cams re ground
Cam followers refaced.
I checked deck height and came up within spec with .25mm shim.
I did the cam timing last week. Set at 1.55. I will recheck today.

So, today I was checking the valve to piston clearance and I got the following results on intake 1. Similar on intake 2. Didn't bother with exhaust yet because I'm too annoyed right now.

Inlet 1
10BTDC 2mm
5BTDC 2mm
TDC 1.25mm
5ATDC 80mm
10ATDC 75mm
15ATDC 95mm
20ATDC 1.60mm

75mm seems just ridiculous. What could cause this? Could I have stuffed the cam timing completely?
I followed Wayne's instructions to the letter I believe. Turned screw until there was just zero clearance and checked it by turning in using 360 degrees as 1mm. Surely something is wrong and its not a build up of machining tolerances?

Any tips on where I can look for the problem? Even if I tore it down I don't know where I would make up the difference other than adding shims? Even then wouldn't an extra .25mm just give me .25mm extra clearance at the valves?? Ugh


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[Adrock]

Just checked the cam timing on that side.
Exactly 1.55 and spot on with the case line.



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[Eagledriver]

Set your timing to 1.25 instead. That will increase your clearance to at least 1mm. Check the exhaust as well. 1mm is plenty, especially on the intake. You will get a couple more HP as well. Those cams were timed as far as.9mm on some cars.

By the way the timing seems to be related to port size from what I can tell. If you have 34 mm ports you want to advance the cams towards the 1.5. Large ports (39mm) go with retarded cam timing of 1.0 mm.

-Andy

[Adrock]

Ok just did the right side and exhausts here are the results.

Intake 4
10BTDC 3mm
5BTDC 2mm
TDC 1.5mm
5ATDC 1mm
10ATDC 75mm
15ATDC 1.10mm
20ATDC 1.60

Exhaust
Left all 2.50mm+

Right
All 2.50mm+

Very much the same results on the intake on 4.

Just wanting to check that I'm doing this right:
Set engine to TDC Z1 on cylinder 1 with valves closed. Turn screw until it just stops moving against valve.
I then turned it 360 degrees inwards. One full turn so it was set to 1mm.
Then I turned the crank. I was able to turn it almost 360degrees before valve and piston got close (around 5BTDC).
Is this where they should meet?

Is it possible I have something back the front? Like a cam upside down? Am I starting at the wrong spot on the whole process?

It seems strange to me that the clearance on the exhaust is more than enough. I set it to 2.5mm and was able to spin the crank freely.

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[Adrock]

Originally Posted by Eagledriver

Set your timing to 1.25 instead. That will increase your clearance to at least 1mm. Check the exhaust as well. 1mm is plenty, especially on the intake. You will get a couple more HP as well. Those cams were timed as far as.9mm on some cars.

By the way the timing seems to be related to port size from what I can tell. If you have 34 mm ports you want to advance the cams towards the 1.5. Large ports (39mm) go with retarded cam timing of 1.0 mm.

-Andy

I think your post made me realise whats up. I'm referring to Wayne's book. Where the cam timing notes are, it's a little confusing because it shows two possible cams for my engine.


I had assumed where it said OR this cam... You used the same settings for the cam shown.
I'm assuming this is wrong though.

What Im thinking now is that these should be set to the 84-89 carerra value of 1.1-1.14 or optimally 1.25 as you mentioned. I'll give it a go and see how what happens. Cheers!

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[Adrock]

Ok...I've got a the timing set to L 1.31 and R to 1.28. Let's see how the clearances go. Fingers crossed.

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[Adrock]

A quick check without stopping at each spot. I can turn the crank 720 degrees with 1.25mm clearance on 1 and 4 intakes. Couldn't turn it over at 1.30mm.

Ran out of time to check the exhausts. At least it improved! Now the question is do I tear the thing down to install another shim or run with the above.

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[356RS]

Adding another shim under the cylinders will increase the deck height and lower your compression ratio. I would think there is still some cam timing issues to deal with.

[Adrock]

Originally Posted by 356RS

Adding another shim under the cylinders will increase the deck height and lower your compression ratio. I would think there is still some cam timing issues to deal with.

Such as...? I assume you're referring to the fact that they don't match on each side?

Everything I've read said they should be as close as possible and within spec.

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[Walt Fricke]

You have typos for those 75mm etc readings, right? You can't turn that screw 75 turns.

One way to check for an upside down cam is to complete the installation of the rocker arms. If you are upside down, a valve is going to hit a piston (no harm, because you are turning slowly).

Another is just to look at #1 intake and #4 intake at pulley Z1. One will be completely closed (firing point in the cycle), while the other will be slightly open (because that's where you set the overlap value). You can check the exhausts, too, because they are the same deal, and you have the 1 & 4s installed. Rockers wiggle on one side, but not on the other.

[Adrock]

Originally Posted by Walt Fricke

You have typos for those 75mm etc readings, right? You can't turn that screw 75 turns.

One way to check for an upside down cam is to complete the installation of the rocker arms. If you are upside down, a valve is going to hit a piston (no harm, because you are turning slowly).

Another is just to look at #1 intake and #4 intake at pulley Z1. One will be completely closed (firing point in the cycle), while the other will be slightly open (because that's where you set the overlap value). You can check the exhausts, too, because they are the same deal, and you have the 1 & 4s installed. Rockers wiggle on one side, but not on the other.

Ha, you are correct, thanks Walt - definitely should have been -0.75mm

I've undone everything and started the cam timing from scratch and confirmed that the cams were in the correct way - so that's all good.
I think the problem was simply that I was using the specs from the wrong cam.

Now that I've adjusted the crossover point to 1.30ish there seems to be more clearance - at least 1.25mm.

I'm just wondering:

*Do I lower the cam timing point closer to the lower end of the allowed limits to hopefully gain more clearance?

*Do I do the unthinkable and tear down to install another shim under the cylinders?

*Do I leave it set as it is and run with the 1.25mm clearance? I know it's 0.25mm less than what the books suggest. But I've also read other people's posts where they have run with 1.25mm or less. Although - I've never read anybody saying that they were successful and their engine didn't die. hehe.

Does anybody have experience with running 1.25mm clearance without any issues?

[Eagledriver]

Minimum clearance from the factory workshop manual is.8mm. You have well over that now. You don’t need to mess with it anymore.

-Andy

[Adrock]

Ok. Double checked everything.
Cams are set 1.25 and 1.29 happy with that.

Intakes are at lease 1.25 but could not get 1.50mm.

Exhausts are at least 3mm.

I'm pretty happy with that I think.

Next!

Thanks team pelican.

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