Deck Height CIS

[Cipotifoso]

Wayne talks about checking deck height for flat-topped and domed pistons on page 145 of the Engine Rebuild book. CIS pistons are not directly addressed; can they be measured via the flat-topped vernier caliper method?

My 2.7 was aligned bored back to standard with (I'm assuming) .50 (has to be mm) machined off of the case mating surface (cases are stamped "050" by Ollies below cylinder #2). They gave me six 1.00 mm base shim to account for this and the decking. If I put the 1.00 mm base shim and the Victor Reinz-supplied .25mm shim on the cylinder, I get a deck height of 1.50mm (varies slightly depending on the measurement) using the vernier caliper method.

If the stock motor had a .25 mm base shim from the factory and now has .50 mm machined off the case mating surface and an assumed .20 mm machined off the cylinder bases on the block (sourced from a different thread as Ollies didn't give me the spec), then there should be about .70 mm less deck height that needs to be compensated for.

My thought is to use the 1.00 mm base shim only and shoot for a deck height of 1.25 mm.

Is my methodology correct?

Thanks,
Dave

Measuring Deck Height:



Ollie's shim:



Victor Reinz shim:

[Cipotifoso]

Talked to Ollies this morning on the phone. They decked .050" (or 1.27mm) off of the cases to align bore it back to standard. Their advice was to stack the 1.00mm shim with the .25mm shim in the gasket kit. Here's the math I come up with:
- Each side planed .025" or 0.635 mm (.050' total removed; assuming half from each side).
- Cylinder spigots trued with another .20 mm removed (I need to confirm, info from another thread).
- Total removed: 0.835 mm
- Spacer provided: 1.0 mm

- If both 1.0 mm and .25 mm spacers stacked, I'm gaining 0.165mm over stock (assuming nothing else has changed). Measured deck height is 1.5mm

- If I leave out the .25 mm base gasket/spacer, then I'm subtracting a theoretical 0.085 mm from a stock measurement, which should bring me to a 1.25 mm deck height.

I'll try to confirm some of my assumptions and will measure cylinder #4 on the right side to see if I come up with the same figures. But if everything is as above, I'm leaning towards leaving the .25 mm spacer out. Does this sound correct?

[Eagledriver]

I see no problem with your reasoning or math. I'd leave out the .25mm spacer.

-Andy

[Cipotifoso]

Some further progress (and setback) in my saga...

After talking with Ollie's again, it was clarified that the "050" on the case indicated .050" from each side (align bore & spigots).

Hmm.... .050" is taken off; a .040" (1.0 mm) base gasket spacer is provided....somewhere there's an extra .010" missing.

So I measure the deck height a million times (hard to get repeatability at an accuracy of a hundredth of a millimeter), and it seems to come up 0.95 mm with 1.0mm spacer only; 1.20mm with both spacers stacked.

After searching and reading all I can find on the subject, I have the following info: 1) Wayne recommends 1.25-1.5 mm in the rebuild book; 2) Experts on this board advise no more than 1.0mm due to more space increasing the probability of detonation at the cylinder edges; 3) CIS engines are probably OK with more than 1.0mm and can fall into the range Wayne recommends. Since I'm at 1.2 mm, I somewhere in the middle and should be OK.

Now I've got the camshafts timed and need to check valve to piston clearance. Since I've got a stock (but heavily machined) 2.7, I figure the shortcut method to do this would be OK.

Cylinder 1: TDC; crank the adjuster for the exhaust valve in two turns; 1.5 on the intake. Turn over the engine carefully....no resistance! I do it a couple times more to be sure.

Crank back out the screws on Cylinder 1, bring Cylinder 4 up to TDC and crank the exhaust in 2.0 turns; intake 1.5. Turning the crank over slowly, it feels good until it feels a bit funny around TDC on the valve overlap but it turns over.
I back out the screws and go to the static method of checking the piston to valve clearance: At about 8 deg BTDC, I get 1.8 turns on the exhaust before it stops.

My concerns: Did I bend a valve? Subsequent leak down testing showed a range somewhat in line with the other non-broken in cylinders (~5%), however; I can hear something by the exhaust and fluid squirted in the port bubbles by the seat (however it bubbles also a little on cylinder #6).

My options? It seems I need another .2mm and the only place to get this would be thicker cylinder base gaskets. If I replace the current 1.0mm/0.25mm stack with custom 1.45mm or 1.5mm, this should give me a 1.4-1.45mm deck height and 2.0-2.05 mm piston to valve clearance. This assumes a linear relationship between base gasket thickness and piston to valve clearance, which a related post I found states that this is not the case. But absent a formula or ratio, I've got to go with a linear solution.

Has anyone else experienced anything like this? Is my approach and proposed solution correct? If you know anything about this, I'd like to hear from you.

Thanks,
Dave

[304065]

I would try to establish more concretely the piston to head clearance using modeling clay. Using the valves as micrometers can be tricky.

I would also measure everything again. You don't mention cylinder height or height group. Are these original heads, have they been surfaced?

The 356/VW folks have a nice steel mandrel machined that is precisely the diameter of the
crank journals, this allows them to measure the spigot height directly with a depth micrometer.

It sounds like what Ollies has told you is that they removed fifty thou of material from the case flange and spigot per side. Fair enough, and that's why they recommended you use an 1mm (0.039") spacer. You are right, the tolerance is reduced by 0.011" if that material is not put back with the spacer.

Next: What is your rod length? Did you have the rods rebuilt?

Next: What is your cylinder height?

Next: What is the depth of the groove in the head? Were the heads machined?

Somewhere there is tolerance buildup along the way of course, since you need to pull the heads and cam boxes off, it's a good opportunity to figure out where it's coming from. The shorter spacer will definitely affect things by .279mm.

So I see your problem- you have too little clearance to the head with a deck height of 1.2mm. Deck height is a relative reference between the theoretical (in your case actual) deck of the piston and the top of the cylinder. Therefore, your heads must have been cut?

Hope this helps having others think through the problem as well. Let us know your measurements.

[Cipotifoso]

Thanks John for your help.

- Rods were completely rebuilt and balanced. I didn't measure them as I didn't have the correct measuring tools. I assume Ollies did it correctly with a center to center distance of 127.8mm.



- Cylinder height: I am using brand new Mahle Nikasil CIS pistons cylinders. I didn't measure the cylinders, but they are stamped "W5." This should theoretically equate to 85.400 - 85.425mm, however; I read in another thread ("Mahle question???") that this applied only to the 2.4 Biral cylinders and was no longer valid for 2.7 cylinders. The 2.7RS cylinder had red or blue dots at one time; mine have none. Here again I can only assume 85.400 - 85.425mm.



- Heads were refaced 0.15mm as per the receipt (maybe this is a large part of the ~0.2 mm piston to valve clearance that is lacking?), however; this is at least the third rebuild on this engine, so I don't know if they were also refaced in the past. I didn't measure the depth of the groove. I assume you mean where it steps down into the cylinder sealing surface. Since I'll be having to do some dis-assembly, I can measure this at that time.

Dave

[Cipotifoso]

Latest update:

I plan to measure everything again this weekend, but indications so far point to needing another 0.2mm height on the cylinders. I discovered in another thread that a good source for custom base gaskets is the Flatout group (flatoutgroup.com). Then I thought about it some more and remembered that Ollies provided 1.0mm spacers with my engine. Would they have other sizes or a source? I called up George and after discussing my problem, he took my address and is dropping 6 - 0.5mm cylinder base shims in the mail. If everything checks out, my plan is to swap out the 0.25mm base gasket for the 0.5mm stacked with the existing 1.0mm. This should theoretically give me a deck height of 1.40-1.45mm and a piston to exhaust valve clearance of 2.05mm.

Oh, I'll also have to pop that one valve out, chuck it up in a drill and see if is bent (that's the method I've come up with so far).

Dave

[Eagledriver]

Your clearance is fine at 1.8mm (IMO). The factory manual for my 72 engine says .8mm minimum clearance. People say 2.0 to be conservative. I use 1.5 as my personal minimum. You may have bent a valve and if you have a leak your valve will eventualy burn. If it's not bent you may just need to lap it in some more.

-Andy

[Cipotifoso]

Eagledriver,

I looked in the '72 factory manual and I cannot find this spec. Can you tell me where it is?

Your feedback is what I'm looking for here: Are my observations based on the available data correct? Am I taking a reasonable approach? Or, is there something I'm missing?

Thanks,
Dave

[Eagledriver]

It's on page E68 of my 1965 workshop manual for the original 2 litre (with supplements for later engines). It's a warning note that says some pistons have less valve clearance and the minimum must be .8mm. It's also noted on Bruce Anderson's 911 performance hand book edition 1 page 128. He recommends 1.5mm for safety.

I have slightly bent intake valves during the timing operation. It doesn't take much. A slight intake leak is not as bad as any exhaust leak. Intake valves don't burn.

I see nothing wrong with what you are doing, I'm just saying you have enough clearance already.

-Andy

[Cipotifoso]

Andy,

I see it now: "Warning: Pistons for engines up to No. 900727 are slightly higher in the valve pocket area. If it becomes necessary to replace the cylinder head of an engine having these pistons, the minimum distance between the piston valve pocket and the valve head must be determined during assembly, and must not be less than 0.8 mm (.0314").

The method to fix this is that "the valve seat must be ground away until an adequate distance is obtained."

I've got Bruce Anderson's 911 Performance Hand Book, Edition 2 (not 1) and it is interesting that I could find no reference to this measurement; he seems to have left this step out entirely (?).

At a minimum, I'll yank the head on cylinder 4 and have the valve checked. I'll consider your input in deciding what to (if anything) about the clearance issue.

Thanks,
Dave