Deck height question!

[Ray_G]

I have just checked the deck height in my 2.7 rebuild. My case was sent to Ollie's and had all the machine work done that was required for the dreaded 2.7. My case was actually not in bad shape, it did not need to be align bored, the case looks like it was decked but only about .010" inch was removed. I put the cylinders on with the base gaskets that Ollie's supplied, they were .020" think as compared to the .010" inch thick ones that came in the gasket kit.
I get anywhere from 062" to about .065" inch all across the whole set of cylinders.
Now, the question is this. Do I need to do the piston to valve clearance check? I am doing a stock reuild, and although I am using all stock pieces they are not all from this engine originally. I am using a differnt set of cylinders, and pistons, and different cams, not from this engine, but stock sizes etc. The pistons are the same CIS type as the engine came with.
Do I need to do the piston to valve clearance check? Seems like a lot of time for stock components.
Thanks in advance.

[Aurel]

You could use the shortcut method: set the valves to the minimum clearance at TDC with the adjustment screw, and do a complete engine rotation to make sure they never interfere with pistons. Do it for #1 and 4, intake and exhaust, and you should be done in 20 minutes or so.

Aurel

[schnellmann]

You should definitely verify your valve clearances. Take your time and don't take shortcuts. You may have stock stuff, but once your case and/or heads get machined they are no longer "stock" in terms of dimensions and tolerances, etc.

Also, you should definitely verify clearance at several points before and after TDC as, depending on the cams you have, piston TDC may not be the time when the piston is closest to the valve. Also, if you're getting close to "safe distance" on one piston, you should definitely measure your clearance on all of them.

Also, depending on the piston dome design you have this shallow a deck height may put the domes themselves in danger of contact with the heads.

Might seem like a lot of time and effort, but consider how much effort it'll be if at your first trip to 6000+ the metal bits inside your engine decide to get "overly friendly."

Wayne's book says to shoot for 1.25 - 1.5mm of deck height IIRC, but a search on "deck height" shows that experts like Steve Weiner and Henry Schmidt say shoot for ~1mm because it lowers the detonation potential.

[p911sc]

I agree with the others. I would check piston to valve clearances on all of the cylinders. Especially with different components coming from different engines. Be careful when you rotate the crank. If you feel any resistance stop and check it out. When I did it, I used a short handled ratchet to avoid accidentally overcranking.
Rick

[Ray_G]

I guess I am going to do the valve clearance check then. The only way I can get my deck height to the 1.0mm number is without copper base gaskets. otherwise the thinnest copper gaskets get me about 1.3 to 1.4mm on the deck height, I have done a search on omitting the base gaskets but it does not seem like something engine builders do.

[cnavarro]

Although there's nothing wrong with a .060" deck, you do not need to use the copper shims - just use Curil T on the cylinder base in lieu of a shim if you want to get to the .040" deck and you should be just fine.

[Ray_G]

Thanks Charles,
I read my own post over, when I am refering to doing a piston to valve clearance check I guess I am really asking if I should pre assemble the engine to do it? I mean I would do the actual check, that won't take long, but it is a lot of work to put it together without sealants and then take it apart again if the clearance it good, and then seal everything. On the other hand if it is not OK then I don't have to clean everything up. I guess I am leaning towards sealing it up and hoping that the piston to valve clearance is good.

[dtw]

Ray - how are you checking the deck height? I'm trying to figure out why your measurement is that high.

[Ray_G]

Me too, just using a vernier caliper, the tail end. From the top of the cylinder to the top of the cis type piston. I measured at the piston pin ends. I get about the same all the way across the set, plus or minus a few thousands. It is more than I would have guessed after the case was decked, but that is what I get.

[schnellmann]

Also, I should say that I just went and re-read your post and for some reason the first time I read it I thought you were measuring in millimeters and not inches. In this case, .065" would probably be too much instead of not enough like I was implying. Sorry for any confusion.

Like Charles suggested, just ditch the .020" base gasket and use some Curil T, or Curil and .010" gaskets. That puts you right in the zone, assuming your deck measurements are the same on all 6 cylinders.

Also, it's only probably 3-5 hours extra work to assemble things without sealants to check clearances, then tear down again. If everything checks out, when you assemble for real it'll go twice as fast and you'll have the confidence of knowing everything will work for sure.

[Ray_G]

Yeah, I went from metric to inches, I worked too many Fokker aircraft and back to MD80's. Opps, some of it is my fault. I am just curious why it is as high as it is. I would think after it was decked that no gaskets on the base would have you low. I don't know. That is what I got, for fun I might try the solder method on one cylinder tomorrow. It won't take but a minute to throw a head on and torque it.

[1982911SCTarga]

I'm confused about this:

Wayne's book says to shoot for 1.25 - 1.5mm of deck height IIRC, but a search on "deck height" shows that experts like Steve Weiner and Henry Schmidt say shoot for ~1mm because it lowers the detonation potential.

By lowering the deck height, isn't compression being increased and, thus, the potential for detonation issues? Am I misunderstanding something?

Brian

[cnavarro]

It partially has to do with what deck height was used when designing the piston.

To give you an example, a 93mm 2.9 10.5:1 piston at a .040" deck requires a 24.6cc dome (assuming a 68cc head). If you take that same piston to a .060" deck, the same piston becomes a 9.9:1 piston.

So, the statement that running a .040" deck lowers the potential of detonation is assuming that at that tight of a deck you aren't exceeding the octane requirements allowed (before requiring twin plugging and/or race fuel). That said, there are a lot of builders who set their engines up for .060" deck and I design the pistons as such to increase the dome volume to ensure they are still netting their target compression ratios. As long as you stay within the realm of .040-.060" deck, you're totally fine.

[schnellmann]

Brian,

I won't pretend to be more of an expert than Charles, but I'll explain the issue slightly differently.

What you say is true regarding compression, but taller deck heights leave larger "dark corners" in which conbustible mixtures will lurk. Optimal deck heights, and their affect on compression ratios and combution are also determined in part by piston dome shape and the location of the spark plug.

And plug position in Porsche heads really factors into this deck height equation. Imagine a piston coming up to TDC and the plug waaaaaay off to the side sparking to start combustion, say ~25+ degrees before TDC. Now as the piston continues to move up the cylinder, the flame front starts to travel across the cylinder. But what happens if the mixture all the way on the opposite side of the cylinder ignites due to compression heat before the flame front reaches it? Detonation.

By the way this problem is the whole reason for twin plugs - they effectively halve the distance the flame front has to travel and start from both sides of the cylinder.

Larger deck heights leave larger pockets of combustible mixture to ignite before its time, particularly in larger bore motors with way offset plug locations such as ours. Sometimes the flame can't get all the way across the cylinder before the mixture reaches ignition temperature. Every ignition stroke is literally a race across the cylinder.

So shallower deck heights are really a risk-mitigation tactic. Less fuel to detonate in the dark corners of the deck height void means less potential for damage if it happens.

Along with the problems I mentioned above, Porsche motors are air-cooled and that leads to wildly varying combustion chamber temps that are heavily influenced by ambient temperatures - thus further increasing detonation risk as the air temp of both mixture air AND the cooling air increases. Basically all engines are water cooled these day simply because that method offers much better combustion temp control (and emissions reduction as a result).

Anyway, I hope this makes sense. It took me a lot of reading on the subject to comprehend this interrrelationship between engine/piston design, deck height, plug position and so forth.

[1982911SCTarga]

Thanks to both of you for helping me better understand this issue. It's a violent world inside our engines, to be sure.

Brian