Cylinder wall thickness vs head sealing

[Wally]

With aluminium cylinders (Nikasils): would a thicker cylinder wall (say 6,5mm) seal better than a thin cylinder wall (say 5mm)?

I know the larger surface area of the thicker wall should give less pressure per square [mm], but it has a greater sealing surface... so what effect prevails?

The question looks a bit like the question of what tires give the best traction: wider or narrow tires...

Thanks,
Walter

[Wesayso]

Just remembered this part:

Originally posted on www.tunacan.net/t4/tour/heads.shtml
The small cylinder bore openings can create sealing issues, especially with higher than stock compression ratios. Flycutting the heads and using later model cylinders (and pistons) from a 1.8 can give you 100cc of displacement and better sealing.

from type IV secrets revealed about the 1.7L Cylinders...

[kdanie]

I've always liked the "wide tire" traction theory. When it comes to cylinder heads I don't know for sure but I think wider would be better even though the actual pressure is more spread out.

When it comes to what you can do with what's available I don't think it's worth worring about unless you go very narrow such as the 105mm type 4 cylinders. Both Jake and Oliver have developed ways to seal cylinders to heads that are very effective.

This is just my opinion so take it for what it is.
ken

[Ron Roberts]

In this application, unlike fat or skinny tires. Total pressure is what is containing combustion, not psi. I think spreading it out with greater sealing surface can only help for another reason. The greater surface and wall thickness should help eliminate any distortion that could break the seal. Also with greater wall thickness would it be O.K. to torque it down tighter? I'm only theorizing here, and have no experience on the subject.

Ron

[Richie]

Hello,

I believe that thicker cylinderwalls are more stable, thus less suspectable to blow through the seal. But giving the studs more torque only stretches them more in my opinion.
I even think that there isn't much difference between 5mm or 6,5mm. The difference has to be bigger to be measurable or noticable on a N/A motor. It's a different story when you start to play with turbo's
Then it's nice if you have the 6,5mm thickness because you have enough thickness for an interlocking sealing ring.
Just my 0.2 cents.

Cheers,
Richard

[Massive Type IV]

Themain problem with head sealing is having different expansion rates for the case, head studs, cylinders and heads. It is made worse by the unequal lengths of the upper and lower head studs, which gives them unequal expansion rates. This problem is multiplied by the "Non square"pattern of the entire stud arrangement.

When the cylinders grow in size, all these problems and their effects become much more apparent.

The reason Nickies work so well is that the studs they are designed with have the correct expansion rates for the engine, since all the materials now grow at more equal rates. When a cast iron cylinder is used it has a very different expansion rate and really can screw things up. alot of machining and hand sealing methods make them acceptable, but they are no where near what an aluminum cylinder is in the sealing department- no matter what you do.

As for sealing surfaces, as I have seen its a very small portion of the answer, many many stock engines have head leaks.......with thick cylinder sealing areas.....The 1.7 just isn't as prone because like I explained above, bigger bores make the problem more apparent.

I'm working on a small bore long rodded, long stroke Turbo configuration that will use a modified 1.7 cylinder and a different piston.....hopefully it will be a good combo and will be a Turbo that will not require Nickies for us to assemble.

as it stands now ANY Turbo or ANY engine with a bigger than 96mm Bore requires Nickies at my shoop.

[njv]

hi
must be thinking rice here!!!

[Wally]

Thanks guys,
I also took in consideration that any material that could be left standing in the head would less weaken the heads and maybe so help to keep the seal intact better.

Would boring out the heads to the point (appr. 113 mm) that one or several stud holes become visable (or shine through) also effect sealing negatively ? or doesn't it matter as long as the cylinder doesn't need to seal in the stud hole itself?

Hope I made myself clear...

Thanks,
Walter

[Wesayso]

Did you take a look on the Klaus site? At the bottom left is a picture of the holes doing just a little more than only "shine trough".... Those are for the steel 103mm's...
I don't know to what size these heads are flycutted...(but would like to know )
A couple of months ago LN Eng. talked about making the Biral 100mm like "thickwall" 103mm cylinders, so maybe that outer size of the cylinder will do good in your application too ...