Itinerant Air-Cooled

I confess. I'm clueless. Something to do with rocker arm geometry. I've never measured my deck height, although Hal and I shimmed up my rocker arm assemblies a few years back, but for the life of my I don't know how Hal knew it was needed, or how he knew when the shims were correct.

I'd appreciate it if someone could enlighten me on the ins and outs of properly measuring, and to what ends, deck height.



thanks,
neal

ruckman101 wrote:I confess. I'm clueless. Something to do with rocker arm geometry. I've never measured my deck height, although Hal and I shimmed up my rocker arm assemblies a few years back, but for the life of my I don't know how Hal knew it was needed, or how he knew when the shims were correct.

I'd appreciate it if someone could enlighten me on the ins and outs of properly measuring, and to what ends, deck height.

thanks,
neal

Deck Height is simply the distance from the top of the piston to the combustion chamber.
It is adjustable by moving the cylinder barrel up or down using the base shim or a cylinder head sealing ring.

Too small of a deck height increases the risk of piston/valve contact at overlap and it can increase compression pressures beyond just the simple mathematical compression ratio, due to the quench area where there is very little space.
Too large of a deck height can cause gutless performance, low compression and poor combustion.

Generally we like .040-.050" of cylinder wall sticking up higher than the top of the piston at TDC. This includes the sealing ring if you are using them. This deck height measurement is utilized as you figure out the displacement and compression ratio and combustion chamber volume.

It relates to the rocker arm valve stem geometry in that the selection of base shims or sealing rings changes the assembled distance of the heads from the camshaft. THIS affects the angle of the rocker arms in a hurry. THAT is when you may need to shim rocker stands to get the valve geometry correct i.e. valve stem is dead parallel to the rocker arm's adjusting screw at half open.
Colin

Cylinder head sealing ring? Is this a type 4 need?


and thank you.


neal

ruckman101 wrote: Cylinder head sealing ring? Is this a type 4 need?
and thank you.
neal

Type 1 engines have used sealing rings and base shims for all number of reasons for getting the dimensions correct. Air-Cooled.Net has some good information and decent $$ copper head sealing rings to cover botched flycutting and whatnot.

Type 4 engines were originally equipped with sealing rings, the factory suggested not using them in 1990 on the 2.0 engines.
Colin