TheSamba.com :: View topic - Why is a “94 case”-cylinder really 97 mm?

Mon Oct 08, 2018 3:38 pm

I’m rebuilding an engine and my mechanic said to order 92mm pistons with thick-walled cylinders that, as the catalog puts it, “fit 94 Case, 94 Heads.” Okay… then how come the outer diameter of the case-end of the cylinders is actually 97mm, and the head-end diameter is 101mm? So, where the number “94” came from.

Mon Oct 08, 2018 3:48 pm

Beacon wrote: So, where the number “94” came from.

That is the inner diameter of the cylinder in millimeters . . . the "bore"! :wink:
To determine the displacement of any engine, you will need to know the cylinder bore diameter . . . it is one of the three variables that is plugged into the formula!

On air-cooled VW cylinders, the top (at the head) of the cylinder is thicker to contain the cylinder (combustion) pressure and heat . . . the bottom (case end) of the cylinder does not require the same thickness as the top since it is under only crankcase pressure.

Mon Oct 08, 2018 5:34 pm

57BLITZ wrote: Beacon wrote: So, where the number “94” came from.

That is the inner diameter of the cylinder in millimeters . . . the "bore"! :wink:
To determine the displacement of any engine, you will need to know the cylinder bore diameter . . . it is one of the three variables that is plugged into the formula!

On air-cooled VW cylinders, the top (at the head) of the cylinder is thicker to contain the cylinder (combustion) pressure and heat . . . the bottom (case end) of the cylinder does not require the same thickness as the top since it is under only crankcase pressure.

So, "94" is an agreed upon term mechanics know means 97mm at the base and 101mm at the head? I guess every trade has it's jargon. But it seems to make more sense to give the actual dimensions.

Mon Oct 08, 2018 6:41 pm

Beacon wrote: So, "94" is an agreed upon term mechanics know means 97mm at the base and 101mm at the head?
Well . . . It most instances it is and someone building a "Big-Bore" air-cooled VW engine should learn it. The size of your cylinder is described by its "Bore" size.
Pistons are also referenced to by the cylinder bore size, however, they are actually a bit smaller then the cylinder inner diameter.

Have a look at new cylinder heads and note how they are described by "Bore" size . . . http://www.cbperformance.com/category-s/387.htm
Standard bore size for these heads is recognized as 85.5mm.
Same for new crankcases . . . http://www.cbperformance.com/Engine-Cases-s/200.htm

Mon Oct 08, 2018 6:52 pm

What 57Blitz is saying is 94mm is the INSIDE DIAMETER of the cylinder.

So, it is indeed an actual measurement.

There are standard size outside diameters the industry has settled on, but it could be anything depending on how the thick the cylinder walls are. And in the past, different size cylinders have had different OD sizes.

And there are thick wall, and thin wall sets. But the ID is what we are taking about when we say 94's. (Or other sizes)

Mon Oct 08, 2018 7:01 pm

Beacon wrote: So, where the number “94” came from.
Comes from your mechanic.
Tradition?
It is true, no part of the piston nor cylinder is 94mm :lol:

Mon Oct 08, 2018 7:39 pm

Don't feel bad. Two by fours are not 2"x4".

Max

Tue Oct 09, 2018 1:53 am

well if you know what your doing it makes perfect cents. so... where does the 92 come from??? it is A or B? or possibly C????

Tue Oct 09, 2018 4:16 am

I found some pics (posted by modock) from an older thread that may explain better...

Look at chart dimensions D (cylinder ID), B (case bore) and C (head bore).

The D-column is what is normally referenced as the piston/cylinder diameter used for calculating displacement. The "thick walled" 92mm cylinder would have a 92mm cylinder ID (D) but have larger OD sizes (B or C) for a 94mm cylinder providing thicker cylinder walls for better strength and cooling.

Tue Oct 09, 2018 4:48 am

Beacon wrote: I’m rebuilding an engine and my mechanic said to order 92mm pistons with thick-walled cylinders that, as the catalog puts it, “fit 94 Case, 94 Heads.” Okay… then how come the outer diameter of the case-end of the cylinders is actually 97mm, and the head-end diameter is 101mm? So, where the number “94” came from.

You need to verify if your case is currently cut for 92s or 94s. They make thick walled 92s that are turned thinner at the base to fit a normal 92mm case, but you need a 94mm cut head. Or the normal thick walls are 94mm cylinder diameter on both ends.

Tue Oct 09, 2018 6:12 am

Ashman40, your chart clears it up. It's just like buying a 2X4 that is not really 2" by 4"
Thanks

Tue Oct 09, 2018 6:28 am

I wanted to make over a dozen wood discs just over 3" diameter. I ordered a 3.25 inch hole saw from Home Depot as the hole wood be 3.25 inch, the discs would be slightly smaller due to wood loss while cutting.

Yes, I'll be cutting those from a 1 x 4, which is really 3/4 inch x 3.5 inches....