lördag 16 augusti 2014

Torque specifications.

Now the assembly of the long block has begun. As you can see  I have chosen to leave the surface of the case as-is to get an "old" look. One thing to pay extra attention to is the torque specifications. The design of the Okrasa heads makes them weak around the upper middle section, thus normal torque specs shouldn't be followed.

I have two sets of instructions for the Okrasa engines, one in English, probably written in 1957 at the time when the TSV1300 was launched. The other one is a German installation manual for Okrasa exchange engines from around 1963. The interesting thing is that the torque spec differ a lot*. See below:

Okrasa English c:a 1957: 2.5 - 2.7 MKg for the upper middle, 2.7 - 2.8 MKg for the rest.
Okrasa German c:a 1963: : 3,3 - 3.5 mkg for the four upper and 3,4 - 3,6 for the lower.
Stock 30/36 hp spec: 3,6 - 3,8 kpm for all
Stock 34/40 hp spec: 3,2 kpm

One observation is that the torque for stock 30/36 hp engines is much higher than for the stock 34/40 hp. My guess is that the extra torque goes into the crushing of the copper gasket. This gasket is only an extra safety measure to avoid exhaust to leak into the heating system and have no other technical function. If the seal between head and cylinder is in good shape there is no need for it. For the newer engine type this gasket was removed.

As my intention is to have a properly built engine in good shape I chose to remove this gasket. I see no point in putting extra stress on the Okrasa heads by having this copper ring (or risking not to seat the cylinders properly if the torque is not enough to crush the rings).

So what torque to use? As the clamp force derived from an applied tightening torque is very much dependent on friction (approx. 90% of the torque goes into friction and only about 10% is actually providing useful clamp force), you really don't know what you end up with anyway. My intention is to get the friction reasonable low and to use the higher torque figure from the older Okrasa instructions. It's guesswork, but as a home-mechanics I can't make any deeper investigations.

*I have written the units exactly as my sources. To make it clear: 1 MKg = 1 mkg = 1 kpm = 9,8 Nm

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