O-ring for Severin KV8090

hello, for the Severin KV8090 I can not find replacement O-rings for brew group and upper piston. 3 pieces three year old Oringe I have measured with the sliding gauge: 37.7x3.2mm. Possibly they are already shrunk? At a foreign spare parts shop (identical in construction Turmix Finessa) O-rings with 36,8x3,3 mm were offered. The groove diameter at the piston is measured approx. 37,7mm.
But what is correct now? I have already installed 37.69x3.53, but this soon braked the piston too much. The smaller standard dimension would be 34,52x3,53mm. Due to the elongation by 9% to 37.7 mm in the installed state, the cross-section is reduced to a cord dimension of approx. 3.33 mm (according to a manufacturer's table from Parker).
This would be very close to the cord dimension of 3.2 mm that I measured. Is a permanent cross-sectional elongation of 9% problematic for silicone rings?
Anyway, the machine works with that now. Can I leave the O-ring with the standard size 34.52x3.53 in with a clear conscience? Otherwise I don't know where to get suitable seals.

QUOTE (jamais @ Sunday, 12 December 2021, 16:48)

hello, I can't find replacement o-rings for brew group and upper piston for Severin KV8090. I measured 3 pieces of three year old orings with the sliding gauge: 37,7x3,2mm. Possibly they are already shrunk? At a foreign spare parts shop (identical in construction Turmix Finessa) O-rings with 36,8x3,3 mm were offered. The groove diameter at the piston is measured approx. 37,7mm. But what is correct now? I have already installed 37.69x3.53, but this soon braked the piston too much. The smaller standard dimension would be 34,52x3,53mm. Due to the elongation by 9% to 37.7 mm in the installed state, the cross-section is reduced to a cord dimension of approx. 3.33 mm (according to a manufacturer's table from Parker). This would be very close to the cord dimension of 3.2 mm that I measured. Is a permanent cross-sectional elongation of 9% problematic for silicone rings? Anyway, the machine works with that now. Can I leave the O-ring with the standard size 34.52x3.53 in with a clear conscience? Otherwise I don't know where to get suitable seals.

O-rings should be installed about 1 to 6% smaller than the groove diameter on dynamic piston seals anyway. With the nominal dimension of the O-ring of 34.52x3.53, this results in an almost optimal dimension anyway. A permanent elongation to 9% is no problem with a silicone ring in this application. The expensive replacement O-rings 36,8x3,3 mm, which are sold as spare parts in the shop of König for the identical type Finessa, are not necessary.
I have even used the identical Tchibo Esperto caffé (which are actually developed and built by Tunbow like all identical) O-rings of size 34.52x3.53. After more than a year still no problem.