bigbird2 Posted November 8, 2013 Author Report Share Posted November 8, 2013 Don't have a picture BUT a head spacer would have exactly the same shape as a head gasket - just thicker. You may be right about the numerical CR calc difference? However, I'd guess that earlier CRs were much lower due to the limitations of air cooling if nothing else. My 250 Ossa MAR was so easy to kick that my 105 lb wife could start it. However it does have a looooong kick start lever. Quote Link to comment Share on other sites More sharing options...
bigbird2 Posted November 8, 2013 Author Report Share Posted November 8, 2013 These are the calcs I used to come up with the 1/3 reduction in CR. Please check and let me know if you agree/disagree? THANKS 72.5 Bore mm 36.25 Radius mm 36.25 Times Radius mm 1314 Radius Sq. 3.14 Times Pi 4130 Pi R Sq = area in sq mm 41 Area of top of bore hole in sq cm 0.3 3mm Spacer height/thickness in cm 12 3 mm spacer hole Voulume in CCs 270 Beta Displacement in CCs 11.4 Divide by Beta Stock CR 24 = Beta Stock Chamber Vol CCs 36 Expanded Chamber Vol in CCs 7.5 NEW CR (Displacement / Expanded Vol) -34% Change in CR Quote Link to comment Share on other sites More sharing options...
0007 Posted November 9, 2013 Report Share Posted November 9, 2013 My brain just burst into flames I have never seen how combustion chamber volume relates to compression ratio but just to mess with you further I would expect somewhat of an exponential change in combustion pressure and temperature when compression is reduced I would still look at a 200 but if you are sold on engineering a change like this I would go with a combination of stacking base gaskets to raise port height as well as a small change in compression Quote Link to comment Share on other sites More sharing options...
nzralphy Posted November 9, 2013 Report Share Posted November 9, 2013 Hi umm I could be wrong but I don't think there is a head gasket as standard. Quote Link to comment Share on other sites More sharing options...
mcman56 Posted November 10, 2013 Report Share Posted November 10, 2013 (edited) I get a similar but slightly different number. The CR is BDC volume/ TDC volume. Head volume is included in the BDC volume so the head volume would be more like 26 cc. The new condition would be 308/ 38 or 8.1 CR. I just tried to paste in somes spread sheet cells but the format got lost. I did run a 1/16" (1.5 mm) head spacer on a 250 Techno. It ran fine. The only unique feature was that it would tolerate being pushed down to very low rpms with a heavy load. Edited November 10, 2013 by mcman56 Quote Link to comment Share on other sites More sharing options...
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