Midget Compression Ratios

A low compression (US spec) 1275cc engine at 8.0:1 would have to have a total combustion chamber of 45.54cc.

To raise the compression to 8.8:1 (UK or Oz spec) you need to reduce the combustion chamber to 40.87cc - a reduction of 4.67cc.

An unmodified cylinder head loses 1cc for every .013 shaved off it, so you need to take .060 off the head. Cost, anywhere from $50 to $150 depending on how good the machinist and his tools are. Many mills (and machinist) will only take .005" cuts, in this case 12 passes over the head. Our mill can take .020" off in one cut - 3 passes, 1/4 of the time spent and a lot less cost.

BUT THIS IS NOT A WISE MOVE. You would be ruining a perfectly good cylinder head.

All 1275cc engines shared the same head, but the difference in compression is due to different dishes in the pistons. The very best way to increase compression to 8.8:1 would be to change the pistons for an MG Midget or Morris 1100S set. Sure, this is a little more work, but not much, and you get the added benefits of having new pistons and rings and new big-end bearings. For very little additional cost you can stick in Cooper S pistons and bring the compression up to 9.5:1 - still fine on premium unleaded fuels

avagoodday
Colin