Compression figures

[BI8759]

This might be a silly question but I've put a reconned 1275 head on the old girl and got an increase in compression to about 165 on all cylinders from 140. Is that to do with the head per se or to do with new valves?

[bmcecosse]

Yes! The combustion chambers are smaller = higher compression ratio. What size engine is this on?

[BI8759]

Its a 1098

[bmcecosse]

I assume you haver checked for exhaust valve clearance etc - sorry if I mentioned this before - I can't keep track of everyone doing this excellent conversion! 165 is good - it should go well!

[BI8759]

er.... not really. Just assumed that it would run, which it seems to

[bmcecosse]

Very dangerous........if the ex valves hit the block you will have bent valves at least......

[BI8759]

I don't disbelieve you but would that already have happened if it was going to? Done a few miles now...

[bmcecosse]

Indeed - but you may be only a very few thou from disaster...... As you say - you seem to have 'got away with it' - but for the benefit of others - the exhaust valves need 320 thou clearance - and this must be checked by measurement before going ahead with this excellent conversion. Some 940 heads have suffered some exhaust valve recession - and so they can 'just' creep inside this figure. If they don't - the valves must be sunk down into the seats until they DO have 320 thou clearance. This figure still allows for the gasket thickness as a 'safety margin' - your valves are probably working right in this slim safety margin !! NOT something I recommend.......