Mea­sur­ing com­pres­sion ra­tios

Classic Motorcycle Mechanics - - WORK SHOP -

To have any un­der­stand­ing of com­pres­sion ra­tios (CR) in a twostroke there’s a need to know the cur­rent CR be­fore any ma­chin­ing gets un­der way. For the pur­poses if this ar­ti­cle we’re only look­ing at the un­cor­rected CR i.e. the vol­umes of the cylin­der/head at top dead cen­tre and bot­tom dead cen­tre. The com­monly heard term ‘cor­rected CR’ refers to mea­sure­ments taken after the ex­haust port has been closed by the pis­ton. With­out go­ing into the maths there are four key vol­umes that need to be con­firmed;

1. The vol­ume of the pis­ton crown. 2. The vol­ume of the cylin­der head. 3. The vol­ume of the cylin­der head gas­ket. 4. The vol­ume of the cylin­der tak­ing into ac­count any over­sized pis­tons/bores.

From these and the vol­ume of the cylin­der(s) a com­pres­sion ra­tio can be cal­cu­lated us­ing all that O-level maths you never thought you’d need again. With multi-cylin­dered en­gines each bore/head com­bi­na­tion needs to be in­di­vid­u­ally as­sessed; there’s never any guar­an­tee they’ll all be the same! As a rule of thumb for a road go­ing stro­ker an un­cor­rected com­pres­sion ra­tio of around 10:1 should be a safe bet with stan­dard pump petrol but if you’d like a lit­tle more of a safety net then the premium grades of su­per-green fuel will give you and your en­gine a lit­tle more wrig­gle room.

A/ With the pis­ton at TDC a piece of thick steel is tem­po­rar­ily stuck to the head with grease. Us­ing a bu­rette, paraf­fin is care­fully mea­sured into the cylin­der. When both drilled holes are filled the vol­ume of the pis­ton crown will be the paraf­fin used mi­nus the vol­ume of the two holes. B/ Us­ing a sim­i­lar tech­nique a clear sheet of thick plas­tic is used to seal the cylin­der head; the vol­ume can then be as­cer­tained. It’s cru­cial here to en­sure that the cor­rect grade of spark plug has been fit­ted or the read­ing will be wrong. C/ Us­ing a mi­crom­e­ter to mea­sure the thick­ness and Vernier to mea­sure the di­am­e­ter the vol­ume of the head gas­ket can be cal­cu­lated. It might not ap­pear to be sig­nif­i­cant but on the 180cc twin each cop­per gas­ket yields 1.1 cc of vol­ume.

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