The piston is not the original and is quite highly domed and would have been fitted to increase the compression ratio of the engine, Blackburne did supply their own high compression racing pistons though at the moment there is no indication as to what engine this piston would have originally come from.
After
the head was removed the bore and stroke was measured: 71mm x 88mm,
which straightaway says one important thing: it's actually a 350!
Removing
the cylinder barrel revealed another interesting find; only the lower
piston ring was fitted. In between the two ring grooves there is quite a
bit less 'varnish' than above where the upper ring would have been,
which suggests the piston was run for much longer with both rings and
then run for a shorter period with the top ring removed. Why a ring was
removed is a bit of a mystery, it may have been that Stan removed a ring
to gain some more engine speed when he was using the bike for
sprinting, though leaving the top ring would have given a bit more
compression. Perhaps he tried the top ring and there was some piston
instability? I think this will always be unknown!
The con rod was found to be very highly polished and the paper gasket between the crankcase and cylinder barrel appears to be from a magazine, a peer into the crankcase revealed plenty of old Castrol 'R' slopping around and a clean looking crankshaft, to be investigated further when we get the time...
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| The highly domed piston crown |
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A view of the underside of the piston , note the undercut machined into the lower inside of the skirt, as well as the highly polished con rod
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