The change to the ''big pistons'' happened beginning with
Engine # 211133
Bore x Stroke WAS
86mm x 68mm =790cc
It changed to
90mm x 68mm = 865cc
.........................................
Here's some voluntary info,,just for what's it's worth
If You are asking,,,and perhaps needing to rebuild an engine,,
or considering Upgrading from the small engine to the larger one....
Triumph parts are Expensive and there's not much available in terms of Used Parts to supply a ''Retrofit Project''.
I'd suspect even Less likely to find the 90mm Cylinders at decent price as they are more desirable.
Wiseco makes a 92mm/904cc Hi Compression Big Bore Kit.
It requires Boring the Cylinders and installing new oversized sleeves.
But it's only a few $100(usd) more than just a plain,stock Rebuild,,,,
and bound to be a LOT cheaper than upgrading a 790 to an 865 using Original Parts.
Wiseco Retail price for piston kits and sleeves is about $620
Here's a place offering them at about $500
http://www.newspeedmaster.com/html/wisec...
Between the lil' extra displacement and the higher compression ratio,,,it would make a Very Strong Running Triumph.
All in a useful riding range,,without revving the daylights outa it.
NO Suggestion to go out and DO that.
Just thought I'd mention it while on the subject of Speedy's engine size.
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