Cylinder

[Janne]

Hello everybody! Does anyone know if it's possible to fit a G80CS cylinder on a 1951 G80? Or maybe I should ask, is the bolt pattern for the head and cylinder base the same?

[Joker_Bones]

[Andy51]

Simple answer is no. Crankcase bolt spacing is the same, but cylinder head spacing is different. As through bolts are required, head won't fit on bolts. I don't know if there is enough 'meat' in the head to allow larger holes to be milled using slot drill at centres matching those in the head, encompassing the original ones, then using spigot nuts on the studs. Andy

[Joker_Bones]

Are you sure about that Andy?
The Dick Casey video above shows that the stud spacing is not the same for iron barrel road model crankcases and alloy competition model barrels

[Andy51]

I stand corrected. I knew it was either the head holes or crankcase holes that were out. Either way, it is a significant engineering task. I recently saw a pre '55 Matchless special with what looked like an alloy barrel, but it was just painted, it matched the head colour perfectly. Longstroke alloy barrels are rarer than hens' teeth and the 2 I have seen on ebay over the last few years went for £££s.

[Janne]

Thanks for the info. Since through bolts are used I guess the head will not fit either. Is there anyone who knows the advance in degrees for G80. I am building a chopper with the G80 engine and a BSA gearbox with Triumph clutch and rear wheel, Honda 500 -72 frame, Honda front wheel with double leading brake shoes, Sportser front end (+6") and tanks plus a supercharger. I have the magneto but I am planning to use a H-D ignition which the advance range is 5-35° that can be modified.

[Pharisee]

.... Is there anyone who knows the advance in degrees for G80.....

Relatively simple geometry, knowing the crank stroke, and conrod centre distance will give you a crank angle of 40.91° BTDC for the generally accepted full advance distance of 1/2" BTDC for the piston. In reality, anywhere between 39°and 41° would be good enough. This was calculated for the long stroke engine. You'll have to work it out yourself if you have a short stroke motor.

[Janne]

Thanks!

[m knight.]

You will need to strengthen your bottom end and big end in particular, the standard engine could snap crank pins, a supercharger would even at low boost pressure surely wreck the standard engine in no time..