Pepping up a G5

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
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Pepping up a G5

Postby Martin.S » Sat Aug 11, 2018 6:23 pm

Hello all. I owned in my youth a G2 CSR and would like to build a replica so hopefully tomorrow I'll be picking up another project - half a G5 basket case - that being frame, casings, crank, barrel, head, gearbox and hubs. So don't be shy in offering any spares.

I believe the G2's engine bore/stroke was over square unlike the 350 and therefore revved much easier. So would it be possible to fit a bigger piston to make the 350 over square and would the bottom end take it?
I've read here of upping the compression by using a 600 or maybe a Triumph 650 piston - how about a Triumph 750 one - any ideas?

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Re: Pepping up a G5

Postby ajscomboman » Sat Aug 11, 2018 7:03 pm

I ran a converted 59 650 twin piston in my G5 and upped the gearbox sprocket to a 19 tooth and it flew along, even with my frame on it and with passenger. They're still quite a square motor and will rev like buggery! The bottom end end is very robust. Other than a trick piston and cleaning up the ports a little there's not much else to do with these motors.

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Re: Pepping up a G5

Postby clanger9 » Sun Aug 12, 2018 6:37 pm

The 350 is already a bored & stroked version of the 250, so it might be a challenge to go much bigger with the displacement.
It might respond well to a higher compression piston: the standard flat-top one is only 6.9:1, so if you could find something that would take it to 8 or 8.5:1, that would help...
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

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Re: Pepping up a G5

Postby Martin.S » Mon Aug 13, 2018 9:38 pm

I measured the skirt of the barrel - something around 77 to 77.5mm diameter, I would of thought a 75mm bore (365cc) would be quite safe and room for a better squish band - just a thought

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Re: Pepping up a G5

Postby Ozmadman » Mon Aug 13, 2018 11:46 pm

Martin.S wrote:Hello all. I owned in my youth a G2 CSR and would like to build a replica so hopefully tomorrow I'll be picking up another project - half a G5 basket case - that being frame, casings, crank, barrel, head, gearbox and hubs. So don't be shy in offering any spares.

I believe the G2's engine bore/stroke was over square unlike the 350 and therefore revved much easier. So would it be possible to fit a bigger piston to make the 350 over square and would the bottom end take it?
I've read here of upping the compression by using a 600 or maybe a Triumph 650 piston - how about a Triumph 750 one - any ideas?

Why?? the bike rides fine, well mine does and will pull well into the 70's. Want a faster 350 with tons of excitement? then get a vintage Yamaha RD350 (39BHP compared to the lightweights 21) air cooled two stroke like I have..
Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed

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