G3 500cc conversion after piston death

[AFSMatchie61]

Hello
New Club member here, on my second G3, an ex-AFS 1961 machine I've enjoyed greatly. Unfortunately the top of the piston came off and the engine awaits rebuild. I was thinking of rebuilding as a 500 For the extra torque but would need different crank, flywheels piston and I think conrod? These parts seem in short supply, certainly not listed on AMOC or AMC Classic spares. I was wondering if anyone knows if other parts also need to be changed and where might such parts be obtained from?
Many thanks
Dave

3E8F6D09-984C-49ED-9BD8-BBBF88FCFF6F.jpeg

[dave16mct]

The crank and conrod won't need changing but may (or may not) have to be re-balanced. Other parts needed are 389 carb, exhaust pipe and silencer, cylinder head, barrel and piston, front sprocket, possibly clutch with more plates and advance and retard springs. There is at least one other AFS bike which has been 'enlarged'
Dave.
PS the standard 350 barrel can be bored out beyond the +60 thou limit if a piston from a another make is used ( or from an AMC short stroke which gives about 410cc). Certainly it's been done on comp bikes, not sure about road models. Perhaps someone's done it?

[AFSMatchie61]

Thanks Dave, that really is good info and gives me pause for thought. If I change the exhaust downpipe and carb it will change The appearance of a very original I and nicely mellowed unrestored bike - to my eye anyway! I enjoyed it as a 350 hugely so might just keep it that way (and it gives me an excuse to get a 650 twin too 😀)
Any particular piston you'd recommend for a 350? I think the original is a wire wound split skirt design.
Thanks 👍
Dave

FA3452A4-2B16-4FC6-A4FF-6007025826BB.jpeg

[nomisimon]

Nice looking bike...I think I'd stick with an over-bored 350 rather than a 500 conversion. I've ridden similar bikes with both engine sizes and thought the 350 was nicer. I'll be looking for a twin next year to go with my 1953 G3LS.

Simon

[Pharisee]

My late brother-in-law, Malcolm "Nobby" Saggers ran a modified 350 Matchless before his untimely passing. That was bored out and used, if I remember correctly, a Triumph piston... possibly from a T140(?) that had a bore of 76mm and would give you a capacity of 420cc.
If also remember that he had grafted on a Velocette cylinder head. A strange mixture but one on which he toured Eastern Europe on more than one occasion, with luggage and a pillion passenger. There are others on here that knew him for a lot longer than I did and may have more reliable information.

[dave16mct]

There is some discussion about the use of various AMC pistons to up the capacity in Jampot February 1999. This can be accessed (above) by clicking on 'club' and scrolling down to find the 'Jampots 1954-2006'.
Dave.

[AFSMatchie61]

Thanks all for your input and Da ve for tge pointer. I think on balance I will retain the bike as standard; all sorts of unforseen consequences usually happen when I embark on ‘improvements'! 😀

That Matchie with a Velo head must have taken some engineering skill to build! Remarkable mix.

Is there a particular 350 standard piston to go for? I'm hoping to prevent a reoccurence of my incident!

Thanks
Dave

[dave16mct]

Ebay for original type Hepolite pistons. Thorntons of Shrewsbury and the club's own spares scheme will be able to supply a pattern piston. Russells sometimes unearth original stuff and are worth a try. The chances of finding a wirewound original are slim but they do turn up. If your original piston was a wirewound one you may have to hone the bore for more clearance. The clearance neede should be written on the box with the new piston(but often isn't). Lot of info on here if you do a search.
Dave.

[bitza]

F W Thorntons are now based in Telford .

[dave16mct]

Yes, just habit calling it that. The old shop's still there. Walked past it lots of times. Just further down towards the river there's an advert, Velocette Motorcycle Sales or something like that still painted high on the wall. Also opposite there's a very old fashioned cobblers shop and he's still there making and repairing shoes.
Dave.