650 Barrels on a 500

[atreyu]

quote:
A 66mm (standard 500) cylinder will bore right out to 70mm-odd if you can find pistons to suit, but the cylinder spigots are extremely thin-walled at that stage,


Groily, Do you think I would get away with 69mm pistons? I've found some that I feel I can make work in my engine but wonder how thin the cylinder spigots will be, would this be a safe size without going "too far"

[Don Madden]

The original "Cooper Twins", 1952-53 550cc were regular G9/20 overbored to 69mm, so should be OK. The 20B/G9B for 1954-55 used thicker cylinders & different cases. Cheers, Don.

[Groily]

Oh Boy, that's a tough one David.

A +60 piston, the normal max for the bore, takes us to 67.5mm. (And I think the factory used to draw their line at +40 way back when).

But hey, it can be done I know, because I had 70.5mm until common sense made me get the barrels resleeved! (I've still got the pistons . . . in very good order . . . not that expensive!)

At that bore, the pistons were wider than the combustion space, so I had to machine a chamfer on the piston crown outer edges to stop them kissing the heads; the comp ratio was unusually high as a result; and I was worried about the spigots, which were thin to say the least (under 2mm and a bit less in places because the things hadn't been bored dead centrally). I ran it on NGK B8ES. It always started and ran OK.

It ran like that from '08 to early this year, did about 4000 km on it, a lot of them at full throttle as it was such incredible fun, and didn't blow it up. Amazingly. But I had spent a bit of time on the bottom end too, which may have helped cope with the insanity up top.

At a 'mere' 69mm you'd have an extra 20 thou all round on the spigots compared to where I was, and hey, maybe it'd be fine. The barrels are quite well supported in the throats, after all, and the heaviest loads are when the piston is within the finning.
But you'll be playing with fire all the same. If anything lets go, you'll be looking at barrels rods and maybe the crank, etc.

Mine was running on twin 626 concentrics, all the valve guides had been shortened 10mm and roughly chamfered to improve flow, and a dremel had been run round the inlet and exhaust ports. (The guides were badly worn in no time as a result, for being too short.) The breathing had been increased (a good thing) with a large pipe off the timing cover (the flapper valve was retained in the drive side).

For all its silliness, it was an extraordinary engine for a '53 twin, capable of pulling an easy 70 in 3rd (AMC box), it wrenched your arms off when it hit about 5000 rpm on full throttle and I'd have put it up against any other period bike of any size in a traffic lights Grand Prix. The noise on an open mega was something else. Oh, and it used oil when maltreated.

I never went too far from home unless I knew She was in a good mood, that I'd left the trailer hooked up to the car and there was petrol in it!

So, the choice is yours! I'm happier with mine re-sleeved to 500 in the sense it won't explode any time soon, but I do miss that acceleration and willingness to rev. It's still quite good on the twin carbs, albeit with new (proper) guides and valves and a lower compression ratio with AE pistons of about 7.25:1.

The wild side of me says Go For It and port the heads, get some carbs, make some manifolding, open up the breathing and make a wannabe G45. The sober side says don't be so bloody daft and if you want more horses get a bigger bike!

[rex.webb]

All this "POWER" !!!!!!!!!!!!!!!!!!!!!!!
Makes me fell verry laid back with
10 BHP
at 3600 RPM
But ther it is Every one to there own.
Ride with Care and Live Long.