Twin not so special

Groily · Post by **Groily** » Thu Jul 03, 2008 10:14 pm

I've come back to this after a few days playing with other marques and a diversion converting a '50s French tractor whose dynamo/reg had melted down to a filthy great alternator. What did I do in a former life to deserve that as a project on a hot June day? (Nice machine though - 3 cylinder air-cooled diesel that has already lasted one lifetime and will last another).

Have at least got the bottom end of the hybrid twin down to its component parts in the meantime. Glad to say the crank is absolutely fine, round and within spec, and excellent radii. Looks very recently reground. Tad of wear on drive side's shells - possibly not helped by that piston kissing head at full stretch. Centre main like new and rods' eyes round. Cams and followers also very good - the latter look darn near new. No weld in cases - nothing has ever gone for a walk. So a v nice basis for a decent motor. The Gods shine on us sometimes.

What continues to bother me is the top end. Why would someone mark a set of barrels '+20' if they were actually about +160? Why do the barrels bear a stamp saying 'B' - in the same relative spot on both - in a seriffed typeface not the sort of thing one gets in a box of letter and number stamps? Per Don's very welcome info re the differences between G9 and G9B, annular grooves on sleeves vs cases etc, these should be overbored standard 500 barrels rather than the real thing. Certainly the drive side case is standard un-machined G9. But why on this earth would anyone mark the barrels so carefully, and is the 'B' a maker's mark or not, irrespective of annular grooves on the sleeves? I'd never have applied such good and careful marking to anything I had messed with just to get it down to the pub and back. Fooling posterity wouldn't have been on my agenda. Wouldn't have known what posterity was even. But . . .

The key question remains - can the barrel bottom spigots survive at a 70mm bore? The centre of the gudgeon pin at bdc, when the load switches direction, is below the level of the bottom annular land on the sleeve; not good perhaps, although I have no idea what the loading would be. If a sleeve and/or piston skirt let go it would be curtains and could well cost me the bottom end.

The horns of these dilemmas can be sharp. Having sorted some a few not-so-bad horrors I could get a new set of shells and reassemble.With a compression plate or whatever to keep pistons and heads at proper remove.
Or, do I try to source some standard barrels (or resleeve the ones I have) get some standard pistons and go from there? Which I know would be fine, but would involve a few dips into the Family Allowance.

The barrel base flanges - which I looked at a bit carefully - bear no obvious signs of having been reduced although I can't say they haven't been - they're quarter of an inch spot on. The heads are exactly as one would expect bar some rough porting, with the recessed ring for the upper barrel spigot exactly as usual, and there was nothing funny about the angles of the rockers or the position of the eccentric adjusters, which were at roughly mid-point. If someone had shaved the barrels or heads to a stupid extent, something a bit cockeyed in that department would surely be evident. However, looking up the bores from the bottom, it's obvious that the pistons would hit the chamfers on the heads. Bit like Niagara falls - lots of water going over cos there's nowt to stop it. Could relieve the chamfers a a bit I suppose to get necessary clearance if a compression plate down below looked like an oil leak waiting to happen.

I shall muse more while I play with other things. It wouldn't really matter if it blew up I suppose - apart from the damn-fool loss of decent original parts if it were all down to my crass stupidity. And vanity of course.

itma · Post by **itma** » Thu Jul 03, 2008 10:28 pm

I suggest

ebay.fr

Don Madden · Post by **Don Madden** » Fri Jul 04, 2008 3:42 am

In the neolithic era, we had the G9B/20B. I paleolithic times the Cooper twin ruled. BSA, Triumph & Matchless were the mounts of choice in those days for our demanding desert racing. There was only a smattering of other brands, although some were very good.

The other makes all had bigger or more powerful engines & Frank Cooper, the Western importer for AMC wanted more. He had various special models built in his shops here to fit the American Market.

One was the Cooper Twin bikes, built by Dick Brown. Dick told me he bored the G9 barrels to 69mm & fitted Harley-Davidson WL, (750cc V-twin), pistons. He did not tell me the details of what mods were done to fit them & I've not seen them myself so don't know more. Frank told me about certain crankshaft oiling mods he had done but I don't know if they were used only on his race bikes.

The Cooper Twins had the alloy mudguards as used by the scramblers & higher bars were fitted. I know little else about them.

I have seen +0.060 pistons for the G9B so the non-grooved barrels can be bored quite a bit.

Cheers, Don. Edited by - Don Madden on 04 Jul 2008 02:43:43 AM

Groily · Post by **Groily** » Fri Jul 04, 2008 9:00 am

To get other bits or dispose of all itma!?
No, no, shall persevere and there'll be a result in due time, which will work. It's just a Q of what configuration to adopt and how robust the Family Allowance is!

Thanks again Don for your wisdom. Cooper twins sound like a whole story waiting to be written up! So a G9B even at +60 (c 70.5mm per Rick's starting point) would have a bit more sleeve thickness on the lower spigots than a G9 bored to 70mm, given the grooves are about a mm deep all round . . . at least 1.5mm or 60 thou more in fact. Hmmm. Nervousness heightens. Looks as if some standard bits will be needed.

I do have some 650 barrels (4 to choose from and all potentially serviceable) if anyone wanted to swap any 500 bits? I'll have a look at them again and maybe put something in the for sale etc slot with info re dimensions. But anyone interested, who has any spare G9/Mod 20 equivalent, pls send me a PM!

Over to the UK later today for a few days of traffic jams and the nanny state. Hope I survive both.

itma · Post by **itma** » Fri Jul 04, 2008 4:22 pm

Bring a stab vest to wear inside the M25

Don Madden · Post by **Don Madden** » Fri Jul 04, 2008 6:28 pm

Here's a thought for you, since the motor is already non-standard. Replace the cases with 1956 or later G9, which have the wider stud spacing. G11 cylinders, pistons & heads will then fit giving 600cc & good spares availability.

I have seen detailed pictures of a 1960 G9 with this conversion.

Cheers, Don.

Groily · Post by **Groily** » Wed Jul 09, 2008 8:08 am

Made it back without needing body armour itma but it was a near thing. Everywhere you go - especially if it involves the occasional search for something potable - there's the risk of falling foul of the neanderthal underclass, which is now to be found even in previously quiet niches of the sceptred isle. Really is a shame, and very noticeable to those who have lived elsewhere for a long time. On the positive side, loads of decent people and places still around, so all not lost quite yet.

Like the G11 idea Don - best thought to date. Shall look at v carefully as soon as I have finished with a nearly-on-the-road B*A . . . Then I'd have a much better (than it's got) set of cases for my Mod 20 in reserve, which would be a welcome by-product . . .

matman · Post by **matman** » Wed Jul 09, 2008 1:32 pm

quote: my H****a 350F to 425cc

Wow, Don,

with the four *s, that must be one of the very rare HODAKA bikes of that time. But a 350 Four?!

I nearly used to buy one (250cc two stroke single) from a GI who served and lived (and played with his band) here around Frankfurt. Chance missed...

Cheers, Jan

Don Madden · Post by **Don Madden** » Thu Jul 10, 2008 3:52 am

Speaking of fours, Jan-Hendrick, look around for my old 1938 Zuendapp K800 boxer four. It was fully restored & I had planned to bring it back to the US with me when I returned in 1954. It was stolen just before I left & was last seen in Weisbaden, not far from you.

My 350 four was a 1972 Honda, not common outside of the US. It would have been a great bike for the German roads back in the 50s.

My 1952 DKW 200 was also a great bike there. Wonder where those bike are now?

Cheers, Don.

Groily · Post by **Groily** » Mon Jul 28, 2008 9:53 am

The 'go 600' thought is a very good thought Don.
Am now coming back to this hybrid having had a week or 2 to think more while getting another marque back on the road.

What I have is:
1961 frame forks and cycle parts, all decent
A good 500 (dynamo)crank
A good set of early (dynamo) cases
A selection of good rods
Some very decent cams etc (marked SS if that means anything?)
All the other engine bits including the nice oil pump (and several others too)
An AMC box/clutch with the heavy-duty dimpled plates.

I also have the previously-mentioned massively overbored 500 barrels and pistons which I can't safely use - the spigot is minisculey thin down below, and the pistons kiss the 500 heads all round up top because they are too wide.

In addition I have the tired original set of Mod 31 cases (rod/rods have escaped at least once in the past), a probably unsalvageable 650 crank (radii look lost to me), more very good cams and followers etc from that engine, plus several sets of 650 barrels, a set of 650 pistons that match one set
AND 2 good complete 650 heads with valves, pushrods etc etc.

I am, I hope, also about to have a decent unwelded set of G11 /Mod 30 cases subject to checking whether they're dynamo or alternator.

However, despite reading the threads on Twin Changes and on Parts Interchangeability, I'm still confused on a couple of important points and would welcome enlightenment from those who know.

Obviously if I continue with the 600 theme, I'll need 600 barrels and pistons (and possibly pushrods although I assume they're the same as the 500?). Plus possibly a crank rebalance. Anyone know what the different factors were between the 500 and 600?

So, major Q - would 650 heads go on 600 barrels, I ask myself since I've got 'em already? Presumably the stud spacing is correct, and presumably the bore will match the combustion chamber/ top spigot recess on 650 heads same as 600? If the answer to that is Yes, then it looks as if a 600 it will become. If not, then probably back to replacing or resleeving the overbored barrels and keeping it at 500.

Grateful for any advice, and for warnings on any other issues relating to cylinder oiling, metering jets, pushrods or whatever. Never ever thought that I'd ever want or need to know all this sort of stuff!