34a/Hawkestone barrel

[249XUU]

Sounds like a daft question but has anyone used a spacer under the base of an iron Hawkestone barrel to get a softer power delivery?.

[Brian Catt]

Not too daft a question actually. I've seen this method used on several "continental" machines. I've seen a works Greeves with a slab of alloy under it's barrel, but that may have been for a different reason.

[Peter Rotherham]

Villiers Cylinder Barrels.

The old chestnut crops up again here. Be very careful as to EXACTLY which Barrel & Rod you are using. The older 31/33A type which are a direct fit onto the 9E engines, or the later 32/34/36/37A Barrels which have a thicker base flange and accordingly longer Connecting Rod. I have seen the early Barrels spaced up by using an alloy plate to allow them to be used on the later Rods. My own TE has a Greeves Square Barrel with a plate instead of a copper Head Gasket to reduce overall compression levels.
If you insert a plate under the barrel you will raise the height of the Exhaust mounts, maybe enough to cause problems, and the effective opening times of the ports. Transfer port sealing is important at this is quite often only just achieved by the top ring at TDC in some 2-stroke engines. Too high a lift and these will still be open at ignition point. I'd do a Head off, measure before considering this venture. Again, too high and the compression ratio would lower, not something I'd think a Scrambles engine would benefit from ?
I'd ask the experts like Adrian Dickerson.....

Any help?

[John Wakefield]

The 34A (Hawkstone barrel) used with a S25 Villiers carb (you will need to make an adaptor as the 34A has a flange mounting for an Amal carb) should give a useful increase in power over a 32A without sacrificing torque & low down power. Lots of 34A barrels came onto the market cheaply in the 60's from the likes of Pride & Clark (thats a name from the passed) having been removed from engines by Greeves to fit alloy ones. I fitted on to a Sprite trials I had at the time & it made it a bit more lively. I think today though the 34A barrels are a bit sort after, worth looking for at autojumbles.
As Peter says fitting spacers under barrel is not recommended as it alters port timing. As Peter says you do need (1/4" I think) spacer under the 31A barrel to fit 32-37A engines. The 31A barrel has a very thin mounting flange compared with the other 32-37A types.

[Colin Sparrow]

Perhaps I'm a bit thick here, but isn't the point of putting a spacer under the barrel to compensate for the use of a too-long rod? The object, surely, is to arrange things so that the piston is pretty much flush with the top of the cylinder at TDC without clouting the squish band.

If it's set up like that, surely the port timings would be as the maker intended...

Signed, Puzzled of Suffolk

[John Wakefield]

Thats it exactly Colin, as I said in my previous post the spacer enables the
31A barrel to fit the 32-37A series engines with the longer conrod. The 31A had basically the 9E bottom end that had the shorter rod.
249 XUU was asking if it was in order to fit a spacer under a 34A barrel to give softer power delivery. Clearly it is not.

[Brian Catt]

I have seen this method used on other bikes, but just to soften a bike it's easier to use a thicker head gasket (I did this with my Ossa SDR). From his original question, I don't think the over long con-rod is the problem, just fierce power delivery.

[riflegreen]

All the production square barrel machines were fitted with decompression plates between head and barrel .

As I recall some were steel and some aluminium .

Chris

[Andy Z]

I have seen this method used on other bikes, but just to soften a bike it's easier to use a thicker head gasket (I did this with my Ossa SDR). From his original question, I don't think the over long con-rod is the problem, just fierce power delivery.

Hi Brian,

Do you still have the SDR? I only ask as I have one too, and I can't imagine there are many of these bikes in the UK. Mine has a Stiletto barrel, which has evil porting! The standard comp. ratio on the SDR is 12.3 to 1, so I'm not surprised you lowered it!

Sorry to go off-topic guys!

Cheers,

Andrew
190

[Brian Catt]

No that went along time ago. I used a trials ring/gasket which was quite a bit thicker than the original one. I would have liked to ditch the stupid 2 needle carb. Got muck in there a time or two, and it took hours to get off, clean and re-fit! It was a bit quick when you lit the fire!

[greeves246]

Hi Brian,

Do you still have the SDR? I only ask as I have one too, and I can't imagine there are many of these bikes in the UK. Mine has a Stiletto barrel, which has evil porting! The standard comp. ratio on the SDR is 12.3 to 1, so I'm not surprised you lowered it!

Sorry to go off-topic guys!

Cheers,

Andrew
190

the ossa stilletto i think has the same as the same porting as the MAR
just the head spacer is diffrent 1mm for stilletto and 1.5 for enduro and 2mm for MAR alters the performance but the stiletto did have a evil powerband 30+ odd screaming horses on the day !

[Mike48]

I'm surprised at your reference to the powerband of the Stiletto, Greeves246, they are my favourite bike, having raced one MANY years ago. I found them very rideable, with good midrange, helped no doubt by the double needle carburettor, which when set up correctly (not by me) gave excellent throttle response through the rev range. Much better than my Bultaco Pursang of the time, even though it had more peak power.

[Brian Catt]

I'll second the wide range of power! On my SDR, although softened, it was amazingly tractable yet went like stink when you opened the throttle.

[ADickerson]

Softer power from a 34A!

Sounds like a daft question but has anyone used a spacer under the base of an iron Hawkestone barrel to get a softer power delivery?.

Just to get back to the original post, it might be useful for us to understand why the question was raised? Pete, you summed it all up perfectly well, and thanks for the compliment, have to confess that I'm not the right person to provide an answer, as I've been working in totally the opposite direction for the last ten years or so! Perhaps the originator could add a little more background to his/her post. I'm sure there's a wealth of advise, some of it might even be useful, just waiting to be past on.

[249XUU]

The original question was raised as my 31a engined Scottish had been pinking and siezing from time to time and the big end developed play over a period of just 6 months.
I replaced the crank with a better one (standard - not full circle) and placed a 1mm spacer between the barrel (iron scrambles barrel) and crank case mouth. I ran the bike up the other day and noticed that it had the effect of softening each power stroke just noticeably and that is with exactly the same ignition settings. I will let you all know how the bike reacts to sustained high revs and if it has cured the pinking issue when I get the chance (young family to look after and all that..).