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73mm bore?

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73mm bore?

Postby Adam_Winstone » Wed Jan 02, 2019 3:10 pm

Hi all,

Does anyone know how close you can overbore to the main cylinder studs before bore walls become too thin and cause issue? We’re all familiar with photos of 4 point seizures that correspond with stud holes but I note that oversize pistons are now available up to 73mm, which would leave the bore wall very thin on the alignment with stud holes.

Anyone have experience of running 73mm bore in any of the kits/cylinders available, whether iron bore (e.g. Rapido) or plated alloy (e.g. TS1, RB, Mugello)?

Cheers,

Adam
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Re: 73mm bore?

Postby Warkton Tornado No.1 » Wed Jan 02, 2019 10:28 pm

With anything in life that we strive to get the best from, there are compromises that must be made for the whole package to work well. The balance of power, torque, economy & reliability must be considered.

Being methodical pays dividends, & the likely problems that can arise from any departure from OEM design parameters must be considered.

"Four point seizures" don't just happen without something in the mix that hasn't been taken into account.

Scootering magazine recently brought to it's readers' attention that the stud & cylinder diameters are relatively important to each other, but I'm certain that if anybody is already clued up on that, it will be you, Adam.

I also doubt that you will have not considered ignition timing, sufficient cooling & piston metallurgy. If you look toward Lambretta Group Four racers, which, whilst the class survives, can still keep going quicker year on year, they have no means of fan cooling. Admittedly, Avgas has a huge influence upon the ability to run a cooler engine, but the trade off is that high octane ('Super Unleaded') petrol produces more power.

However, I believe that it is all too easy to overlook the inlet & outlet of our tweaked motors.

Personally, I don't marvel @ what I read about power outputs on Lambrettas fitted with restrictive exhaust bleeds, common to many 'Clubman' exhausts, because the smaller tailpipe diameters are an easy way to enhance torque. That can so easily be proven on the dyno, but 'real world', WOT rides of any duration can cause meltdown. That's nothing clever for me to say, as any decent two stroke tuning manual will verify. Somehow, though, I reason that many established Lambretta Tuning Emporiums consider that 'we' are immune from such a basic rule.

Another consideration toward augmenting cooling on a modified Lambretta, other than a LH footboard fitted scoop to improve air passage to the fan, is to think carefully about the carburation/inlet, particularly the size. I don't doubt that the cylinder cowling cannot be improved & wait with anticipation to see the images of the modifications performed by a regular contributor to this here Forum.

Perhaps I am alone, but my belief is that it is possible to go too small with the Venturi size. After all, that cool, stratified charge must vary in it's effect by volume, & if a bigger carb means easier breathing, I'll take the option for the likelihood of added cooling. It goes without saying that any filtration must be adequately sized, but I have voiced opinions about that on this Forum before.

I reckon that with reasonable care, a good choice of piston & taking into account all elements, I would definitely take the risk of a 73 mm bore on a cast iron barrel that had attributes worth saving. That feeling includes Rapido, which I consider a slightly flawed kit that can respond well to 'de' or 're'-tuning with the win-win results that 'stroking' can deliver.

I hope that you go ahead, & it would be brilliant, & perhaps a big 'ask' if you were to post images on this Forum showing your build in progress :D
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Re: 73mm bore?

Postby Adam_Winstone » Thu Jan 03, 2019 12:08 am

Thanks for the well considered response. That's all I can respond with at the moment as I'm on a tiny phone screen (LOL).
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Re: 73mm bore?

Postby Fast n Furious » Thu Jan 03, 2019 2:30 am

As my old dad used to say:- Candles that burn twice as bright, only burn for half as long! ;)
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Re: 73mm bore?

Postby hullygully » Fri Jan 04, 2019 12:39 pm

think a mate of mine ran a 72.5mm steel linered TS1 with no problems :shock:
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Re: 73mm bore?

Postby Warkton Tornado No.1 » Fri Jan 04, 2019 3:48 pm

hullygully wrote:think a mate of mine ran a 72.5mm steel linered TS1 with no problems :shock:


You make an excellent point, there, that I should have made with my earlier response.

I had a Rapido barrel subjected to catastrophic blow-up & it was salvaged by Langcourts, returning it to original bore size, matched ports & everything you could hope for. They matched the liner to the piston that was sent with it giving a spot-on clearance of 3-1/2 thou.

All things considered, that might be the better option, although the cost nowadays is likely to be £100 plus.

"Swings & roundabouts" though as you save on the cost of reboring, honing & a 'special' piston & further savings can be had if you can juggle the geometry with, say, a TS250 piston & cylinder base packing to suit.....

Food for thought :idea:
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Re: 73mm bore?

Postby Adam_Winstone » Fri Jan 04, 2019 10:06 pm

^... yep, I am considering options at the mo', hence looking for input from others who have already gone down the route of big oversizes. I have run 72mm a couple of times already and had no issues with them (in the right barrel), and 71 - 71.5 in TS1250 oversizes, but and just wondering whether the 72+ (up to 73mm) is perhaps just pushing it that step too far?

Thanks for the thought provoking responses so far :)

Adam
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Re: 73mm bore?

Postby phil23fair » Mon Jan 14, 2019 5:24 pm

I'm running a Harry Barlow tuned Rapido 250 with a 72.4mm piston without any problems
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Re: 73mm bore?

Postby Adam_Winstone » Mon Jan 14, 2019 9:25 pm

^... Ta
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