62mm stroke crank

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carlos fandango
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Tue Jul 14, 2009 7:34 pm

I was going to get an rb250 but now im worried about the reliability of the kits, re. the exhaust bridges being the same as the rb225.
So i have plan B, involving a 62mm crank. ;)
Does the crankcase need any mod. to fit a 62mm? I think ive heard about it needing a clearance groove machining in the crankcase for the con rod big end ? if so what size?
Thanks Russell
When i was young my Mom said "if you haven't got anything nice to say, say nothing at all " ..................... and people wonder why I'm quiet around them !

ricalnic
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Tue Jul 14, 2009 8:03 pm

CF I think it is only the RB22 that has the additional exhaust ports. RB20 and 25 have single port.

rgds

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drunkmunkey6969
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Tue Jul 14, 2009 8:52 pm

ricalnic wrote:CF I think it is only the RB22 that has the additional exhaust ports. RB20 and 25 have single port.

rgds

Only RB20 has single port, RB20 & 25 have bridges.


ricalnic
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Tue Jul 14, 2009 9:49 pm

I stand corrected, sorry. tks DM

DJ 240
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Wed Jul 15, 2009 6:22 am

HI yes you do have to mill out engine casing I am just building mine now, but dont go to mad because the casing becomes thin ?
my mate Vinnie has this set up with a few more mods and it is a quick bike very Iam using a long rod in mine as suggested to me
buy Dave Webster

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garry inglis
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Fri Apr 24, 2020 8:00 am

Rb info

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Warkton Tornado No.1
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Fri Apr 24, 2020 12:49 pm

Lambretta tuning being all about compromise within the standard geometry, a longer stroke crankshaft will only fit if:

The big end outside diameter has been reduced compared to the norm (I have a Terry Shepherd (RIP) item waiting to be tried) although Tameni 62 mm's may clear;

The housing bore is increased in diameter to clear. This in itself is no bad thing IMHO as HPC is likely to result in less power with an engine of this nature.* Then there is also 'windage' which has nowt to do with curries & beer. This modification can be achieved adequately & neatly with flap wheels etc;

A combination of both of the above.





* anybody cynical about the benefits of lowering primary compression can Google & find tests on a two stroke where the only modification was to lower the primary by 'just(!)' re-rodding the crankshaft with a longer rod. I'm sure that in the very near future, the current, excellent articles in Scootering will acknowledge this aspect, but it is pertinent in that increasing/optimising transfer entries does help toward what some of us believe to be a beneficial scenario. Quantifying appropriately is the difficult part & I personally can only look toward the likes of Japanese air cooled technology for what I believe to be better examples of infrastructure. Available rods & pistons is why the Grampian/Mitaka data is an automatic 'go to' when it comes to the planning stage of any engine ;)

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