Crankshaft advice for a moron

[Mike - - Bristol]

Right guys , my long , long term S2 project will eventually need an engine. So to save a little money (to start with and until I blow it up), I will use a spare Mugello V2 barrel and have some local bods port and play with it , I have a 26mm phbh and a PM or Clubman pipe to go on it.
And back to my question. I want to use 60mm stroke crank but not too sure about which conrod length to use. What difference does it make and what changes would go along with the final choice.
The MBGM full circle 60x110 looks promising but any advice would be appreciated .

[olliewtf]

usually conrod choice is dictated by the compression height height of the piston you choose. for example, a suzuki piston, or any jap piston, has a shorter compression height (distance between little eye and top of piston) than a standard lambretta configuration. therefore you need to make up the gap by increasing the length of the conrod.
However, you can also use a longer rod to move around your port timings with the addition of different thickness base packers. a 110 rod is most commonly used for this purpose. longer rods ie 116 or 117 are used for jap piston conversions.
if you are getting your barrel tuned ask the tuner what he (or she) recommends. it may just be be worth opting for a longer rod as it is more versatile in future applications.
there is also a theory that the longer the rod, the smoother the engine, due to the longer stroke causing less vibrations, particularly at higher revs.

I have heard mixed reviews about the bgm cranks so ask around before purchasing. i have no experience of them so my opinion is null and void.
personally, i needed a crank for a jap conversion and wanted it to be 60mm. so harry barlow built me one and it was dead cheap for what it was.
also bare in mind that some 60mm cranks, mine included, need a trench cutting for big end clearance. I achieved this with a dremel. not pretty, but functional.

best of luck!

[sean brady scooters]

firstly ,con rod length does not affect the stroke length !..that is pre determined by the spacing of the big end from the center of the crank .
the smoother running that you allure to is from using a con rod that measures usually about twice the stroke ..so ideally 58mm stroke would be 116mm con rod .
as per compression heights of pistons ,then again things have moved on ..far better now to utilise pistons with more height above the pin to help lower primary compression ratios in conjunction with longer rods and packer plates to allow expansion chambers to scavenge efficiently .

[olliewtf]

woops i said stroke instead of length! two very different things.
and yes i remember that argument about the doubling affect, and was wrong to suggest that unless its exactly double, it has any different effect.
that last part is interesting however.
although i always assumed that most jap pistons had shorter heights than standard lambretta pistons. having said that, at the age of 23, i have very little experience of the range of pistons suitable for conversions.
can you answer me in a sentence, if at all possible, how increased primary compression helps expansion scavenging? is it due, and excuse my basic terminology here, to the increased thrust of the charge encouraging quicker and more efficient scavenging?
the thing that gets me about 2 strokes is the appearingly infinite variables you could throw into the mix; i remember when i tuned my ally barrel i was torn about opening up the transfer feeds, on the one hand i wanted to think that i could keep an increased primary compression, on the other i thought i could increase surface area and copy other tuned barrels. i opted for number 2 on the basis that someone explained to me very cleverly, and now i will completely deconstruct the brilliant answer into useless tat, that lambrettas tend to have high primary compression anyhow, so lose a little for larger gain further down the line.
Im still a novice so excuse me if ive mixed up my bananas from my oranges.
and soz about the thread drift.
its all good.