Lambretta 60mm crank

[paulnobodyimportant]

I keep seeing the mention of these cranks, so what are the pros and cons of these over a standard 58mm crank.

Plus, what are the benefits of fitting a new conrod to a SIL crank. Is it that they are less liable to bend and you may have a better big/small end. Only saying this as I was thinking of modifying my SIL crank for a standard 200 engine re-buildand wondered about the need to change the conrod etc.

I appreciate that it may be better to go out and buy a new black MEC crank, and if so, its back to my first question about the 60mm crank.

[corrado]

Re. Conrod ...Big end bearing is very suspect in a SIL crank and if you're going to change the bearing for a better one then you may as well "kill 2 birds" and get a Yamaha 110 conrod fitted at the same time. It means that you need a 3mm packer on the base of the cylinder but that lends itself to altering the port timings just by altering the thickness of the packer without touching the barrel itself. SIL crank webs are reputed to be very strong [better than the MEC] according to some tuners. The SIL conrod leaves a lot to be desired in such a critical area. Cost wise it'll probably cost less to uprate the bearing and conrod than the cost of a new MEC crank. Easy option is just to fit the MEC crank though.
Re. 60mm .... That's one for those in the know.

[rosscla]

It will alter your port timings and squish, you'll gain a few cc and you might need to fit a packer at the base or a different head gasket to get the settings where you want them. It might also be advantageous to consider one with a different rod length and quality, this would give more flexibility, but again you'd need to think about what you're trying to achieve.

Some of them need a groove machining in the casing as the big end of the rod might catch, your piston may touch the casing at BDC.

In relation to vibration, this will depend on the quality and balance of the crank you're considering, again there is a school of thought that says the rod length should be twice the stroke, so 58 / 116 , 60 /120 ought to give less vibration, I think this is something to do with piston acceleration or velocity but that's getting a bit technical for me...someone who knows what they're talking about will surely be along in a minute...

The key thing would be thinking about why you're considering changing and what you expect from your motor and how much your expecting to spend as top quality cranks can get expensive quickly